How to install a dormer window: 8 Ways to Frame a Dormer

Things to Consider Before Adding a Dormer to Your House

Things to Consider Before Adding a Dormer to Your House

  1. Angi
  2. Solution Center
  3. Roofing

Updated August 12, 2022

Miles Nelson /

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If you’re looking for a relatively simple way to change the look of your home, you can’t go wrong with a dormer. Adding a dormer not only refreshes the look of your house from the outside, but also expands the indoor living space. A dormer allows you to utilize the attic space in a whole new way, whether it’s a cozy reading nook or making it comfortable enough to be a spare bedroom. 

But, there are some things to consider before you begin your dormer project.

1. The Existing Roof

You should consider the shape, style, and angle of the existing roof before deciding on a dormer for your home. The roof of your new dormer should match the existing roofing material to create a consistent look. In addition, you should install roofing underlayments on the roof of your dormer to guarantee it’s well protected.  

The roof section around the dormer may also need some repairs, upgrades, or replacement to ensure the new material will match perfectly and looks its best. You also may want to clean up your roof before installing a dormer. This way, you will get the full benefits of your roof’s new addition.

2. The Size of The Dormer Addition

The dormer should fit the scale of the house with a sense of proportion to the current roof. Otherwise, a dormer may end up being a dominant feature that could actually reduce the value of your house. 

The planned function of the space will help to determine the size of the dormer. However, the dormer generally should be small enough to break up the roofline and fit the roof without looking out of place. A dormer creates more headspace in an attic with a low ceiling while the window brings light into the room, so a window seat or a writing desk would fit nicely in a typical dormer.

3. The Permits

Before adding a new dormer, you will likely need to obtain a permit. A dormer permit costs an average of $1,160, or between $390 and $1,930, depending on local laws and regulations. It’s wise to look into these costs before getting started.

4. The Cost of Adding a Dormer

If you’re looking to install a new dormer to your existing roof, it typically costs between $2,500 to $20,000, or $115 per square foot, depending on the size of the dormer you want to install.

Many options make a significant difference to the total cost, including the material, the size of the window, and the siding option. The cost of adding a dormer is different from the cost of installing a new roof, so be sure to talk to a pro to get a more customized estimate.

5. The Style Of The Dormer

There are different styles of dormers you can add to a building. The shape and structure of your existing roof will determine the dormer style, as it should match your roof’s structure, suit the aesthetics, and improve the curb appeal, which will in turn increase the overall value of your property.  

Here are the different dormer types to choose from:

  • Gable dormer: Two planes sloped in the opposite directions creating a triangle-shaped dormer

  • Hipped dormer: Three sloping planes which usually match the hipped roof of the house

  • Flat dormer: One single horizontal flat surface covering the three-walled dormer

  • Shed dormer: A single flat roof that is sloped in the same direction as the roof

  • Wall dormer: An extension of the wall above the eaves level of the roof

  • Eyebrow dormer: A small arched window that sticks out from the slope of the roof

6. Dormer Windows

If you’re looking for natural light and ventilation in your attic, dormer windows are a great choice. The dormer window can come in various sizes and styles, but it’s usually smaller than other windows you’d find in your home.

Installing the right kind of window can be energy-efficient and help reduce utility bills like heating and lighting, as a window regulates the amount of light and heat coming in through the dormer.

You should also consider how to maintain and clean the dormer windows, either from the inside or the outside, depending on how the dormers open.

7. Dormer Draining System

You should also consider installing the right gutter. Not having a gutter on your dormer may lead rain to drip off and cause seepage into the walls or damage to the roof. The way water drains from the dormer gutter makes a big difference as well. You shouldn’t install an open-ended gutter that drains to the roof, as this may increase moisture levels and create mold over time.

The best practice is to install a downspout, which takes water from the dormer’s gutter to the lower gutter. This will help eliminate damages to the roofing caused by the water running off. However, if you don’t like how it looks, putting a metal tray under your downspout will lead water to the lower gutter without running on the roof. The metal tray is a safer option, and, bonus points, it blends well into the roof, so it’s aesthetically pleasing.

You should install roof flashing along the seams where the dormer’s roof meets your home’s roof to prevent water from entering the attic space and provide extra protection to your attic.

