How to build foundations for an extension: Building Foundations: A Step-By-Step Guide

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Building Foundations: A Step-By-Step Guide

(Image credit: David Snell)

Building foundations function to distribute the weight of your home, stopping the plot’s subsoil from spreading and the structure facing an unequal settlement — both of which could lead to structural problems for your home down the line. 

If you’re the project manager for your self build or taking on the work yourself, it’s critical to double check all measurements for the building foundations and make sure everyone on site is working from the same plans to avoid costly mistakes that are difficult to rectify. 

The type of foundations system you choose may depend on the type of soil you’re laying your building foundations into, however a good team should have a level of flexibility in case they discover some surprising ground conditions during the excavation. Arm yourself with our guide to foundation systems and soil types, as well as keeping some budget back as a contingency for the foundations.  

Whether you need a refresher on the process, or just want an overview about what foundations will entail for your build, take a look at this step-by-step guide for laying out building foundations.

What Materials Do I Need for Building Foundations?

To start laying the building foundations, you’ll need:

  • mesh reinforcement in the foundation concrete
  • compressible material for lining the trenches
  • a polythene slip membrane
  • cut metal pins to use as markers ready on site
  • several rakes and pairs of wellies (you’ll be surprised how many people turn up for work in trainers)

(Image credit: David Snell)

Be aware of any lead-in times. If the engineers have designed a foundation that requires reinforcement cages, these may have to be manufactured off site.

Also think ahead to getting the services into the house, so get some lengths of pipe or ducting to take through the concrete.

How to Lay Building Foundations

1.

Start Building Foundations by Marking Out a Level Datum Point 

Mark out a level datum point. (Image credit: Homebuilding & Renovating)

Whats is a datum point?

A datum point is a point of reference from which further measurements can be made. The point can be based on the finished floor level, an existing building or a benchmark.

First you will need a level datum point. If you are building an extension, this is usually the finished floor level or damp-proof course (DPC) of the existing building. For new builds, this will be marked on the plan as a fixed point such as a manhole cover or a marker in the road or pavement.

The datum point is important: all your height measurements are relative to this point, so any errors could cause problems later on, particularly if there is a height restriction placed on the building.

2. Calculate Material to be Removed for Building Foundations

Calculate how much material needs to be removed and cleared as a starting point for your foundations and oversite. (Image credit: Homebuilding & Renovating)

Next, calculate how much material needs to be removed from the site to end up back at finished floor level once all the layers of flooring materials (insulation, concrete, screed, etc) have gone down. Time taken with a site level at this stage will make things easier later on when the digger may not be able to reach any areas that have not been dug deep enough.

3. Use String Lines to Mark Out Your Building Foundations

Start marking out the walls, starting with one corner point (Image credit: Homebuilding & Renovating)

Start marking out for the walls. Set up timber hurdles at the end of each wall, out of the way of where the digger will need to be. These carry the string lines that represent the wall faces of your building. Where space is limited, use corner pins instead and set up the hurdles after the concrete is in.

4. Mark Out the Trenches for Your Building Foundations

Use marker paint to mark out your footings (Image credit: Homebuilding & Renovating)

Begin lining up your footings by referring to your datum point. When it comes to extensions, never assume an existing building is square; pick the side you need to be square to and set up a line, then square off that line.

The easiest way to do this is using Pythagorus’s theorem (the square of the hypotenuse is equal to the sum of the square of the other two sides). If you measure the length of your first wall and then secure a tape measure at each end, by working out the length of the hypotenuse you can use both tapes to give you a third point exactly 90° to your first line.

Alternatively, most modern laser levels have a square function which does this for you.

5. Mark All Corners of the Building Foundations

Once you have two lines set up at 90°, other measurements can be taken (Image credit: Homebuilding & Renovating)

Once you have two lines set up at 90°, all other measurements can be taken from this point, with all corners and junctions marked. A lot of digger drivers are quite happy for you to mark ‘centre of dig’ lines, but you may prefer to mark both sides of the trenches.

Check the width your walls and allow 150mm both sides, then use a string line as a guide to mark the trenches with marker paint.

6. Start Digging The Foundation Trenches

Digging the trenches can begin (Image credit: Homebuilding & Renovating)

Now you’re ready to start digging. Have your site level set to the depth you need to dig and check it often. Ultimately it is down to the building control inspector to decide how far down you need to go; they will inspect at this stage and advise.

7. Introduce Depth Pegs and Pour

Introduce depth pegs in the trenches to show where you need to fill concrete to (Image credit: Homebuilding & Renovating)

Once Building Control has visited, you are ready to pour the footings. You should place depth pegs in the trenches to show you how deep to fill, but you may find that marking the sides of the trenches with marker paint also works well, with a final onceover with the site level and a concrete rake to get them spot on — the more accurate you can be with your levels the easier (and cheaper) the next stage will be.

Also make sure that ducts are installed for any pipes or cables that need to cross the trench below the concrete.

Begin pouring the footings using either a pump or labourers with barrows (Image credit: Homebuilding & Renovating)

If your site is difficult to access, it may be worth considering a concrete pump. This can work out more cost-effective than a few bodies with barrows.

Top Tips for Laying Building Foundations

Keeping the Site Tidy When Laying Building Foundations

Before you fire up the digger you’ll need to decide what’s going to happen to the spoil. If it’s staying on site you may need to hire in a dumper so that it can be stored in a suitable place where it won’t interfere with the build.

Otherwise, you’ll have to organise lorries to take it to a tip. The turnaround time, or distance to the tip, will dictate how many lorries are needed to maintain continuity on the dig.

You will also need to decide whether you’ll be able to load the concrete directly into the trenches or whether a pump will be needed. If you do decide on a pump — always a good idea if more than three loads are anticipated — then it will need to be booked and the concrete suppliers advised so that their mix and turnaround time can be adjusted to facilitate a continuous pour.

Getting Lorries to Site

If the concrete is being directed from a lorry into the trench then you need to make sure that there is hardstanding and stable ground for it to park.

