How To Add A Dormer

Adding a dormer to your house has many benefits depending upon the size of the dormer such as - extra space, extra room, extra lighting, and aesthetic exterior appearance.

A dormer is a window set vertically into a gable that juts out from a sloping roof. The word dormer comes from the French word dormeor, meaning sleeping room and from dormir meaning to sleep. A dormer has its own roof, which is either flat, vaulted, or pointed.

Tools and materials needed:

Ladders/scaffolding/toe boards




Lumber (amount of lumber needed will be determined by the size of dormer)

Window assembly

Roll felt roofing



Moisture wrap paper


Drill, with drill bits and driver bits

Circular saw

Reciprocating saw

Miter saw

Measuring tape


Plumb bob

Speed square

Leather gloves

Safety glasses

Building a dormer is not a job for a beginner. I strongly recommend you have some good carpentry skills before attempting this project. As with any project always check with your city building code and get any permits and special instructions needed before building. Some cities will actually require a copy of the blueprint for your dormer.

Safety is always a must so use proper scaffolding, ladders, toe boards, glasses, gloves, and other types of equipment to help you while on the roof.

Determine where you want the dormer and the size of the dormer. The location and size will be determined by the size of the room or area below the dormer. You will also need to take into consideration what structural obstructions might be in the way such as electrical wiring and roof supports. Once you have this information draw out your blueprint, make a list of materials needed, and purchase them. (This plan is taking into consideration that you have a couple extra helpers. If not precut all the wood and construct the dormer on the roof instead of on the ground pre-fitting each section before taking up to the roof in sections.)

Once you have this done and have your building permit it is time to begin work. The first step is to remove the shingles and felt paper from the roof where you are putting the dormer. Be careful not to damage the other shingles while removing the others. In addition, look for nail heads that will help you determine the exact location of the roof trusses. Depending upon the size of your dormer, you will have to remove a portion of one or more trusses. Do not cut any opening until you have the dormer frame in place though. Once you have the shingles removed and the space marked out, trusses located, it's time to begin building the frame.

You will have to make sure you know the slope of your roof so you can cut the right angles in the studs. Using a speed square and tape measure determine the slope of your roof. Mark the studs and cut with a miter saw.

Next, you will build the window frame. There will be a header at the top, uprights at the left and right, and a sill at the bottom. Take the measurements from your window assembly. Once the window frame is built, fit the window to the front wall. Do not attach at this time just make sure it fits.

Now you will cut your boards for the frame and sidewalls. After they are cut, put the sides together as you would do any standard wall frame. When you have the dormer in place, you will add the wall studs. Once you have the dormer put together on the ground making sure everything fits properly - measure and cut your trusses for the roof. After you have this fitted, you can begin by attaching your bottom frame (a 2x4 wood frame the size of the dormer) to the roof using 16-penny nails.

Once this is done bring up your wall sections, trusses, and window assembly. Attach the front wall to the bottom plates. Next, attach the top plates, which will go from the roof to the top corners of the front wall. The top plates should be angled to match the roof slope and a little longer then the actual size of the dormer so they extend past the front wall. You can trim them later. Plumb the front the wall and then nail the top plates into place securely. Now you will place your trusses and attach them.

Once all the framing is done, it is time to cut the hole. You will need a reciprocating saw for this job. Make sure, while cutting the opening you do not cut the trusses. Cut up to each one removing the existing material. Once the hole is cut, you will use a plumb bob to mark the corners for the ceiling cut. Drive a nail at each corner and then chalk your line and cut the hole in the ceiling using the reciprocating saw. Now you will strengthen the existing framework around the hole you cut by adding to the trusses and rafters on both sides. After the existing trusses and rafters have been strengthened attach a rope with a pulley to them one at a time and using the reciprocating saw cut out the truss or trusses that need to be removed. Lower them down carefully one at a time.

Now put in a header between the two side trusses right beneath the hole you cut. Next, you will put in the 2x4 studs for attaching the sheeting, siding, and drywall. After the walls are framed use ½" or ¾" plywood sheeting for the outside walls. Then you will put on the roof sheeting. Use no less than ¾" decking plywood for the roof.

Now you will use no less than 15-pound roofing paper (felt) and then apply your flashing and shingles. At this time, you will also tie the existing roof into the new dormer using flashing around the base and caulking. Next, you will wrap the walls with moisture barrier paper and insulation. Be sure to caulk any gaps and around the window/s where trim pieces will go or siding trim. Now you will add the siding to match the existing house.

Once this is done it is time to finish the inside work and enjoy your new space!

© High Speed Ventures 2011