It’s a great idea to inspect your roof to ensure your current gutter can handle the increased water coming from the dormer.

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Learn more about our contributor

Marwa Hasan Marwa is an architect and content creator with a passion for home decor. When she’s not working, you can find her learning a new DIY skill, gardening, hiking, or watching a new TV show.

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What to Know Before Adding a Dormer to Your Home

With home prices through the roof, homeowners are looking for sensible ways to add square footage to their homes. If you have an attic space that you want to finish but would like to add natural light to the room, a dormer house addition might be the right fix. 

What is a dormer house addition? Have you seen the homes with windows popping up out of the rooftop, typically in box shapes? Those are dormer windows. There are several types of dormer house additions that may appeal to you. 

Types of Dormer House Additions

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A dormer house addition can add more room if you would like to finish an attic space and need more room to stand up along the slanted-roof room. Or maybe you would like to bring ventilation into a room with windows. Let’s take a look at the types of dormers that may be a great add-on to your home. 

Gable Dormer

A gable dormer is a common type of dormer. It is named for the gable or the triangle shape that is at the top of the roof while both sides slope down. Gable dormers are typically built along the slanted sides of the main roof, called eaves. They do not fill the whole slant of the roof. You can have one, two, or three windows in a row that come out as gable dormers. 

Gable dormers can provide natural lighting to the room. Keep in mind that if you are building the dormer so that you can stand up inside the room, you will need to duck between each window. On the positive side, the valleys between each dormer will put less pressure on the roof as water will have clear run-off pathways to drain.  

Gable dormers give the home a colonial revival look, with the windows coming up out of the roof and adding dynamics to what may be a flat, dark surface on the front of the house. It is not only a great structural choice to add a gable dormer house addition, but it is also a design choice that increases curb appeal. 

Wall Dormer

A wall dormer stays flush with the outside wall and raises the existing wall up into the roof in a triangle shape. If you have a flat side to the exterior of your home, a wall dormer may break up the monotony of the single flat roof. You could also add a dormer window to bring in light to the inside space and ventilation. 

Shed Dormer

Shed dormers are rectangular, with the whole rectangle shape coming up out of the slanted roof. It appears as though it were a lean-to-shed breaking through the top. Many people line the shed dormers with dormer windows for a beautiful, architectural look. 

Shed dormers are used to extend the standing room in an attic space. For example, maybe you would like to convert the attic to a bedroom or two but need to be able to walk around the bulk of the room. A shed dormer will add a substantial amount of extended ceilings so that you can fully use the space. 

Hip Roof Dormer

A hip roof dormer is similar to a gable dormer, but with some added structural issues along the way. When you add a dormer to a hip roof, you will need to reinforce the roof to be sure it can hold the weight of the new addition. 

With any dormer house additions, it will take the expertise of a home remodeling team to be sure that your home’s structure stays strong and unwavering. While it can be a luxury to add usable square footage to your home, you want to do it in a safe and sensible way. 

Why You Should Think Of Adding A Dormer To Your Home? 

The number one reason someone chooses a dormer addition is to add square footage to one’s home. With home offices being all the rage, wouldn’t it be lovely to finish the attic space in your home and set up your desk right in front of a  dormer window with a view? 

Here are some examples of dormer house additions that provide additional square footage while adding dynamics and excitement to the outside of your home: 

  • Build a second level under existing vaulted ceilings
  • Finish an attic space and extend the ceiling for height benefits
  • Add light and ventilation to an existing attic room
  • Add light to your vaulted entryway
  • Change the boring curb appeal of a boxy home

You can invest in your home by working on a dormer house addition. You don’t have to break the bank with this simple addition, and you will end up with more liveable space to spread out in your home. And that will, in turn, add value to your home at resale. 

Other Things to Consider

For dormer additions, it’s not as simple as breaking a hole in your roof and adding a window. Before starting on your dormer house addition plans, you have some things to consider. You will always benefit from the expertise of a trusted home contractor to make sure all the details are presented so that you’re not caught by surprise in the middle of your home addition. 


With home remodeling work being widely popular right now, you may run into delays with securing permits. Your professional home contractor will know just what to do to secure the correct permits for your home so that your project moves in a  safe and timely manner. 