Most mixes can be ‘pulled’ around the trenches using rakes, but if the sides of the trenches are unstable this may cause collapses and unwanted contamination of the concrete. (Shuttering the trenches prior to pouring can aid in this instance.)

Lorries have extension ramps but these can usually only reach for around 4m at most. Unless the lorries can off-load at various convenient points around the building, then it’s best to use a pump.

Adding a Basement When Building Foundations

If your ground conditions appear difficult, you might do well to consider building a basement. If you are expecting to spend, say, £30,000 on getting out of the ground, then you are maybe halfway towards the cost of a basement and you may find that you are able to add considerably more value to the house than the additional cost of a basement build.

Bruce is a third generation builder, based in the South West, with over 30 years’ experience in the building industry. He specialises in new build homes, extension and renovation projects, and his even built homes for self build industry guru David Snell. Bruce has recently completed his latest self build.

Foundation Solutions for Home Extensions

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Build It

Get your extensions project off on the right footing with Tim Doherty’s in-depth guide

by Tim Doherty

30th June 2019

So you’ve got a great design in the bag and you’re keen to crack on with your extension.

First things first: you need to decide how to build it.

This process starts with the foundations. Specifically, you need to understand what supports the existing house. This will have a major impact on the design of the new footings.

Combine this with knowledge of soil conditions, trees, service/drain locations and more, and your team can plan the best solution for your project.

Typical existing foundations

The long hot summer of 1976 heralded a turning point in foundation design.

Following a significant flurry of claims for subsidence, insurance companies enforced deeper foundations in order to stabilise new properties.

Before that, houses built in the 1930s to 1970s may have had foundations little more than 600-800mm in depth. Properties from the Victorian /Edwardian periods frequently only went to depths of just 450-500mm.

In fact, the Victorians, famous for their solid brick walls, used to step the brickwork out by a quarter -to- half of the brick’s width over several courses. This meant that the bottom of the wall sat on a wider brickwork footing. This footing may in turn have sat on a shallow strip of concrete, but not to any significant additional depth.

Houses built  post war may have used more concrete and less brick, but they still used shallow foundations. However, Building regulations have since changed, and they now require foundation depths of circa 1m. This change was a bid to eliminate superstructure movement.

If you have an old house with an existing cellar, this will probably have shallow foundations under the base of the external (retaining) walls.

If the cellar has shallow foundations, so too will the walls above-ground. So, the cellar might have depth of 2.5m, and sit immediately next to a non-cellar wall foundation of only 500mm depth.

Nowadays, it is common practice to transition these changes in depth over a short length of walling.

Topography, trees, services & drains

Next on the list should be a practical assessment of levels, planting and services on your property.

Drains, in particular, have a nasty habit of being right in the way of your extension. Traditionally they sit close to the existing building and often wrap around several sides. Sometimes drains on your land also cater for neighbouring properties. Here you’ll be under an obligation not to compromise overall performance.

An installation that serves two or more properties is technically a public drain.  Under regulations introduced in 2011, the water authority is responsible for its maintenance. Plan ahead for drain diversion, and make sure that the water authority agrees with your proposal.

The same applies to services (gas, electric, water and broadband) leading to your property. Always make sure these won’t be compromised by the addition. Speak to the network distributor if necessary.

If large trees are in close proximity to your home or extension, this can also be a problem. The NHBC (www.nhbc.co.uk) has a very useful guide. This guide shows how deep your foundations will need to be according to the height of common species and how far away the house is from the tree.

Topography is also important. Stepped foundations help if the ground is sloping. Here, again, there are clear minimum standards to conform to in terms of the depth and length of steps, and how far they need to overlap.

The Party Wall Act sometimes requires neighbours to come to an agreement if foundations come close to boundaries. Any agreements must be met prior to foundation excavation.

The Act provides a framework for preventing and resolving disputes .

Find out more:  www.self-build.co.uk/party-wall

Soils

The type of subsoil will also influence the depth and width of foundations required; and sometimes affects more fundamental aspects of the foundation design.

The ideal conditions to find are well-compacted sandy gravels (with the right mixture of porosity and compressive strength), chalks or sand/siltstone. Generally, these will allow for straightforward foundations.

Clays, on the other hand, will shrink as they dry out in summer and expand when they soak up moisture in the wet seasons – so footings generally need to be deeper and wider. You may need to install specialist products, such as clayboard, in order to stabilise the foundations – or even an engineered solution.

You’ll probably need to appoint a structural engineer if the soil adjacent to your property is soft, organic or generally unstable (such as filled, made-up ground).

Can you use trench fill or strip footings?

The easiest type of foundation to build is an excavated trench. This is when the excavation is mass filled with concrete (hence the name trench fill).

The concrete is taken all the way up to 75mm below the top of the excavated trench walls. After-which,  a course of blockwork will be installed up to damp proof course (DPC) level. This gives you a minimum of 150mm clearance above external ground level.

Strip footings work according to the same principles. However, the depth of concrete is restricted to the bottom of the trench only, up to depths of around 200-400mm. The rest of the foundation wall is constructed in dense concrete blockwork, directly under the above-ground external walls, with a DPC separation at floor level.

Concrete costs mean trench fill is a bit more expensive, but most people go for it as it’s so quick and easy to install. Widths are usually 450mm or 600mm to match excavator bucket sizes.

The external wall system, which is typically around 300mm thick, then sit directly in the middle of the foundation. The trench sides should always be vertical, so you can be sure the concrete is substantial all the way to the base.

Those doing the work will often dig the trenches by hand if access is tight. It’s fine to go for something between those two widths (unless your engineer says otherwise).

The wall must be central to the top of the foundation and the setting must be accurate. Those laying the foundations will use string lines and profile boards to accurately set out the site.

If the excavation depth is more than 1.2m (3m being the maximum with these standard foundation types), and depending on soil conditions, then the sides of the trenches may need to be supported temporarily.

Pumps might also be necessary to remove any water in the bottom of the excavations.

