How a Skylight Is Properly Installed on a Shingled Roof
A skylight goes in by cutting a hole through your roof deck. That part takes about an hour. Everything that decides whether the hole stays dry for the next twenty years happens afterward, in the order the membrane, metal, and shingles go back down around it.
Most homeowners never see any of that. The crew works up on the roof, the interior finishing happens later, and by the time you walk into a brighter kitchen the flashing is already buried. Knowing what should have gone on up there is the only way to catch a shortcut before it shows up on your ceiling.
What Makes a Skylight Different From Other Roof Penetrations
Every vent stack, exhaust cap, and chimney interrupts water running down the slope, and each one gets metal shaped to carry that water back out onto the shingles. A skylight is the largest interruption on a typical house. It also sits partway up the slope, so water reaching it has already collected off everything draining above.
Sealant on its own will not hold that back. The waterproofing is metal and membrane laid in a specific order, each piece lapping over the one below it, so there is no seam facing uphill for water to work into. Flashing goes underneath the shingles rather than over top of them. There is a solid technical breakdown of how roof flashing is sized, layered, and fastened if you want to see how the pieces fit together.
How the Installation Should Go
This is a correct sequence on an asphalt shingle roof. Metal roofing follows the same logic using panel-specific flashing.
Layout is confirmed from both sides. The position is marked on the ceiling inside, transferred up to the roof, and checked against the rafter spacing before anything is cut.
Shingles come off well past the opening. Stripping back only to the edge of the cut leaves the flashing nowhere to lap into. Courses need to be opened up on all four sides.
The deck is cut and the opening framed. Headers go in between the rafters to carry the load the cut rafter was handling. On wider units this is structural carpentry, and in Oakville it may require a building permit.
Membrane goes in before the unit does. Ice and water protector is wrapped into the opening and lapped several inches onto the surrounding deck, so there is a waterproof layer under everything that follows.
The skylight is set and fastened. The unit is fixed to the framing with the hardware the manufacturer specifies. Substituting fasteners here is a common way to void a warranty.
Flashing is installed from the bottom up, woven into the shingles. The sill piece goes on first at the low side. Step flashing runs up both sides, one piece per shingle course, each one covered by the course above it. Head or saddle flashing closes the top edge, where water pressure is highest. Counter-flashing finishes the perimeter.
The interior shaft is framed and insulated. Framing, insulation, vapour barrier, drywall, paint. Skipping insulation in that shaft will give you condensation problems every January.
Deck-Mounted and Curb-Mounted Units Are Flashed Differently
A deck-mounted skylight sits low against the roof plane, and its flashing is woven straight into the shingle courses. It is the more common choice and the cheaper one to install.
A curb-mounted unit sits on a wooden frame built up off the deck, raising the glass a few inches above the shingle surface. That height gives the flashing a vertical face to seal against and keeps the bottom of the unit clear of standing water and drifted snow. Curbs add to the cost, and in an Ontario winter they usually hold up better. Any roofer quoting your job should be able to tell you which one they have priced. Click here to learn more about what a skylight installation costs in Ontario, including how the mounting type affects the total.
Where These Installations Go Wrong
Almost every skylight leak traces back to a decision made on installation day.
Flashing laid over the shingles instead of under them. Water finds the seam and runs beneath it.
Sealant used in place of proper flashing. Caulk dries out, cracks, and needs redoing every few years, which is not a maintenance schedule anyone actually keeps.
Step flashing spaced too far apart or cut short, leaving gaps along the sides that open up as the roof moves.
Fasteners driven through the flashing where the manufacturer never intended them, punching holes in the layer meant to stop water.
An old flashing kit reused during a roof replacement, when the metal is already fatigued and the sealant has aged out.
Shingles not stripped back far enough, forcing the crew to shoehorn flashing into courses that were never opened up.
Most of these come from crews who install skylights occasionally rather than regularly. At Silver Oak Roofing, the skylight work we take on is flashing integration on shingle and metal roofs, repair when an existing unit starts letting water through, re-flashing during a roof replacement, and emergency response when one starts leaking mid-storm.
What Ontario Winters Add to the Equation
Freeze and thaw cycles put more stress on a skylight opening than rain does. Meltwater comes down off the warmer part of the roof, reaches a colder edge, and refreezes. Ice builds up behind that and works into any joint depending on sealant instead of overlapping metal.
Snow is the other factor. A unit mounted flush with the deck collects drifted snow along its upper edge, where it can sit wet against the head flashing for weeks at a time. That is why the head flashing is usually the first piece to give out on Oakville roofs.
Condensation causes a lot of false alarms. If the shaft between the roof and the ceiling is poorly insulated, warm indoor air reaches the cold glass, condenses, and drips. It looks exactly like a leak. Rule it out before anyone starts pulling shingles.
FAQ
How long does a skylight installation take?
Most single-unit installations take one to two days on the roof, plus the interior finishing. Framing changes, a deep shaft, or a steep roof with difficult access will stretch that out.
Can a skylight be added without replacing the roof?
Yes, as long as the surrounding shingles have enough service life left. If the roof is within about five years of replacement, doing both together saves opening the same section of roof twice.
Why do skylights leak at the top rather than the bottom?
Water coming from above hits the head of the unit first and with the most force, and snow tends to pile against that edge. The sill flashing at the low end sees much gentler conditions.
Should skylight flashing be replaced during a roof replacement?
Yes. Flashing ages at roughly the same rate as the roof around it. Reusing a fatigued kit under new shingles is one of the more common reasons a leak turns up shortly after a re-roof.
Can a skylight go on a metal roof?
Yes, though the details differ. Panel profiles need flashing shaped to the ribs and sealed to the panel system rather than woven into courses the way shingle flashing is.
Conclusion
A skylight installation is finished work you cannot inspect afterward. Once the shingles are back down, the flashing underneath is out of sight, and you find out how well it went the first winter that tests it.
Ask whoever quotes the job how they plan to flash it, how far back the shingles are coming off, and whether the existing flashing is being replaced. If you already have a skylight that is staining, drafting, or letting water in, reach out to our team and we will take a look at the roof around it.
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