How to build a dormer

Updated February 21, 2017

A dormer is not only a point of interest on the roof of your home, but also a less expensive way to add some headroom to an upper story without having to actually raise the entire roof of the house. In some applications, especially with a cathedral-ceilinged room, the dormer can act as a skylight.

Nail sheeting to the roof, but leave a gap at the top. There you will attach a ridge vent, allowing proper ventilation. Nail two strips of 16.8 kg (15 lb) roofing felt to both sides of the roof sheeting. Seal the window opening by putting a bead of caulk around the window opening and install the window. Make certain you pick out a window that will fit in with the other windows on your house.

Put step flashing (the end result looks like scales) around the lower corners, and a piece of flashing at the foot of the front wall. This helps ensure that rain flows around the dormer and into the gutter, rather than through any gaps that may be left between the dormer and the roof. Attach wood to over the subfascia, and install soffits beneath the overhang of the dormer. Finish the outside of the dormer with shingles or clapboards that fit in with the rest of the building. Finish the inside by installing plasterboard.

Find out what the local planning requirements are concerning the size for headers.

Measure the roof. This allows you to determine where you'd like to place your dormers as well as making sure it will fit in with the room beneath. Proper placement will also insure that the dormer does not look out of place with the rest of your house.

Take off the shingles and felt paper from the roof where you will be building the dormer and study the exposed nail heads. This will indicate where the roof trusses are located.

Start building the dormer, beginning with the front wall. The studs at the bottom will have to be cut at an angle to match that of the roof. Determine the roof's slope by measuring up 3.6 m (12 feet) on the roof, then figuring out how high about the starting point the roof has risen. Some roofs have a 9/12 slope, meaning that the sloop would rise up 2.7 m (9 feet) for every 3.6 m (12 feet). Use your measuring square to determine the angles at which the studs must be cut. For a 9/12 slope, set the measuring square at "9". Use the mitre saw to cut the angled studs.

Build the window assembly with a header at the top, uprights for the interior and exterior, and a sill. Put the window assembly inside the wall and nail it firmly into place.

Attach a 5 x 10 cm (2 x 4 inch) board with nails to the bottom of the front wall, and then attach the board assembly to the roof with 9 cm (3 1/2 inch) nails.

Fasten the top plates. Nail one end to the roof (the ends will have to be beveled to match the slope of the roof), the other to the front wall. If the top plates extend beyond the front wall, that's fine. Cut them later. Be certain the front wall is plum, and that the plates are level.

Nail the bottom plates to the roof. One end will extend to the top plate, the other to front wall's base.

Use your level to determine the placement of the intermediate studs. The studs can be 60 cm (24 inches) on centre. Mark it with your pencil, then cut the studs to size, and nail them into place.

Cut the rafters to size using your mitre saw. The ends need to be at a 45-degree angle.

Make a 90-degree cut with the jigsaw where the rafter will rest on the plate.

Angle the ridge beam at one end so that it will fit properly, then nail it to the roof and the front wall rafters. The beam should extend beyond the front wall.

Do a compound mitre cut in the ends of the last rafters so the will be flush with the roof. Attach the rafters to the ridge beam and the top plate.

Measure, then cut the roof beneath the dormer framing with either a reciprocating saw or circular saw.

Determine where the ceiling should be cut from below by using a plum bob. Mark the corners by driving nails that can be seen from below. Cut through the ceiling with a reciprocating saw.

Double up the trusses and rafters. Add more lumber to the truss and affix them with nails and glued plywood strips on the sides.

Tie a rope around the truss, then cut through the middle truss with a reciprocating saw. The rope will act as a safety measure: it keeps the truss from falling down to the floor below, perhaps crushing someone.

Build a header in the front wall by installing cross framing between the two side trusses beneath the roof opening.

Fasten up 5 x 10 (2 x 4 inch) studs with nails. You will use these for attaching the drywall. Make the opening flush so that the drywall fits properly.

Install strand board sheeting with sixteenpenny nails on the outside of the dormer. Begin with the dies, and install the front piece last. The front sheathing should be all one piece. Install subfascia underneath the rafter ends. The subfascia acts as a nailing framework for the fascia and soffit trim.

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