Engineered rafts

Where trench fill or strip footings aren’t suitable, your structural engineer might opt for a raft.

The reinforcing bars and mesh tie the integrated foundations and floor slab together, and these are strongest at the sides and mid span. It’s unusual for an extension to use a raft.

As the name suggests, this type of foundation ‘floats’. Purpose-designed movement joints connect the new edition to the existing dwelling to create the raft.

But engineers are solution-oriented people, and they will let you know if an engineered raft is the right move for your project and the site conditions.

Piles & pads

Troublesome sights may require concrete pad or piled foundations, with purpose designed engineered ground beams spanning from one pad/pile to another. Ground beams can then suspend any concrete of timber floor joists.

The choice between pads or piles is based upon soil conditions, with the latter being a more extreme, and thus expensive, option.

Typical domestic piles can reach depths of 6m to 8m – or even further – to reach good ground bearing strata. Mini piles work on the same principle but often only reach depths of 3m to 4m. There are several ways that you can install them. Many can twist into positions, much like a cork screw.

The engineer will have to approve the eventual design and some piling firms have their own engineering services to offer a full package.

A major cost is the pile caps, which sit on top of the piles to give a level base for the ground beam. Concrete pads, by comparison, are often only one metre square and are built to normal depths of 1m to 1.5m, with reinforcement typically added to tie the ground beams to the top of the pads.

Those installing a pad such as this will usually do so by hand, as it can be difficult shaping a pad of this size correctly

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30th June 2019

Groundworks & Foundations – Build It

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Structural Choices

We explain everything you need to know about getting your groundworks done

by Build It magazine

13th January 2021

You can’t afford to be complacent about the setting out of foundations – getting it wrong is all too easy. If you end up having to have extra trenches filled with concrete, this means extra cost for you right from the outset.

How to get foundations right

It doesn’t just matter to you that your first steps are correct, it matters to the local planning authority that your home is built in the position shown on the approved plans. Even when dimensions haven’t been specified, planning enforcement officers can scale off the drawings to check that your home is where it should be.

The distances to boundaries and the levels are both important from a planning aspect, because of the issues of overlooking and overshading neighbouring buildings.

tHE pOLITICS OF PLANNING

You and your builder are the only people who will check the setting out at the start, so take the time to get it spot on. With pegs driven into the corners and chalk lines sprayed on the ground between them, you can start digging the foundation trenches.

Mark the centre line of the foundations to avoid confusion; the wall positions will be set out with pegs and string lines later when the foundations have been concreted.

Self Build Education House – Groundworks & Basement Floor Slab

Build It magazine’s Self Build Education House is a real-life, access-all-areas project that will give you a unique insight into the process of creating your own bespoke home. The guide has a comprehensive section which outlines the process of laying foundations.

READ MORE 

Foundation types

Trench fill foundation

Preferred by many self builders and by most large-scale operators, trench fill avoids the need for bricklaying below ground.

The concrete is poured to within 150mm of the surface ground level, saving time and trouble. This means your project will get out of the ground quickly.

The sides of the trench play as much a part in supporting the load as the bottom, and so this foundation must only be used in stable ground, where the trench sides are firm and capable of bearing loads.

Clay and chalk soils are ideal for trench fill foundations.

Strip foundations

Usually wider, strip foundations use less concrete than trench fill because the base layer of concrete is thinner. Strip foundations are typically 300mm thick.

The exact thickness will be determined by the soil conditions, design of the building and the number of masonry courses in the walls up to damp-proof course.

If you have a sloping site, you may need to step the foundations to keep them level. Steps should overlap at least the width of the trench when concreted and that means shuttering across them. Use plywood and retaining pegs.

Strip foundations are often necessary in softer soils, such as sand, since they spread the load of the building out over a greater area.

Building control and home warranty inspections

You must notify your building control officer at certain stages and await their inspection. You can assign the task of giving notice to your contractor, but check for yourself that the works have been inspected and approved before covering that stage and continuing.

Most building control authorities are happy to book inspections from a phone call, email or fax, so long as they are given a day’s notice. The same goes for your insurance warranty inspector, who will also need to inspect at some stages.

Building control inspection schedule for groundworks

  • Commencing the work Give two days’ notice in writing
  • Foundation excavations Give one day’s notice by phone, email or fax
  • Oversite preparation Give one day’s notice by phone, email or fax
  • Drains laid in trenches Give one day’s notice by phone, email or fax
  • Drains testing Give one day’s notice by phone, email or fax

Foundation depth

Foundations formed in rock and stony grounds can often be shallower than those dug in “shrinkable” soils such as clay. In the latter, at least 1m deep is normal.

Nearby trees or shrubs may extend this depth to a good deal more. Your building control officer will guide you in these cases.

Once the trenches have been dug and inspected, you will need to hammer pegs into the sides to mark the level for top of concrete. Timber pegs are safer than steel pins.

Pouring the concrete

Make sure the ready-mix lorry can access the site. Readymix can be placed by pump if the trenches are difficult to get to or the truck’s chute can’t be extended far enough.

Pumps must be booked in at an early stage. One reason for doing so is because there may not be that many of them in your area – they’re often tied up with bookings on civil engineering works.

Once poured, the concrete will need to be hand-raked into level against the pegs.

Ground-bearing floors

Beneath a ground-bearing concrete floor slab, the ground must be properly prepared with an oversite treatment.

The site should have been stripped clear of topsoil and vegetation before the foundations were dug. It’s from this reduced level the infill construction begins.

Hardcore should be used in a layer, at least 150mm thick, but no greater than 600mm, in selected aggregate. It’s then compacted down in layers with a plate compactor. If this task is done badly or with the wrong material, settlement is to be expected, causing the slab to crack.

Check out your options for oversite material locally. Hardcore rubble needs to be clean, broken brick, concrete or stone that has been crushed or is naturally available in less than 100mm particle sizes.

Approved aggregates are available from merchants and some offer recycled glass and concrete crushings.

On sloping sites, where part of the oversite may exceed 600mm deep, it may be acceptable to use lean-mix concrete in layers with the hardcore to make up the difference, instead of switching to a suspended floor type.

