How to Install Dormers

Dormers give cramped rooms more usable space.

A dormer is an extension that protrudes from the roof of a house, most often including a window. Dormers provide additional headroom that increases the usable space in the attic, and the window gives improved lighting and ventilation; together, these effects help an attic to be converted into a living space such as a loft. With the right tools and skill, you can install a dormer in your home.

Things You'll Need

• Hammer
• Nails
• Window
• Tape measure
• Level
• Waterproof membrane or builder's felt
• Roof sheeting
• Double or triple roof rafters
• Caulk
• Metal flashing
• Polyurethane foam
• Fiberglass insulation
• Aluminum tape
• Circular saw
• Nail gun
• Window trim

Instructions

Installing the Dormer

1. Create a plan for the installation of your new dormers. Visit your local building department to obtain any permits for the construction, if necessary.

2. Use a hammer and nails to secure sheeting to the roof. Apply two strips of builder's felt on either side of the sheeting.

3. Place metal flashing around the corners and middle of the bottom of the opening on the roof where the dormers will be installed. This is to prevent rain from dripping through any gaps between the dormer and roof.

4. Use a tape measure to measure the width and height of the portion of the roof where the dormers will go. This is necessary to make sure there is enough room for them to fit properly on the roof.

5. Remove the nails from the roof with the back of the hammer. Remove the shingles and old felt paper from the spot.

6. Install roof rafters on either side of the dormer. Use double or triple rafters so that the walls of the dormers can rest on them. This will leave space uncluttered beneath the rafters.

7. Put up each wall of the dormer in the designated area for installation. Make sure the studs at the bottom match the roof's angle.

Installing Dormer Windows

1. Use a tape measure to determine the area on the roof where the window will be installed. Measure the window's top, center and bottom, and be sure to record the results so that you don't forget them.

2. Apply a waterproof membrane or builder's felt on the window's existing housewrap. This extra insulation is necessary for added protection to prevent leakage during rain or snow.

3. Fold out the window's nailing fins in perpendicular positioning to the sides of the window frame. Ensure that the gaps between the frame and studs are equal. Hammer nails in place at the corners, but make sure not to drive them fully in.

4. Check every side of the window with a level. Measure the window diagonally from corner to corner to ensure that the measurements come to within 1/16th of an inch. If this is correct, secure the fins by nailing them into place.

5. Apply a layer of waterproof membrane on the window fins. This is necessary for the giving added protection to the window in insulating the fins.

6. Apply a thick bead of caulk around the window frame. Press the metal flashing into place where you just placed caulk around the window.

7. Add more caulk to the gaps on the inside of the window. Apply polyurethane foam over the caulk or apply fiberglass insulation and seal it with aluminum tape.

8. Add trim to the window. Measure the trim with a tape measure and cut it to proper size to match the window frame by using a circular saw. Affix the trim in the window frame and secure it with a nail gun.

Tips & Warnings

• While measuring the window, if you find you have more than an inch difference between it the roof area, you will have to nail in wood filler strips.

• Metal flashing can be installed on the dormer window over the waterproof membrane or builder's felt for added protection against the rain and snow.

References

• Cal Finder: Installing Your Dormer Window
• House Logic: Install an Attic Dormer Window