How to Install Skylights in Gable Roofs

Mark the corners for the ceiling opening -- which should be larger than the roof opening to allow excellent light pass-through -- on the ceiling joists in the attic above the room. These marks are provisional, allowing you to flare the top of the light shaft in your final layout after installing the skylight.

Touch the point of a plumb bob in turn on each of the two ceiling opening corners nearest the eave. Pull the plumb bob’s string up to the roof deck (the plywood or OSB sheathing underneath the shingles that's visible from the attic). Mark the two points where the string touches the roof deck. Measure up from these marks the distance called for in the rough opening specifications. Mark the upper corners of the rough opening called for by the skylight manufacturer. Drive nails up through the plywood deck and the shingles at the four corners of the rough opening.

Climb a ladder to the roof. Remove shingles from around the rough opening with a pry bar so you can seek the roof deck. Set the shingles aside. Snap a chalk line between the four nail tips poking up. Knock the nails out of the roof deck with a hammer and nail set. Set a circular saw deep enough to cut the roof deck only and not deep enough to cut out any rafters under the deck. Cut out the roof deck along the lines with the saw, carefully lowering the saw onto the chalk line to begin each cut. Remove the roof deck cutout.

Return to the attic. Mark the rafters where they will accept doubled headers at the top and bottom of the rough opening. Align the top header perpendicular to the roof deck; align the bottom header vertically. Nail horizontal 2-by-4 supports across the rafters above and below the rough opening if you are cutting out any rafters, which is likely if your skylight is more than 14 ½ inches wide. Place the 2-by-4s high enough and low enough to clear the headers and wide enough to extend one joist past either side of the rough opening. Cut intermediate rafters and toenail two 2-by-6s -- or whatever lumber matches the rafter dimensions -- between the end rafters to create each double header.

Return to the roof and install the skylight according to the manufacturer’s instructions. For a pan-flashed skylight, for example, you apply roof cement to the rough opening. Press the skylight into place. Nail the provided 2-inch nails at each corner, driving the nails into the rafters below through the provided holes, and 1 ¼-inch nails in the provided holes between the corners. Flash and reshingle the roof according to the manufacturer’s instructions.

Return to the attic. Revise your marks on the ceiling joists for the light shaft opening if necessary so that they are at a 45-degree angle to the top of the finished skylight opening. Support, cut and frame the ceiling opening as you did the rough opening in the roof. Frame the light shaft by running studs at 16 or 24 inches on center from the roof double headers to the ceiling double headers and toenailing them in place. Add vertical supports toenailed every 16 inches on the sides of the light shaft.

Insulate the attic side of the light shaft with batts or rigid insulation. Finish the inside of the light shaft with drywall or a similar finish.