Who does the groundworks?

If you’re hiring a main contractor to deliver your build, they may have an in-house team or at the very least will appoint trades they trust for the groundworks phase. Nevertheless, you may find it reassuring to ask for references for site operatives you haven’t directly appointed.

If you’re working with a package or turnkey home supplier, bear in mind that most of these companies require you to sort the groundworks yourself – before they come on site to do their bit (the house shell and onwards).

“Your frame supplier or brick and block contractor isn’t going to inspect the groundworks as they go along,” says Charlie. “They’ll turn up on site expecting the work to have been done properly, so it’s crucial you find the right person for the job. Otherwise, you’ll end up with a lot of standing time, while the trades wait on your groundworker to rectify their work.”

If you’re knocking down an existing building to make way for your dream abode, then it can make sense to use the same contractor for both this and the groundworks. This way you can avoid any bickering between trades about whether the site has been left in an adequate condition (with any rubble or debris properly cleared away) post-demolition.

Take care not to push out the external walls if a round stone fill is used. Stone just rattles around and doesn’t exactly compact. Like water, it finds its own level very quickly. Lap and tape down 1,200 gauge polythene for the damp-proof membrane and make sure it is dressed up over the walls so it can be lapped with the damp-proof course later.

The insulation boards can be laid out over the damp-proof membrane and covered by an additional layer of polythene before placing the concrete. These boards should also be cut to an upstand around the external walls to encase the concrete slab fully in a warm tray.

Angular hardcore needs binding with a thin layer of sand to protect the polythene damp-proof membrane from puncturing and also to create a level bed for the insulation to sit on.

Levelling

 the concrete

The hard work is in raking it out and tamping it into level using a straight length of timber that will span the distance between the walls on either side.

Tamping the concrete helps to consolidate it in the same way that vibrating it does and, combined with a gentle sawing and tapping motion, will bring the surface to the level and finish needed.

Planning the drainage system

Levels are just as critical when laying the drainage system. The invert level is the bottom of the pipe and from this point the system falls are determined.

The trenches need to be carefully dug to the right depth and gradient to avoid huge amounts of bedding being necessary to support the pipes.

Gravel is used to a depth of 100mm to 200mm thick for both bedding and surround material supporting and protecting the pipes at the correct fall. A gradient of 1:40 is ideal, but plastic drains will tolerate much shallower falls if it’s necessary.

Look for at least half a bubble out on the spirit level and make sure they fall the right way. Eyes can be deceived when looking at drains in trenches, particularly when the ground slopes the other way.

Drainage tips

  • Lay pipes to an even gradient
  • Lay pipes in straight lines and avoid bends
  • Locate the soil vent pipe at the head of the run
  • Provide rodding points or inspection chambers at bends and changes in gradient
  • Connect water closets and soil vent pipes direct to inspection chambers
  • Use at least 100mm of gravel to cover the pipes before backfilling the trench and don’t leave the trench open for longer than is necessary

Drainage trenches to run services

Using deep drainage trenches for running in other services makes good sense. Gas, electric and water cables all have to be provided at your expense, so laying them in position now will save time and money later. Getting the supply companies to connect them when the house is complete can be a slow procedure, so get the job registered, priced and booked in well ahead to avoid delays later.

Drains testing

For peace of mind, a water or air pressure test should be carried out before drains are backfilled to ensure all the joints are watertight and everything works. Your building control officer may want to witness the test at this stage or when your self-build is complete.

The reason for the latter is that drains can become displaced or damaged with careless backfilling and site traffic.

Drainage connections are usually best left until later, but if you do connect up at this stage to the mains system, make sure you fit manhole covers and fill gulley traps with water. The air will have turned foul on site before you’ve hardly started.

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Published

13th January 2021

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Pat Canova / Getty Images

Nobody can deny the need for a good house foundation. Improperly built foundation footings—unlike poorly installed gutters or gapped hardwood flooring—can eventually bring down the house.

So, building codes naturally have a few things to say about foundation footings. Like any code recommendations, these are not how-to guides to building footings so much as they are parameters that you need to observe and should be adjusted according to your local code requirements.

Site Grading

The ground immediately adjacent to the foundation footings—on the exterior of the foundation—should slope down at a five percent minimum slope. This must continue for at least ten feet.

Load-Bearing Value of Soil

Foundations must rest on firm, undisturbed soil. ​Undisturbed soil is soil that has never been turned over, tilled, graded, hoed, dumped, dug, scraped, compacted, amended, or anything of that nature, by human by or machine. Undisturbed soil is significantly stronger than soil that has been disturbed.

Code refers to load-bearing values (LBVs) as “presumptive.” This means that a soil test is the only way to really know the load-bearing value (LBV) of the soil for the footings on a given site.

Disturbed soil can eventually be classified as undisturbed after enough time; a soil test can determine this.

Soil Type LBV Per Square Foot
Bedrock 12,000
Sedimentary Rock 4,000
Sandy Gravel or Gravel 3,000
Sand, Silty Sand, Clayey Sand, Silty Gravel, Clayey Gravel 2,000
Clay, Sandy Clay, Silty Clay, Clayey Silt 1,500

Depth, Width, and Thickness of Footings

Depth of Footings

Footings should extend to a minimum depth of 12 inches below previously undisturbed soil. Footings also must extend at least 12 inches below the frost line (the depth to which the ground freezes in winter) or must be frost-protected.

These rules may not apply to accessory buildings (such as sheds) if they are below a certain square footage specified by your local building code. Also, deck footings may have a different depth requirement. Some decks, such as those not attached to the house, may not have the same depth requirements.

Tip

Areas with expansive clay soil will require caissons (or concrete columns) to be drilled down to bedrock at pre-determined locations, with some sort of expansion material between caissons to allow for soil expansion and contraction. The foundation walls will then be poured on top of them.

Width of Footings

Footing width can vary according to the structure, site, and conditions. Under code, generally, one-story buildings with footings on undisturbed soil with LBVs between 1,500 and 4,000 should have a minimum width of 12 inches.

