Running pipes in concrete floor: Plumbing Under a Concrete Floor

Plumbing Under a Concrete Floor


By David Crosby

Issue 156


Synopsis: With a slab-on-grade foundation, there are no floor joists in which to run plumbing pipes. Everything must be buried in the dirt prior to pouring the concrete. This article details how to locate the pipes accurately, pitch them correctly, and backfill the excavation so the slab wont crack.

In slab-on-grade construction, a concrete slab serves as a floor (and at times the foundation) for the structure above. It is common throughout the Southwest because slab-on-grade construction is fast and strong, and because it offers a high thermal mass while being a potentially less expensive route than traditional woodfloor construction. Concrete is easy to tile over, and here in New Mexico, tile floors are king. Also, the advent of radiant heat and color-tinted concrete make a finished concrete floor an attractive, extremely durable alternative to traditional flooring.

With a slab-on-grade floor, there are no joists to house plumbing, so we bury the pipes under the leveled building pad and pour the concrete floor right over them.

Because the plumbing rough in happens prior to the concrete pour, layout accuracy becomes crucial long before the first wall is built. The expression “set in stone” takes on new meaning when you consider that a layout error could place a drainpipe in the middle of a room rather than in the intended wall.

Verifying the plans now saves headaches later

Before starting any excavation work, I make sure that I fully understand the plans. I confirm that everything that’s supposed to fit inside the footprint actually will and check that wall thicknesses are appropriate throughout the drawings.

For example, if the bathroom wall that houses the plumbing, also called the wet wall, is drawn as 2×4 framing and a vent stack is supposed to go in there, I’ll see if I can substitute a 2×6 wall instead. The vent stack is usually a 3-in. pipe that supplies air to the drain, rises up a wall and exits through the roof. The air keeps water in the traps and allows liquid and waste to drain. Think of putting your thumb over a full straw; it won’t drain unless it’s vented. Plumbing a vent stack inside a 2×6 wall is just easier, and the plumber will thank me.

Start all the measurements at the same outside wall

Quite simply, the layout involves transferring dimensions from the plans to the building site, which is why I take the time to verify the plans. Before running a tape or snapping a chalkline, I gather all the interior-wall information from the plans. Then I begin the layout. For this layout, I measured from the north stemwall (the concrete wall that forms the perimeter of the slab), having verified that it is square to the other walls and runs parallel to the bathroom’s wet wall. All my measurements were taken from the north wall, also called the common point. Because all the measurements originate from one point, it’s easier to verify them later.


For more photos, illustrations, and details, click the View PDF button below:



Get home building tips, offers, and expert advice in your inbox

View PDF

Plumbing in Concrete Slab

Many homes have been built on concrete slabs. This is an efficient building practice, but can create serious plumbing problems in the future, as the home ages. When installing the plumbing system during construction, the pipes are laid prior to pouring the concrete, and after pouring, fully encased. In some cases, the pipes are laid in the dirt beneath the slab. Unfortunately, should a pipe crack, break, or burst, you have a real problem on your hands.

Under Slab Plumbing

If you have concerns about the state of repair of under slab plumbing or have noticed any leaks, particularly in an aging home, our team at Benjamin Franklin Plumbing in can help you identify the source and get it repaired efficiently. Your bathroom plumbing diagram under the concrete slab may no longer be available, but we have equipment that will reveal piping, whether in the bathroom or any other area in which they are placed in the concrete slab. If you are remodeling your bathroom on your own (more power to you!) or want to install a new bathroom, there is plenty to understand before you begin. If you are having a new home built, with pipes being laid in the concrete slab, it is well worth having every detail checked by a professional before the concrete is poured – for obvious reasons!

Rough In: Be Careful During Cement Pour

The plumbing system build starts with a “concrete slab plumbing rough in.” This is when the plumbing contractor puts the pipes in place, prior to the cement being poured. Your plumbing contractor should be present when the concrete is poured to ensure no pipe or vent is shoved or kicked out of place. Installing plumbing in the concrete slab requires skill and accuracy, as well as a full knowledge of the building codes in the Cedar Rapids area.

If you are in the process of building a home on a concrete slab foundation, whether you are doing it yourself or working with contractors, ensure you work in close collaboration so that everything is done right. Nothing is more critical when it comes to the plumbing system. Before any rough-in for under slab plumbing, you need a professional plumbing diagram. Although you can look these up, it is well worth the time and money to have the plumbing section performed by a professional. Any leak, error, or other mistake will, in the end, likely cost you thousands to repair, because it will involve cutting through the concrete slab, creating dust, noise and an uproar you can live without.