If you run into permit delays, you can’t do much about it. Instead, work with a contractor that can give you a fair estimate while putting in the work to  get the permits as quickly as possible.  


When you place an entire structure on the eave of your roof, you may run into drainage issues. The roof is built at a slant to stop water and debris from building up on your roof and creating damage to the materials used in the structure. 

You may need to install a gutter or other drain assistance to ensure that the roof stays in good condition and is free from any debris or buildup if water gets trapped up there. Talk with your contractor about the obstacles that a dormer addition may pose to roof drainage. 

Roof Condition

Before you start construction by adding a dormer house to your roof, you will need to look into the roof’s current condition. You will want the roof to be in the best condition and ready to handle a construction project. If it is time to replace the roof, do both projects together so that you can build around the new dormer addition. Often we will have to replace the roof regardless of its condition based on the scope of the project.  

A new roof will ensure minimized issues with the cuts and bends in your roof for years to come. The last thing you want to do is add a dormer to your home and then deal with leaking in your new home addition. Instead, get an inspector to check out the roof before starting on a dormer addition. 


How much does it cost to install a dormer house addition? The truth is that it could range anywhere from $20,000 – $30,000. Here are some factors that go into a dormer house addition cost: 

  • Square footage
  • Dormer windows and how many
  • Reinforcement of existing structure
  • Finishing remodeling costs
  • Roof replacement

Meet with your contractor to get an accurate estimate on your dormer addition project. They will be ready to tell you what your home addition will require, and the expected prices for materials and labor. Most of the time, a dormer addition is cheaper than a full-on second-story addition since it utilizes existing space in the home. But if your project is extensive and an ample amount of square footage is being added to your home, the price comparison could be similar to other types of home additions. 

Once you have considered all the details that will come up during a dormer house addition, you can be prepared to expect the unexpected when construction starts. Look into a dormer addition to add square footage to your home and save you from an exhausting and unnecessary move. Your dormer house addition can enhance your home’s exterior and create the feel of a major makeover with one or several beautiful dormer windows.

Contact Tandem Contracting for Your Add-On Needs

When you are unsure about how to add more liveable space to your home, do not discount a dormer house addition. Our team at Tandem Contracting is ready to help with your home addition. You don’t have to uproot your life and move when you have beautiful possibilities right in front of you. Please meet with our team to discuss your add-on needs.  

Tandem Contracting has quickly made a name for itself as a premium home remodeling contractor in Morris County. We build long-term relationships with our clients, always getting a call back for the next project because homeowners are so thrilled with our work. So contact Tandem Contracting today to get started on your dormer house addition. 

Detailed instructions for the installation and finishing of do-it-yourself skylights

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  • What calculations need to be done?
  • Getting started, tools
  • How to mount?
  • How to install and adjust the frame?
  • Preparation of insulation and drainage
  • How to install salary?
  • How to install glass?
  • Interior decoration
  • How to properly insulate a window?

The attic floor can be made from an ordinary attic, but this will require the installation of skylights, since without them the room will not be lit, it will not be able to be ventilated and used as a living space. It is not necessary to pay big money for the work: you can always handle it yourself if you know the basic principles.

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What calculations need to be done?

Before installing a roof window, you need to correctly calculate its dimensions, choose the right model, otherwise you will simply waste your money and the window will not suit you. You need to choose a design, first of all, based on what material your roof is made of. You need to think about where the window will be, at what angle. Usually, the window area is calculated based on the area of ​​​​the room: for every 10 m² of floor, there should be at least 1 m² of glazing.

It is worth considering that the higher the opening under the roof window, the more light it will give. Do not place the window too high: it will not be convenient to use it. The only exception is the roof, the angle of inclination of the sides of which is 15-20º.

If the roof has round slopes, windows installed 1-1. 40 m from the floor will look best and provide good lighting. It is acceptable to put the window lower, for example, 80 cm from the bottom surface, but this is not always aesthetically appropriate. If it is above 1.9m, it will be inconvenient to use. Putting one large window or several small ones is already a personal choice of the owner of the premises, it is worth noting that the more windows and the larger their area, the brighter the attic space will be.

