1 Cut & frame the opening.

Follow the manufacturer’s specifications for cutting the hole in your roof. From the inside, drill a locator hole where you want the center of the skylight to be and then work on the roof to cut the opening with a circular saw.

Fig. 1, Cutting and framing an opening in the roof for a skylight.

Frame the opening so it is structurally sound according to building codes. Be sure to support the ends of roof rafters before cutting and removing them! Install header joists perpendicular to the rest of the joists (Figure 1).

If the skylight is mounted above an attic, you’ll also need to cut and frame a hole in the ceiling of the room below it, and frame a light shaft through the attic. Note: If you’re not familiar with basic carpentry practices or you are installing a large skylight that will require the removal of more than one roof rafter, consult a professional builder.

2 Cut back the roofing & attach the skylight.
Using a utility knife and a straightedge, cut the roof shingles about 3 inches back from the opening on all four sides. Set the skylight in place, centered on the opening, and attach it to the roof with nails or screws.

Fig. 2, Positioning Roof Paper

3 Slip in the underlayment.
Cut strips of roofing paper about 8 inches wide and slip them under the shingles (Figure 2). Install the bottom piece, then the sides, and then the top.

The idea here is to overlap the pieces down-roof with the pieces that are up-roof so water will shed properly. Sneaking the paper under the shingles will be tricky. Use a flat pry bar to lift all the roofing nails that would get in the way, taking care not to damage the shingles.

4 Install bottom & step flashing.
Install the bottom flashing, a single piece that wraps partway around the skylight and sits on top of the roof shingles (Figure 3). Drive roofing or flashing nails horizontally into the skylight, not vertically into the roof.

Next, slip individual pieces of step flashing under the shingles. On each side, start at the bottom and work up. The step flashing pieces must overlap each other by 4 inches or so. Attach the flashing pieces to the skylight, not to the roof.

Fig. 3, Side/Bottom Flashing

5 Install the solid flashing pieces.
These pieces are designed to fit tightly onto the skylight and ensure that water cannot enter between the step flashing and the skylight. Attach the bottom piece first and then the side pieces (Figure 4). Install the top piece (called the head flashing) by slipping it under the roofing and attaching it to the skylight.