Two-story buildings require a minimum of 15-inch wide footings for 1,500 LBV soil. For 2,000 LBV soil or greater, two-story buildings may have 12-inch minimum wide footings.

Levelness

Requirements for levelness are different for the top and bottom of the footing:

Top of the Footings

The top of the footings must be level, with no exceptions.

Bottom of the Footings

The preference for the bottom of the footing is that it is level. Exception: If building on a sloped grade, you can step the footing by as much as one unit vertical per 10 units horizontal (or a 10-percent slope).

Spread Footings

Spread footings help distribute the load carried by the footings over a wider area. The spread part is a base that looks like an upside-down “T” and transfers the weight across its area. The spread footing should be no less than six inches thick. It should project, on both sides, no less than two inches.

Strength of Footing Concrete

For foundations and slabs, except for garages, the minimum strength of the concrete should be 2,500 psi. Vertical walls exposed to the weather may be required to have 2,500 psi concrete minimum strength, on up to 3,000 psi, in some severe weathering conditions.

Building Code Basics

These code specifications are derived from the International Building Code (IBC) for 1- and 2-story residences. This summarized guide is intended to give you a general sense of code requirements for foundation footings. The IBC encompasses the International Residential Code (IRC) but includes provisions for commercial buildings as well as for residential buildings. The IRC is equally suitable for residential buildings. In the IRC, the chapter relating to foundations and footings is chapter 4.

Keep in mind that each construction project is unique. For example, the soil is different from place to place, and thus the load-bearing value of the soil will change. Also, code rules are set and enforced at the local level, typically through each city’s building department. Most local code authorities use the IBC and/or IRC as their model codes but may omit, modify, or expand any code specification to suit local conditions and legal requirements. While the IBC and IRC are merely suggested guidelines, the local code is the law of the land that you will need to observe.

Watch Now: How to Tell If a Wall Is Load-Bearing

House Foundation Types, Uses, and Pros and Cons

Every house is built on a foundation, but not every house is built on the same type of foundation. Home foundation type is based on factors such as house design, geographical location and climate, soil and moisture conditions, and the project budget.

Click Play to Learn About the Types of House Foundations

Foundation types vary, but likely your house or home’s addition does or will have one of these three foundations: full or daylight basement, crawlspace, or concrete slab-on-grade. These are serviceable—and sometimes even flexible—foundations that work for millions of homes across a variety of climates and needs.

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    Full or Daylight Basement Foundation

    BanksPhotos/Getty Images

    A basement foundation is the deepest of the three major foundation types. A full basement matches most or all of the floor space of the level above and it is generally at least 7 feet high. Newer homes typically have taller basements to facilitate conversion into living space. 

    The primary advantage of a basement is all the extra space that it provides for storage or for living. In some homes, finishing a basement can nearly double the home’s living space. A basement can be conditioned (meaning that it is heated or cooled) like the rest of the house or it can be unconditioned.

    Full Basements

    A full basement is one that matches the perimeter of the house above. So, if the ground floor of the house is 800 square feet, the full basement’s square footage is also 800 square feet—more or less.

    A full basement consists of structural foundation walls that bear on foundation footings running along the perimeter of the basement. Footings usually extend below the frost line, the depth to which the ground freezes in winter. 

    A full basement with a ceiling 7 feet high or higher is a valuable asset to a home. It can be converted into habitable living space or turned into a home gym, children’s play area, home theater, or just left for storage.

    Daylight Basements

    One variation of the full basement is the daylight basement, sometimes called a partial basement. Built against a slope, the daylight basement has one or more sides that are completely embedded in the ground, from floor to ceiling. However, as the slope descends, one or more sides of the foundation are exposed and can have large windows and doors to bring in daylight. 

    Daylight basements are often more practical for conversion into living space than full basements. They offer the possibility of adding a separate entrance, a necessity if you plan to rent out the space. Daylight basements, too, avoid many aspects of basement living, due to the increased natural light and air. Mold and mildew are easier to manage in daylight basements than in full basements.

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    Crawlspace Foundation

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    Foundations that create a crawlspace consist of short foundation walls that stand on footings. The walls may be very short—creating a space that you literally have to crawl through—or they can be in the range of 4 feet tall, providing room for storage and sometimes a furnace or other equipment. 

    Typically, crawlspaces are unheated spaces and may be ventilated with small vents that penetrate the foundation walls to promote a small amount of airflow through the crawlspace. Moisture control and water leaks and seepage are other considerations with crawlspace foundations.

    Tip

    For foundations in areas with certain types of clay soils, caissons (or deep holes filled with concrete) are often drilled down to bedrock. The foundation is then placed on them in lieu of footings. This allows for the expansion and contraction associated with certain soils.

    Crawlspace foundations are less expensive than full basements because they require less excavation and less foundation wall material and labor to build. Like full basement foundations, crawlspace walls typically are made with poured concrete or mortared concrete block.  

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    Concrete Slab-On-Grade Foundation

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    A slab-on-grade foundation is a solid concrete slab that rests on the ground. Grade refers to ground-level, and slab refers to the monolithic concrete pad.

    Slab-on-grade foundations are most commonly used in climates that do not experience ground freezing and thawing, as this can lead to cracks in the concrete and shifting of the foundation.

    Slab-on-grade foundations tend to be less expensive than the subgrade (full basement or crawlspace) foundations and can offer better protection against termite infestation (also common in warmer climates).

    One notable downside of slab foundations is that water supply and drainage pipes are encased in the concrete. In the event of a problem, the concrete slab must be cut into to access the pipes. With a slab foundation, it also means you won’t have storage space for your home or additional living space. It is also not feasible to retroactively add a basement or crawlspace.

Building regulations: extensions: Foundations | GOV.WALES

Guidance

In this guide

4. Foundations

Foundations are required to transmit the load of the building safely to the ground. Therefore, all buildings should have adequate foundations (normally concrete), which will vary from one project to another depending on the circumstances of each case.

