Install a Curb-Mounted Skylight With a Flashing Kit
A skylight turns a dim hallway or a north-facing kitchen into the brightest room in the house for a few hundred dollars in parts. The roof work is a long day; the flashing is what decides whether it ever leaks.
Skylights earned their leaky reputation in the 1980s from two habits: units set flat on the deck with a smear of tar, and step flashing skipped because the roof was steep and it was late in the day. A curb-mounted skylight with the manufacturer’s flashing kit fixes both. The curb lifts the glass above the water, and the kit builds a layered, shingle-style shed that never depends on sealant. This is real roof work with a hole in your house at the end of day one, so plan for dry weather, a helper, and a harness.
Sizing to the rafters, and when to call a pro
Standard curb-mounted units drop between rafters on 16- or 24-inch centers; a 22-1/2-inch-wide unit fits a 24-inch bay without cutting anything. Go wider and you must cut a rafter and add doubled headers, which usually means a permit and sometimes an engineer. Trusses are off limits entirely. Pitch below 3/12 needs a taller curb and special flashing, and tile or metal roofs need different kits than the asphalt method here.
Lay out from inside and drive locator nails
In the attic, pick a bay clear of vents, wiring, and plumbing. Mark the rough opening on the deck underside: the skylight’s curb dimension plus 1/4 inch each way. Drive a 16d nail up through each corner so you can find it on the roof. Add 2×6 headers between rafters at the top and bottom of the opening with three 3-inch structural screws per end.
Strip shingles and cut the opening
Snap lines between the nails. Strip shingles about 12 inches back around the opening, working a shingle ripper under nail heads so surrounding courses survive. Cut the deck with a circular saw set to plywood depth and finish corners with a reciprocating saw. Roll ice-and-water membrane around the hole, bottom strip first, then sides, then top, each lapping the one below by 6 inches.
Build and set the curb
Assemble a 2×6 curb on the ground to the skylight’s spec, corners screwed and checked square; diagonals should match within 1/8 inch. Toe-screw it to the deck and rafters every 8 inches and confirm it is level side to side. Run a second layer of membrane up the curb’s outside face and onto the deck, bottom to top so every lap sheds downhill.
Flash from the bottom up
Reinstall shingles to the bottom of the curb. Set the kit’s sill flashing over them and nail only through its vertical face. Then alternate: one shingle course, one step flashing piece tucked under the next course and lapped 3 inches over the piece below, up both sides. The head flashing goes over the last step pieces and under the course above, so water never meets an upward edge. Sealant belongs only where the kit specifies, usually the curb corners.
Set the skylight and finish the shaft
Drop the unit over the curb and fasten through its flange. Inside, frame the light shaft from roof to ceiling, flaring it 10 to 15 degrees toward the room to spread light dramatically. Insulate the shaft walls with R-15 batts, add a room-side vapor retarder, then drywall, corner bead, and two coats of flat white paint.
Test before you close it
Before drywall, run a hose above the skylight for 15 minutes while a helper watches the shaft with a flashlight. Start low and work uphill so any weep is easy to isolate, and fix it while everything is open.
Falls are the most common serious DIY injury. Clip a harness to a ridge anchor on any pitch over 4/12, keep the roof dry, and never work alone. Do not open a roof if rain is possible within 48 hours, and check whether your jurisdiction requires a permit for a new roof opening; many do.
Every piece of flashing laps the one below it. Water only has to be told once.
Keeping it dry for decades
Clear leaves from the uphill side of the curb every fall; a dam of debris is what turns good flashing into a pond. Check the curb-corner sealant every few years and wash the glass from a ladder, not the roof. A properly flashed curb unit outlasts the shingles around it, and the same flashing method goes right back on when the roof is replaced.
Keep the momentum going
All articles
The One-Morning Gutter Tune-Up That Prevents Basement Water
A seasonal gutter maintenance routine: safe ladder work, clearing and flushing, resealing end caps and seams, correcting pitch, and extending downspouts.
Replace Damaged Asphalt Shingles Safely, One Tab at a Time
A step-by-step guide to replacing wind-torn or cracked asphalt shingles: breaking the seal, pulling eight nails, nailing on the line, and hand-sealing.
Install a Tubular Skylight in a Single Afternoon
A step-by-step guide to installing a tubular daylighting device: finding a clear rafter bay, cutting the roof and ceiling, flashing the dome, and assembling the reflective tube.