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Getting started, tools

If you want to install skylights, do-it-yourself installation is quite possible. You will have to act according to the instructions:

  • First of all, you need to prepare the window opening, then install the frame.
  • Install hydro and thermal insulation.
  • Build a drainage system.
  • Set salary.
  • Insert double glazing.
  • Install vapor barrier.
  • Mount slopes.

To carry out the work, you will need the following tools:

  • Screwdriver or small drill.
  • Set of self-tapping screws, nails.
  • Beam and wooden slats.
  • Building level.
  • Circular saw.
  • Insulation materials.

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How to mount?

Before installing a roof window with your own hands, you need to choose a suitable place for it, mark it for preparatory work. After the area is outlined, you need to make holes in the roofing material with a drill. This is done from the inside of the roof. Holes are needed so that the material can be cut through without damaging the rest of its parts. The hole is made using a circular saw, the cut must first be made diagonally, cutting out triangles, and they are carefully pulled out and an opening is obtained. Do not cut along the contour: you can damage the roofing material.

Distance from the bottom line of the window to the line of the roofing

Once the opening for the roof window has been formed, it is possible to proceed to the next stage of installation – the installation of the frame. It is worth noting that it is not necessary to disassemble the roof. An exception is the seam roof, in which the material will have to be dismantled.

Device opening in the existing attic

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How to install and adjust the frame?

Before inserting the roof window , you need to pull out the double-glazed window. This will not only make the task easier, since the frame weighs much less without it, but it will also make the job safer. The double-glazed window is inserted after the completion of all work. To begin with, fix the brackets located in the tender part of the structure. They need to be fixed securely, while the upper ones should not be completely tightened. They should hold, but be relaxed: this is necessary so that the frame can be adjusted.

You need a spirit level: measure the inserted frame with it. If it is skewed, it is leveled with the help of plastic corners, which are sold complete with a window. After you make sure that the frame is level, the screws of all fasteners can be tightened to the end. 9

back to contents water will get in. Materials are usually fixed with a construction stapler.

A drain chute is used to remove excess moisture. It is located at the top of the frame, for this, two pieces will have to be removed from the cladding, which will correspond to the size of the gutter. After that, you will need to cut off the material for waterproofing. A gutter is carefully inserted under the edge and fastened to the crate. In order not to damage the tightness, the part is placed at an angle.

Installing a waterproofing apron

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How to mount a flashing?

Proper installation of skylights requires the installation of a special flashing. One of three types of construction can be used, choose it depending on what type of roof is on your roof. The following variants are distinguished:

  • Rebated, if the roofing is flat.
  • Special for roofs covered with any corrugated material. Most often, when ordering a window from the manufacturer’s company, such a salary is included in the kit. You only need to specify what kind of roof you have.

You need to work with both types of salary in the same way, so you don’t have to rack your brains on how to install roof windows , if you don’t have a flat roof. The salary is set as follows:

  • First fix its lower part. At the same time, it is necessary to make sure that the waterproofing with an apron extends not only beyond the frame, but also the roofing material.
  • Work on the sides next. The edges should be laid on the window frame.

Roof window installers, as a rule, fasten the flashing to the frame lathing, after which the outer part of the structure is covered with insulation. Any one can be used. Most often, you don’t have to look for insulation and fasteners anywhere, since they are sold together with the window, and you just need to assemble and install the structure.

Before installation, be sure to check every detail, sometimes manufacturers put parts in the wrong size or something is missing. Some parts may be broken in transit. You need to solve these problems even before installing the window. It is advisable to install a double-glazed window after all internal work has been carried out or it is recommended to protect the glass with a film.

After the salary is fixed, you can take on the installation of double-glazed windows.

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How to install glass?

After the window is installed, it is necessary to insert the double-glazed window into the frame, which was removed before all work was done. To do this, you need to insert it into the frame, secure it with glazing beads and seals, usually instructions are attached to the window. The process of installing a double-glazed window may differ if the manufacturer has provided a different method. You need to install the double-glazed window last, as there is a risk of breaking it during the work. The presence of glass on the frame may interfere with any action.

It is advisable to install a double-glazed window after all internal work has been carried out or it is recommended to protect the glass with a film.