Repairing Pipes Under a Concrete Slab

Pipes under a concrete slab can corrode over time, particularly in homes with case iron pipes, those impacted by tree roots, or those that have shifted when the foundation was repaired. The first sign of an under-slab leak can be an uneven floor area, wall crack, or doors that have begun to stick. Some homeowners notice the problem by hearing water running, even when all systems are off. Repairing the pipe may involve slab penetration (jackhammer), or tunneling. No matter how complex the problem, it can be solved efficiently and correctly by a professional plumber.

Our team at Benjamin Franklin Plumbing Cedar Rapids has specialized equipment to test and identify, as well as fully repair leaks under or within a concrete slab. Let us help you protect your home

Features of laying aquatherm pipes in concrete

Send an inquiry

Fusiolen is a material developed 35 years ago by the German company Aquatherm. The main purpose of its creation is to eliminate the shortcomings of polypropylene, which is actively used in construction. Piping systems made of this material are superior in many respects to similar products of competitors, which is due to a number of features of fuziolen:

  • Homogeneous structure;
  • High impact strength;
  • Low coefficient of thermal expansion.

Due to its high technical and operational characteristics, fuziolen pipes are actively used for pipeline installation in various fields of application.


  • Issues
  • How do Fusiolen pipes behave in concrete?
  • Application of Fusiolen pipes in screed without corrugation and insulation
  • Do pipes break down from chemical additives in concrete?
  • Repair of pipes in concrete
  • How is the linear expansion of joints and exit points in concrete tolerated?
  • Recommendations for laying in concrete
  • Other features of Fusiolen pipes
  • Instead of conclusion
  • Questions, comments, feedback


Some manufacturers of piping systems are discussing whether Fusiolen pipes can be used in cast-in-situ concrete, even without insulation. To dispel all myths, hoaxes and speculations on this topic, Aquatherm has provided unambiguous data based on tests. These are set out in Aquatherm Technical Bulletin 201207F-AQTTB and an official letter from the concrete installation plant.

The bottom line is that due to its physico-chemical characteristics, Fusiolen provides all the benefits described below.

  • The plasticizers and stabilizers in Fusiolen provide increased thermal stability as well as resistance to dynamic expansion.
  • High-molecular compounds of the material itself and ethylene in its composition neutralize the negative effects of chlorine and other chemicals, providing high-class resistance to aggressive environments and aging.
  • The manufacturer has vast experience in the development and production of engineering systems, including those installed in monolithic concrete without insulation, and provides a guarantee for such solutions.

How do Fusiolen pipes behave in concrete?

With hidden Fusiolen pipes, due to the low linear expansion and thermal conductivity, there is no need to use additional insulating materials. Linear thermal expansion is prevented by laying the pipe in solid concrete or plaster. At the same time, resistance to abrasion and physical and mechanical stress is ensured, since the material absorbs slight compressive and tensile deformations due to the high modulus of elasticity.

For the Aquatherm heating system, there is no need to use a protective corrugated pipe before pouring the pipeline into the screed, due to the low linear expansion (Aquatherm – 0.035 mm/mK, PEX – 0.15 mm/mK) and low heat losses (Aquatherm – 0.15 W / m K, PEX – 0.35-0.43 W / m K), the thermal conductivity is also 3 times lower compared to PEX, which makes it possible to lay pipelines without insulation or with its minimum thickness).

What are the consequences of using Fusiolen pipes in a screed without corrugations and insulation?

Laying pipes without insulation significantly saves the height of the floor screed, significantly reduces the overall construction costs. Additional savings are provided by the use of Aquatherm distribution blocks, instead of a tee system or PEX manifolds. The minimum height when using the Aquatherm system is only 30 mm, while for PEX it is 50 mm when using blocks and 64 mm when using tees.

If we talk about the change in temperature of cold and hot water in the pipeline, you can use the results of laboratory studies of Aquatherm pipes: an hour the temperature rises by only 5-6 °C, and over the next 10 hours by 3-4 °C.

In cases where cold and hot pipes are laid in close proximity, data on a long heat exchange process are also obtained:

Chart 2 – Starting Time

Chart 3 – Ending Time

After 10 hours of stopping cold water, the temperature of the solid remained almost unchanged, and the temperature of the pipe increased by only 7.83 °C.

Do pipes break down from chemical additives in concrete?

Fusiolen is highly resistant to damage due to the presence of ethylene in the material structure. Chemical resistance is confirmed by passing tests and certifications in the chemical industry (see table of chemical resistance).

How to repair such pipes in concrete?

The repair is done with a small darner (plugging holes) to use just a couple of square centimeters, in contrast to the PEX system, where you need to change both the pipe section and fittings, make large damage to the screed, which is much more expensive and takes more time.

How is the linear expansion of joints and exits from concrete tolerated?

