Adding Dormers To An Attic

A new dormer can transform a small, dark attic into a bright and spacious living area.  Adding one or more windows improves light and ventilation, and can increase the useable floor space by as much as 30 to 40 percent.

If you compare the cost of building a dormer to adding on a complete room, a new dormer is an appealing project indeed.

Adding a dormer demands careful planning and skillful carpentry.  If you have relatively little building experience, you would be wise to seek professional assistance for at least part of the job.

You may want to work alongside the contractor, learning as you go.  Or have the contractor handle the more difficult aspects, such as cutting into the roof and framing the shell.

It is a good idea to ask the advice of a designer or architect.  At the outset, you need to know if the project is structurally feasible.  If the attic is undeveloped, you must verify whether the foundation, first-floor walls, and floor joists can carry a new floor load.

You should also know whether the dormer walls will be sufficient reinforcement when you cut a hole in the roof.  Usually they will be.

You must plan the size and shape of the dormer carefully to maximize the available living space.  The optimum size reflects a delicate balance between your needs and the structural and aesthetic possibilities of the space.

Another reason for obtaining design help is that the dormer will dramatically alter the exterior appearance of the house.  Good design is essential.

colorful dormer

The dormer should match the style of the existing architecture.  Generally, the same type of windows, siding, and roofing should be used, and any details, such as overhangs and fascia should be repeated.

Most important, the dormer should blend smoothly with the rest of the house, complementing the overall design.  In fact, it should not even look like an addition.

Planning The Dormer

The first step in planning the new dormer is to determine the size and slope of the existing roof so that you can draw an accurate end view of it.

Once you have an accurate drawing of the roof, experiment with different dormer dimensions to see how they will fit into the roof.

Several factors come into play at this point.

One is where you want the top of the dormer roof to connect to the house roof.

The best-looking connection is usually just below the ridge, but if you need more headroom or if the house ridge is so high that the resulting dormer roof will look like a ski jump, relocate the connection for better appearance.

The slope of the dormer roof influences overall design.

The general appearance of the roof must be kept in mind, as well as code requirements for different roofing materials.

For example, if you plan to roof the dormer with wooden shingles to match the existing roof, the slope must be at least 4 in 12.  If you want less of a slope, then you will have to choose a different roofing material.

Shed DormerThe minimum slope for composition shingles is 3 in 12; roll roofing can be as low as 1 in 12.

If the dormer is high enough so that you cannot see the dormer roof, it may not matter what roofing material you use, but it is best to match the roofing with what is on the rest of the house.

A third consideration that will effect the size and shape of the dormer is the minimum ceiling height.

Most codes require that at least one half of the ceiling in a room be 80 inches or higher, and that the ceiling over the other half have a minimum height of usually 48 or 60 inches.


Since the dormer will be part of a larger room, you have to calculate how high the dormer ceiling must be to satisfy the minimum height requirement for the total square footage of the room.

Dormer Shapes

There are two basic shapes for a dormer.

Although the shed shape is easier to build, your choice should depend on how well the dormer blends in with the other architectural details on your home.

Gable dormer.  While a gable dormer has charm and appeal, reflecting the basic lines of the house, it cannot be very large.

It adds light but little floor space, and often requires two or more dormers to balance the exterior proportions of the house.

Shed dormer.  A shed dormer adds more living space than a gable dormer, and can even extend across the entire length of the roof.  The front can be built directly over an existing wall, but in most cases it looks better when it is set further back.