Skylight installation tips

A properly installed skylight will not leak. Use these skylight installation tips to install a skylight that doesn’t leak and follow the manufacturer’s recommendations.

The simplest skylight installation is when the skylight aligns with ceiling joists and roof framing. Obstructions, such as ductwork or pipes should be avoided, as this will mean significant additional work to move them.

If possible select skylights sized to fit neatly between rafters or span several so framing and curbing can be more easily accommodated. Be sure sufficient tile, shingle or shake extends below skylight to maintain roof drainage.

Parallel or splayed openings

An opening must be made in exterior roof and drywall ceiling. When a light shaft is required, the opening in the ceiling can be splayed or offset to maximize light entering through the skylight.

Positioning opening

In attic, mark center and four corners of opening with 16- or 20-penny nails driven through the roof. Drill pilot holes for the nails. This will show the opening from the roof. For tiled roofs, carefully measure position for the skylight on the roof and remove tiles from the area. Keep at least one row of tile between skylight and roof edge.

Lining up lower opening

Inside attic, drop a plumb bob from the nail in center of roof opening to be sure two openings will line up. This may not apply to offset or splayed openings.

Building a curb

If the skylight requires a curb, build it now. (Some skylight models come preassembled with curbs.) The skylight will likely need to sit at least 4 inches above roof sheathing, so using 2 by 6 is simplest. The curb’s inner dimensions should match those of the planned roof opening. When complete, it should drop neatly over corner nails projecting through the roof. Once it is nailed together, use a piece of scrap wood placed diagonally from opposite corners to keep the curb square.

Cutting roofing materials

If you’re using a curb, place it on the roof and mark a line around outside. Set fence on the circular saw to cut through only the roofing material, cut around the mark and pry up material. As much as possible, keep shingles complete to use later. Snap chalk lines from corner nails, reset saw fence to cut through sheathing, cut along the lines and pry out cut sheathing.

Cutting exposed rafters

It will be necessary to remove the exposed rafters. From inside, nail 2 by 4s at least 1 foot from top and bottom of opening. To support roof once rafters are cut, you will need a doubled header and footer. Cut four pieces of wood the same size as the timber used in the roof to fit between the uncut rafters at sides of the opening.

Making headers and footers

Cut exposed rafter, allow for thickness of doubled header and footer. If you have created a light shaft, angle cuts to match the splay of the light shaft. Install first header, nailing at center and ends into rafters. Nail second header to first at 6-inch intervals, using 8-penny nails. Repeat for footers and remove temporary 2 by 4s. Even if the skylight fits without cutting a rafter, install a header and footer to support skylight.

Skylight installation that doesn’t extend to rafters

Install jack rafters at each side of skylight opening, nailing to header and footer.

Skylight installation: flashing

If there is a curb, install flashing at top, sides and bottom to prevent water from creeping under the edges. Use top flashing preformed to curb. Slide flashing under tiles at top of the opening. Position curb into this header flashing until it is exactly over the roof opening. Toenail curb to header, footer and rafters. Slide preformed side flashings under head flashing and under side shingles. The preformed foot flashing slides under side flashing but on top of roof tiles to allow water to flow over tiles and down the roof. If you buy flashing and cut it to size, cut so top and sides overlap and solder joints.

Skylight does not use a curb

This skylight has its own flashing system. Tack skylight to headers, footers and rafters; test that it opens and closes. Finish-nail and follow manufacturer’s instructions to assemble and install flashing. Replace roofing material and shingles.

Cutting ceiling for skylight installation

Cut the opening in the ceiling using a reciprocating saw. If the light shaft spans one or more ceiling joists, install headers and footers as above. If several joists are removed, add a sister joist outside existing joists to either side of the opening to support the ceiling.

Constructing light shaft

With top and bottom openings framed, assemble 2 by 4 studs on 16-inch centers around the opening. At each corner, use two studs as a corner, and drywall. Insulate the shaft.