Molecular homogenous bonding (fusion) provides double strength at joints. Special Aquatherm L-shaped pipes simplify installation, they have attachments to the radiator with a platform, the platform has a specialized mounting hole for fixing to the subfloor. As a result, there are no movements of the tubes, it is convenient to connect devices, you can not mount radiators in advance.

Other recommendations for laying in concrete, screed

Aquatherm pipes must be laid in a channel of sand or fine earth and compacted in such a way as not to damage them. A minimum distance of 75 mm is recommended between the pipe and the underside of the floor. All pipes must be laid at right angles to the concrete floor. Any piping that crosses the slab structure must be laid at right angles to the slab surface and must be insulated with a flexible plastic material at the intersection, at least 6 mm thick for the entire length of the intersection with the slab (Aquatherm Technical Bulletin 1012B-AQTTB1)

Other features and benefits of Fusiolen pipes and fittings

  • Service life up to 100 years according to German DVS certification;
  • 30% better hydraulic properties than metal – contains ethylene to improve glide; link to hydraulic calculations that are better than conventional ppr – technical catalog green pages 93-135
  • Cold stability
  • LEED, NSF Environmental Certification, Environmental Certification

In lieu of

Fusiolen Aquatherm pipes are excellent for laying in screed, cast-in-situ concrete, do not collapse, do not damage, have a stable geometry. They can be laid without insulation and corrugations, pipes absorb stress due to linear expansion, have increased strength at the joints and a number of practical solutions for more convenient installation.

Pipes Aquatherm GmbH

aquatherm green pipe

Fusiolen pipes made in Germany.

  • Coefficient of linear expansion is 0.035 mm/m
  • Low thermal conductivity – only 0.15 W/m*K
  • Operating temperature 95 degrees, pressure 10 bar
  • Guarantee 10 years and 20 million Euros
  • Service life up to 100 years (DVS certified)

aquatherm blue pipe

Fusiolen pipes made in Germany.

  • Coefficient of linear expansion is 0.035 mm/m
  • Low thermal conductivity – only 0.15 W/m*K
  • Oxygen tightness according to SNiP 41-01-200 and DIN 4726
  • Operating temperature 95 degrees, pressure 10 bar
  • Guarantee 10 years and 20 million Euros
  • Service life up to 100 years (DVS certified)

Questions, comments, reviews

Send request

Electrical wiring under the screed in pipes

Always freshly roasted coffee!

Different roast levels!

Tasty Coffee

In many rooms, electrical cables are laid on the floor surface before the screed is poured. First of all, this type of electrical wiring is convenient in the absence of suspended ceilings, behind which electrical cables are also laid. In this case, the volume of punching horizontal strobes on the wall surface is significantly reduced.

Most often, electrical wiring under the screed is carried out in HDPE pipes, which are strong enough and at the same time allow bending. The use of light corrugated pipes for such wiring is not desirable, as they can easily be deformed. Often, screeds on ceilings are poured after a considerable period of time after the installation of electrical wiring. Therefore, pipes must withstand significant mechanical stress, as there will always be inattentive builders who will walk on them with their feet. But strong HDPE pipes, after being attached to the ceilings, should be protected by covering them with plywood shields.

The diameter of the HDPE pipes must allow the cables to be pulled freely. Cables should be laid parallel to the walls. It is recommended that the distance from the cables to the walls be constant in all rooms of the construction site. This will greatly simplify the search for cables in case of repair.

At the intersection of cable lines with heating pipes, it is necessary to provide thermal insulation by separating them with heat-insulating material. It is desirable that the heating pipes along their entire length have thermal insulation.

In open electrical wiring indoors, when crossing cables with pipelines, the clear distance between them must be at least 50 mm (clause 2.1.56 of the PUE). In this case, hidden electrical wiring is laid and this requirement of the PUE does not apply to it.

When using non-metallic pipes, before installing electrical wiring, it is advisable to study the requirements of paragraphs 3.50 … 3.55 of SNiP 3.05.06-85 (this SNiP is now registered as SP 76.13330). These items are shown in Fig.1.

Plastic pipes. Gasket rates

Fig. 1 Paragraphs 3.50 … 3.55 SNiP 3.05.06-85

It follows from paragraph 3.52 that a protective layer of concrete solution is not required at the intersections of pipes. But this applies to pipes with cables. The protective layer of concrete mortar at the intersection of pipes with cables and heating pipes is not specified by the norms. But, it should be borne in mind that the heating of the cable cores due to their proximity to the heating pipes leads to a decrease in the allowable long-term current (this is described in the article Choosing the cable section). Therefore, the thermal insulation of the heating pipes and the layer of heat insulator between the pipes should be chosen as maximum, of course, within reason, as this increases the required height of the screed.

Figure 2 shows pipes with cables and heating pipes for a residential area. The heating pipes are made of a material based on polyethylene and are thermally insulated along the entire length.