These foundations can be cast as deep-fill (filling most of the trench) or shallow-fill (where the minimum thickness to transfer the load to the soil is provided).

There are other types of foundations that may be used if the ground conditions do not make trench fill practicable. It is advisable to contact a structural engineer or speak to building control for further advice.

Factors to be taken into account of when designing a foundation:

Type of soil

The type of soil that the foundation will sit on is important for two reasons:

  1. It should be able to bear the weight (load) of the foundation and the extension – different soils have different load bearing capabilities.
  2. The way it reacts to variations in moisture content (such as in prolonged rainy or dry seasons) can lead to the soil expanding or contracting. This is a particular issue with some clay soils. These changes mainly occur up to a certain depth (typically about 0.75m) therefore foundations should be made deeper so they are not affected by ground movement (although see “Trees” below).

Adjacent structures

It is important to ensure that the excavation for the new foundation does not undermine adjacent structures. In general it is good practice to excavate at least to the same depth as the bottom of the foundation to the adjacent building. If the excavation runs alongside an existing footing then care will be needed – for example, by excavating and concreting the foundation in shorter sections to avoid undermining a whole length of an adjacent structure (see also guidance on the The Party Wall etc. Act 1996).

Trees

Trees will draw moisture from the ground around them and beyond through their root system. As moisture is drawn from the ground it will have a tendency to shrink. How much the ground will shrink will depend on the following factors:

  • Type of soil – Clay soils shrink more than other types of soil. Therefore excessive movement of the ground could cause damage to the foundation and the structure it supports.
  • Size and type of tree – How large a tree or shrub will grow (its mature height), and the tree type will determine how much moisture it generally draws from the ground.

The presence of trees in clay soil areas can mean foundations need to be significantly deeper than might be first expected, although if the trees are far enough away, there may be no impact. Note: If existing trees are removed or significantly reduced in size, all or some of the moisture in the root system will be released over time into the soil and, if the soil is clay for example, could cause swelling of the soil and damage to nearby foundations and structure(s) supported.

Drains and sewers

As the weight (load) from the foundation of a building is transferred to the soil it spreads downwards outside the footprint of the foundation at a typical angle of 45 degrees. If a drain or sewer is within the area covered by that 45 degrees area there is a risk that it could be affected by the load from the foundation and possibly crack. Therefore,the foundation excavation should normally be at least to the same depth as the bottom (invert) of the deepest part of the drain, sewer or its trench.

Size and construction of new building

The foundation will need to support more weight (load) from a two storey building compared to a single storey. This has a significant factor in determining design, particularly in respect of its depth and width. This is directly related to the bearing capacity of the soil supporting it. The width of the foundation is also governed by the wall thickness.

Ground condition

Generally the topsoil is taken away and good undisturbed ground is found i.e. ground that has not been built on. In some cases there are areas which have previously been backfilled, such as above where drains have been laid or to level a site, which consist generally of soft, mixed soil with foreign objects. The foundation can not be poured until undisturbed ground has been found.

Landfill sites

Some properties have been constructed on landfill sites which may require a more extensive form of foundation like piling as the depth of undisturbed ground could be many metres deep. An alternative may be a “raft” foundation. A structural engineer will be able to advise you further.

For health and safety reasons, care should be taken when working in trenches due to the risk of collapse causing potentially serious injury.

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90,000 how to make quickly, reliably and durable

Content

  • Preliminary work
  • Rules for the foundation
  • Hard accession
  • Separate construction
  • frame extension
  • If a brick or wood house

is early or late in almost any other households need additional living and working space. When planning the construction of new buildings by joining the existing main house, it is important to take into account all the loads and the impact of the inevitable shrinkage on both parts of the structure. Before starting the construction of any such structure, it is necessary to equip the foundation for the extension. In case of errors during construction, deviation and distortion of the structure is possible. That is why the most important stage of the process will be the design and installation of the foundation for the future premises.

We must not forget that the implementation of any major projects requires coordination with regulatory organizations. To build an extension to an existing building is a rather responsible task, as well as the construction of a full-fledged house. An accurate calculation and a well-planned action plan are required. A number of professional tools are involved. Most likely, you will have to get helpers. And yet, subject to the technological process, it is possible to do it yourself, without involving third-party specialists.

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Preliminary work

Preparatory stage

In order to determine the type and dimensions of the building foundation, a pit 100 × 150 cm or 100 × 100 cm is pulled out near any wall. The strip foundation is measured in width and height. For a columnar, the parameters of the pillars and the depth of their laying are important. Enough attention should be paid to this issue, since marking the foundation is one of the most important stages.

The size of the base is measured using a steel rod, the end of which is bent 90°. The rod is placed under the sole, placing the bent part strictly horizontally. By turning it, they detect the moment when the curved edge rests against the wall from the inside, a mark is placed on the metal. The distance from the bend to the mark, measured after the rod is removed, will correspond to the width of the tape.

At the time of excavation of the pit, the type of soil on the site is also determined. From the data obtained will depend on which foundation to choose.

If there is a project of an old house, preliminary measurements are not required. But it is necessary to know the geological characteristics. Careful planning at the initial stage of work, taking into account all the features of the area, will help to avoid mistakes.

After the analysis, you should decide what design the future foundation should have. They are distinguished by several types:

  • columnar;
  • tape reinforced concrete;
  • pile and screw pile.

With rare exceptions, the extension has the same type of foundation as the house. This ensures the reliability of the entire structure.

back to contents0005 In the process of attaching two bases, it is impossible to dig the entire perimeter at the same time if it is more than 3 meters. This will make the building less stable.

  • When a building is placed on a concrete slab, another slab is poured next to it. Similar ones are attached to a pile, strip or column foundation. The difference in characteristics will create an excessively large stress on the existing foundation, which will lead to greater shrinkage, and sometimes the collapse of the entire structure.
  • The depth of the foundation for the extension must be planned somewhat less than for the main building. This creates a place for future shrinkage.
  • Further work is carried out only after shrinkage of the poured concrete. It usually takes about six months.
  • Coupling can be carried out in the following ways:
    • Separate installation and subsequent formation of an expansion joint;
    • Rigid bond with common reinforcement.