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Interior finishing

It is important not only to install the roof window correctly, but also to carry out the appropriate finishing. If the cladding is not made and it is not properly insulated from the inside, the window may be cold, condensation will begin to form in the room, and the heat will go outside. To combat these phenomena, sealant, insulation and lining are used; which one to choose is a personal matter of the owner of the premises.

Quite often, the manufacturer offers plastic parts along with the window. This is an inexpensive and easy way to solve the problem. Plastic cornices will close the joints, give the interior a complete look, and prevent drafts from entering the room. You can use drywall, lining for the same purpose.

Finishing and insulation of a roof window

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How to properly insulate a window?

It is very important that the insulation of the structure is carried out correctly. Some builders use mounting foam and do not use insulation, but this approach is not entirely correct, since the material narrows in winter and expands in summer, so the structure may squint.

Mineral wool will serve as the most convenient insulation for a roof window. It can be easily torn off and put back into place. Do not place heaters under the attic windows. Unlike a regular window, a roof window is tilted, so condensation will form on it due to rising warm air.

Conclusion: knowing how roof windows are installed, you can perform this procedure yourself, significantly saving the budget. The procedure is simple and requires only strict adherence to the instructions.

DIY installation instructions

There are no extra meters in the house: this is the golden rule of interior design science. Every day new ideas appear on how to literally equip a personal office, dressing room, greenhouse or bar out of nothing. And in this regard, the most valuable, spacious and comfortable additional space is the attic.

Moreover, it is possible to turn almost any attic into a residential one, and the problem of lack of light and air can be easily solved by high-quality installation of a roof window on gable walls or slopes. And it doesn’t matter which place you have chosen for this element and how many windows you want to install – for the roof, such an “operation” is completely safe, if you take into account all the recommendations of experts and ensure reliable waterproofing of the frames.

Let’s study all the rules that apply to this process.

The concept of living space under the roof once appeared thanks to the bohemian youth who loved the atmosphere of serenity and lightness. Therefore, to this day, this space is equipped so that it is filled with light and spacious. And this is already directly affected by the beneficial installation of a roof window or windows, taking into account their height, number and location.

So, for example, in terms of location, the advantage is given to the sunny side. Those. It is advantageous to place roof windows where the sun will look as much as possible. Moreover, inclined slopes have many advantages in this regard, because windows at an angle let in 40% more natural light than strictly vertical ones.

This is important to consider when choosing a wall, because many people think that a window in a straight wall will look more familiar than something that hangs over the furniture and is psychologically less comfortable. On the other hand, sometimes the windows are directed to the west specifically in order to meet a beautiful sunset with family or friends under the roof (or do yoga):

The second point: slanted windows remove shadow niches, so such a room seems especially bright. At the same time, a straight window in the pediment will leave shadows at the corners of the slopes:

And, finally, the more windows, the brighter, which is quite logical. Therefore, with regard to the number of windows themselves, their area, according to the rules, should be no less than 1/10 of the attic floor:

Now, before you install the roof windows on the selected side of the roof, decide on their height and parameters. So, they are placed too high only if two rows are planned at once.

Otherwise, the windows are placed at a normal distance from the floor (90-112 cm), otherwise a depressing atmosphere cannot be avoided, as in the basement, where the windows themselves are always almost under the ceiling. You should not place the roof window too low either, because a standing and sitting person should have a better view of the outside world.

The height of the roof window is also affected by the slope of the slope. So, the smaller the slope of the roof, the more gentle slope, the higher the window should be.

But remember that there should always be access to a window, at least to open it, and maximum to escape through it during a fire:

You must have wondered: why two rows of windows at all? To watch the stars!

We go further. The choice of window must be successful not only to create the desired level of lighting, but also for the truss system itself. It is good if the width of the window is 2-5 cm less than the distance between the rafters. Then it will not be necessary to finish off additional bars, and it is convenient to fill such a niche with insulation (although it is quite possible to do without it).

the distance between the rafters is usually 60 cm, almost all manufacturers produce their windows with a box with a width of 550 mm or more. But if the window of an unusual shape does not work out, then the truss system in this place is simply adjusted to the width of the existing window, although this is not very good.

