Installing a Skylight

Use a stud sensor to find the rafters. Plan how you will frame the opening (see options on opposite page). Use a drywall saw (or a reciprocating saw, if the ceiling is plaster and lath) to cut a hole wide enough to accommodate the framing (including the headers, which may be single or double 2xs).

Unless the unit is narrow enough to fit between rafters, you will need to cut at least one rafter. Before you cut into a rafter, support it temporarily on either side. For each support, screw a piece of 2x6 to the ceiling and cut a piece of 2x4 to fit snugly between the floor and the 2x6.

Mark the rafter to be cut for the rough opening dimension. Draw lines either 1-1/2 or 3 inches to account for the thickness of a single or double header at each end. Use a square to draw a right-angle line along each rafter and cut carefully using a reciprocating saw, then a handsaw. Do not cut into the roof sheathing.

Install framing as needed. Use 2x lumber that is the same thickness as your rafters. For double headers cut the pieces and nail or screw them together on the floor, then install. Check that the opening is the correct size and ensure it is square.

Drill a 3/4-inch hole up through the roof at each corner. Use a reciprocating saw to cut through the roof alongside the framing. Cut slowly; you may have to cut through several nails.

Caution: Arrange a safe place to work on the roof. Roof jacks are often the best solution. Gently pry up -- but do not remove -- roofing all around the opening. Remove nails so you can slip in roofing felt all around and flashings at the bottom and top.

Trimming below the skylight will call for some custom woodworking. The easiest approach is to carefully measure (and then measure again) the depth of jamb you'll need. Prefab a finished jamb and confirm the fit on the bottom of the skylight. Confirm that it will fit inside the framing. (See Step 14 for final installation.)

Cut strips of roofing felt (tar paper) the width of the rafters plus 8 inches. Starting at the bottom and working up, slip the felt under the shingles and the existing felt, wrap around the rafters, and staple against the rafters. You will need to cut slits at the corners.

Slip the head flashing piece under the shingles at the top. You may need to pull or cut some roofing nails to do this.

Have someone help you lift the skylight through the opening and position it. Partially drive a couple of screws or nails below the opening to temporarily support the skylight. Check from inside the room to see that the unit is centered, then drive screws through the brackets to secure the skylight.

Install step flashing, working from the bottom up. Slip each piece under a shingle, aligned with the bottom edge of the shingle, then drive roofing nails to attach the flashing and the shingle. Flashing and shingles should always overlap the next piece lower on the roof.

Install the bottom flashing piece (also called counterflashing) following manufacturer's instructions. Part of the bottom piece may slip over the step flashing and part may slip under. Drive screws to attach the bottom flashing.

Seal the joint between the roofing and the skylight with caulk or plastic roofing cement.

Shim your prefab jamb box in place using 8d finishing nails. Fill any gaps between the framing and the jamb using caulk or foam spray insulation.

By using beefy casing you may be able to avoid drywall patching; otherwise, patch and tape drywall as needed. Then cut and install picture-frame style casing.