    To correctly pour the foundation for an extension, you need an error-free calculation. The choice of communication method depends on the type of land in the area. This is also affected by the dimensions of the attached structure.

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    Rigid connection

    A similar method is used in the following cases:

  • if there is a shallow strip foundation, the plinth of which forms a single whole with it;
  • when building shrinkage is predictable;
  • when bringing an extension of several floors under one roof with the main house.

    • In the case of the column version, the fittings are fixed on the plinth, if it has sufficient dimensions. When the length is not enough, it is more logical to choose another method. Violation of this rule is fraught with cracks and unsteadiness.

    How to start joining strip samples:

    • A ditch is cut to size. A layer of sand is poured to the bottom to ensure stability.
    • Holes are drilled in the foundation. Each next row is drilled with an offset relative to the previous one.
    • Metal rods are driven into the holes with a longitudinal slot at the end, into which wedging wooden inserts are inserted.
    • With the help of protruding pieces of reinforcement, the foundation for the extension is formed.
    • After the frame is ready, the concrete is poured.
    • Further work may only be carried out after the structure has shrunk. This standard is observed for any filling options.

    The slab foundation is connected in this way only if its thickness is more than 400 mm. The possibility is preserved even when the slab protrudes beyond the basement of the house. The protrusion must be more than 300 mm. The fittings are exposed and welded to the metal frame of the base of the extension. Such protrusions are often found in buildings made of foam blocks.

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    Separate erection

    When working with a tape base, the territory is marked out and a ditch about 600 mm deep is dug under the tape. The bottom of the trench is covered with sand or gravel to create a cushion. The next step is to install a solid formwork according to the given dimensions. Reinforcement is made with metal rods and cement mortar is poured. A free-standing foundation for an extension to the house guarantees durability and stability of the result.

    Adjusting the base to a wooden house

    Using this method, a void is left between the wall of the old house and the foundation of the extension. The width varies from 100 to 150 mm. Inside, a layer of waterproofing and thermal insulation is laid. It is imperative not to miss this moment and make such a deformation seam that prevents shrinkage and protects against damage. The new sole is slightly higher than the old one. Over time, it will settle to the same depth as the main structure. This will ensure the reliability and durability of the entire structure.

    In addition to the strip foundation, a columnar foundation is also used, closely adjoining the main building. The piles are preliminarily waterproofed and installed on concrete pads. Next, the strapping of the required dimensions is mounted. This method reduces construction time, allowing construction to continue in just a few days.

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    Frame extension

    The foundation for the frame extension does not carry significant loads. Basically, such designs involve the use of lightweight materials. In this case, the correct solution would be to use a columnar view.

    The work takes place in several steps:

    • holes are dug in the ground, located below the freezing level;
    • the bottom is filled with sand and gravel;
    • fittings are installed;
    • formwork installation works;
    • cement mortar is laid;
    • poles can be completed with brickwork.

    Glass extension

    It is possible to make an extension to the house using a variety on screw piles. They are screwed into the soil and cut off to the desired height. In order to increase strength and durability, concrete is poured, then the heads are welded.

    This type is characterized by speed of execution. After its installation, it is possible to proceed to the construction of a structure from timber, OSB or plywood. The finished structure cannot be further weighted with any elements. The whole process should strive for minimalism.

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    If the house is made of brick or wood

    In this case, tape variations of reinforced concrete are used. The base for an extension attached to a brick house is installed in the following order:

    • The territory is marked according to the developed plan.
    • Dig trenches below the freezing level of the soil.
    • The bottom is covered with a sand-gravel mixture to a height of 100-150 mm.
    • A rigid connection to the existing base is installed and an expansion joint, if necessary.
    • The foundation for the extension is reinforced with a reinforcement frame. Most often this is a design of four segments with jumpers.
    • Formwork is exposed.
    • Concrete mortar is poured.

    The foundation for an extension to a wooden house is made in the same way.

    The best option for building an extension is the one in which it is built together with the main building. Unfortunately, it doesn’t always work out that way. The need for expansion may arise due to family circumstances, hobbies of the owners of the house, and also depends on financial issues. It is quite possible to attach a room to the house on your own, if you follow all the rules and recommendations.

    How to make a foundation for an extension to a house – builder’s secrets + video

    Is it true that the foundation for an extension differs from the main one, what are its features? Many people are looking for the answer to this question, because when adding additional rooms to an existing house, you want these rooms to also have a reliable foundation. We have collected the most important tips and instructions in this article.

    Wouldn’t the extension be complete without a foundation?

    Everyone knows what role the skeleton plays in the human body, similar functions are assigned to the foundation. In fact, this is the basis that is responsible for the reliability and durability of the building. Also, it is the foundation that provides heat and dryness in the room. If the base is of poor quality or does not fit the specific conditions, then dampness will appear inside, the floors may deform and even deteriorate. It also levels the ground level and prevents the penetration of carcinogenic gas from the soil into the dwelling.

    Outbuilding base

    The choice of foundation must be taken with particular care. After all, it is necessary that it freely withstand the load and correspond to the type of soil. At the same time, there is no need to overdo it, for example, if you can fill in a strip or column foundation, then there is no point in replacing these simpler options with a complex monolithic base. Firstly, it is too expensive and will not justify the investment. Secondly, the first ones are much easier to perform and you can do them yourself even without outside help, but you won’t be able to lay a concrete slab on your own.

    It is mainly built of stone or concrete, but there are also wooden structures. If the building is lightweight, then you can give preference to a shallow base located above the freezing point. But most often its depth should be below this level. According to its purpose, it is divided into carrier and combined. The latter, in addition to standard load-bearing functions, must also provide seismic protection. There is also a special type, it includes “swinging” and “floating” foundations, their pressure corresponds to the pressure of the excavated soil.

    Which type of base to choose from?