Cutting out the hole in the roof depends on which roofing has already been installed. For example, if it is piecework, then individual elements can be easily dismantled without affecting the entire roof covering:

If the roofing is one-piece, sheet, then you will literally have to cut a hole in the roof:

Here is a diagram of the location of additional bars for installing a window:

Each roof has its own specific rafter pitch, which is designed based on the whole set of possible loads. But sometimes the window has to be inserted much narrower, or much wider than this step. And for both cases there are technical solutions, as well as for non-standard situations.

If the window is narrower than the rafter pitch

If it so happens that the window clearly already has a rafter pitch, then here’s what to do. The frame will need to be supported by something, and therefore, at a certain distance, parallel to the crate, you need to install mounting rails and fix the box on them:

It happens that the roof window is chosen well, quite narrow, for example, in the Gothic style. In this case, it will be necessary to supplement the roof truss structure somewhat by installing an auxiliary fragment of the rafter next to the window and special counter-rails for fixing it.

The counter rails themselves should be positioned at a distance of 30-50 cm from the top and bottom edges of the frame (if you install them even closer to the frame, there will be problems with the window finish). These elements will hold the roof window in place.

If the window is wider than the pitch of the rafters

Here you will have to tinker more than in the previous version. Indeed, now it is necessary to affect the rafter system itself, namely, to cut the rafters, which is highly undesirable. The structure itself will need to be reinforced with auxiliary beams. Is it worth saying that it is better not to touch the rafters at all, because each of them has its own load from the roof?

But since it already happened, and one large window cannot be replaced by two small ones, then you will need additional horizontal beams. You need to place them at a distance of 30-50 cm from the lower and upper edges of the window, and vertical ones – 2-5 cm from the window frame itself (for decoration):


Non-standard installation options

Complicated installation options for roof windows are less common than standard ones, but still there. This is due to the wild imagination of modern architects, who, in the struggle for their clients, almost turn houses upside down (although such projects already exist). And in unusual projects, the windows are arranged as it turns out, and we will tell you about some of these options.

For example, installing a roof window into a roof with decorative roofing is quite problematic. The fact is that it is extremely important that the waterproofing layer of the roof (which is always under the finishing roofing) is securely connected to the salary and waterproofing window apron.

And for this it is necessary that the window itself is located in the same plane of the roof. Why a special stand bar is installed under the window frame – that’s the whole solution to the problem.

Rounded ramps are also being built today. It is also possible to install a window on them, only it will be recessed into the very plane of the roof.

But for those who like to contemplate the stars, there are also solutions: installing a window right in the roof ridge. So the lighting, of course, is maximized.

This is done in two ways: either two windows are placed on both sides of the ridge beam, at a distance of 50 cm from the top, while the beam itself remains intact and continues to perform its bearing function further. The second option is to initially build a rafter system without a ridge beam, with hanging and layered rafters. And a special ridge element is installed between the windows.

Roof windows are also often installed in combination to create a winter garden under the roof. In this case, only 100 mm are left between the windows horizontally and vertically, and the number of windows themselves remains unlimited. They are placed in a ladder, trapezium or in the form of a triangle. Usually for this purpose, salaries are made by special order.

Many manufacturers make their windows quite easy to install, under the standard rafter spacing:

The next step is to install a system of drainage gutters, which are also called window flashing. This element is necessary for the removal of atmospheric precipitation.

Almost all modern skylights are supplied with an external special insulating flashing and all the necessary fixings, which are sufficient. But to be sure of the tightness of the window, experts usually advise to still use additional insulating materials.

The flashing is assembled as follows: the lower element is mounted first, then the side parts and the upper one. Then the clamps are nailed, and the lower part of the gutter is snapped onto them:

Next, the trim is attached to the frame, and a sealant for winter sealing is laid around the perimeter of the window. And, finally, the entire structure of the gutter with a seal is pressed with an overlay:

Moving on to window waterproofing. So, insulation is often laid between the rafters and slopes from the inside of the attic room, and the outside of the window box is insulated with a cuff.

For example, modern sealing cuffs are made of a special vapor-permeable material, similar to what is laid under the roof. As an additional insulation, a wool belt is also used, which is pushed between the window frame and the roof bars. It is also possible to integrate expanded polypropylene directly into the box.

Inside the room, the roof window is covered with a vapor barrier apron.