    In this paragraph, we will consider what types the foundation for an extension is divided into. Tape can rightfully be called universal, because it is perfect for both heavy and lightweight structures. It is located only under the outer and outer load-bearing walls. It is mainly poured from concrete; to make the structure more reliable, it should be reinforced with iron rods. If you are going to make this type of foundation with your own hands, then it is better to give preference to the prefabricated version. The disadvantages include high cost.

    Universal strip foundation

    But the columnar foundation, on the contrary, refers to budget options. But it is advisable to use it only when it comes to lightweight structures, for example, you need to tie a frame extension to a wooden house. The supports are special pillars located at intervals of 1.5 to 3 meters. These pillars must be placed at the intersection of load-bearing walls. It is best to make such elements from brick or reinforced concrete, but larch bars can also be used. However, such supports are expensive, and wood is not durable.

    If you are going to build a fairly massive building, and weak soil prevails, then you should give preference to the pile type of foundation. This design consists of supports connected by means of a strapping beam.

    How can two foundations be connected?

    The foundation for an extension can be rigidly attached to the base of the dwelling, resulting in a single structure. Such a decision is relevant if the site has a weakly or non-heaving type of soil. It is also worth giving preference to this option when you are going to make an extension of several floors and want to connect it to the main housing through one roof.

    If the base is a slab, then a solid foundation must also be poured for additional housing. At the same time, it is necessary that its thickness be at least 400 mm or the base of the residential building should have protrusions. In this case, it is possible to weld the reinforcement of the slab to the frame of the new foundation. In addition, if you wish, you can do everything with your own hands.

    Monolithic foundation slab

    Tape-to-tape connection requires a sand cushion and reinforcement with a longitudinal slot. Metal rods are driven into specially prepared holes in the base of the house and form a frame for a new foundation. Next, the joint is made by means of a release about 40 cm long.

    You can also build a separate base for an extension, which will be located in close proximity to the house. A sheet of roofing material should be placed between the two bases. This material acts as an excellent waterproofing, you can also place a layer of thermal insulation or get by with a simple tow. In this case, it is very important that this layer allows the new base to not collect water during rainfall and not damage the foundation of the house. From the outside, special decorative overlays are fixed to the wall, covering the seam.

    Laying the foundation – we do it according to science

    Now let’s move on to the practical part and consider how to make a foundation for an extension with your own hands. This detailed instruction will help save on the services of a master, because it is designed for independent construction work.

    Building a foundation for an extension with your own hands

    How to make a foundation for an extension to a house – a step-by-step diagram

    Step 1: Choosing a base

    The new foundation must fully match the type of the old foundation. In addition, the load and type of soil are taken into account. For example, if we are talking about a massive extension to a brick house, then it is necessary to lay a tape or make a pile foundation. But if the additional room is adjacent to the wooden housing, then it is possible to get by with a cheaper columnar option.

    Step 2: Calculations

    Regardless of whether you prefer a fixed type of connection or a separate foundation, the depth and width, as well as the dimensions of the foundation pillars, if any, should be the same as the foundation of the house. Determining these parameters is quite simple. It is necessary to dig a hole next to the housing wall and measure the dimensions of the base. If there should be no questions about how to measure the depth of a structure, then in order to measure the width, you should take a rod, bend one of its edges by 90° and push it horizontally under the foundation. Then we turn the rod until its bent part rests on the back side. After that, you need to make a mark on the bar and pull it out. By measuring the distance from the hook to the mark, we find out the width.

    Step 3: Preparatory work

    Preparatory work must be carried out before the foundation for the extension can be poured. If you decide to give preference to a rigid connection, then we dig a trench of a given size and drill holes in the existing base. Moreover, their diameter must exceed the thickness of the reinforcement. Metal rods should also be prepared. It is necessary to make a longitudinal slot on their end and insert a special wedging insert into it.

    Step 4: Forming the frame

    We drive the reinforcement with wedges into the prepared holes. The number of rods is taken at the rate of 20 pieces per square. Thus, the frame of the future foundation is formed. To ensure a secure connection of the following parts, it is necessary to leave the edges of the reinforcement about 300 mm long, which will subsequently need to be welded .

    Step 5: Installing the formwork

    Now you need wide wooden boards. We form the formwork of the given dimensions. We securely fix its walls with props, earth, cinder block, etc. You can close the inside of the formwork with polyethylene. The latter is especially true if you are pouring only part of the foundation and you will have to reuse these boards.

    Step 6: Pouring mortar

    Prepare a mixture of cement, sand, gravel and water. It is very important to keep the proportions right. For example, for the M300, you can take 10 kg of cement, 30 kg of sand, 40 kg of gravel, mix everything well and pour 40 liters of water. If the concrete is too dense, then you should dilute it with a liquid, but only add it in small parts so as not to spoil the solution. We pour concrete into the formwork and wait a few days until it completely hardens. We moisten periodically, otherwise cracks may appear.

    Step 7: Forming a column foundation

    Making such a foundation is relevant when a lightweight extension adjoins a wooden house. We drill wells so that they are below the freezing level of the soil. We form a sand cushion and install fiberglass reinforcement. This material has excellent properties and does not require additional treatment with waterproofing compounds. Formwork of specified dimensions is also formed and poured with concrete. This foundation for an extension is the easiest to perform, and everyone can create it with their own hands.

    Step 8: Construction of a separate foundation

    But when the number of storeys of the main and adjoining housing is different, then a separate foundation should be made in close proximity to the former one. The distance between the two bases is from 2 to 5 cm, depending on the number of storeys of the new premises. The higher it is, the wider the seam is selected. We fill this gap with thermal insulation materials or tow. Then a special decorative overlay is attached to the wall of the housing. In this case, it is very important to correctly mark the base. To do this, we stretch the cord on the pegs around the perimeter of the future site and check if the diagonals are equal. If the heaving type of soil prevails on the site, then the floor of the extension must be made so that it is not lower than the flooring of the main housing by the amount of possible deformations.

    • Author: Mikhail Malofeev