Skylight leaks almost always trace back to flashing, not the glass unit itself. Here’s how skylight installation actually works, the code rule on which mounting type you’re even allowed to use, and where leaks most commonly start.
Curb-Mounted vs. Deck-Mounted: The Code Line
Under the 2024 IRC, roofs below a 3:12 pitch require curb-mounted skylights; a deck-mounted unit on a low-slope roof sits directly in the path of standing water, which is both a code violation and a near-guaranteed leak. At 4:12 pitch and steeper, either mounting type is permitted. Curb-mounted skylights sit on a raised frame that uses gravity to direct water away from the opening, and they’re considerably easier to replace later since only the curb-level flashing is disturbed. Deck-mounted units sit flush with the roofline for a lower profile, but the flatter transition makes achieving a perfect seal harder, and replacing one requires stripping back roofing material and re-flashing.
The Flashing Sequence That Actually Prevents Leaks
Flashing goes in a specific order, bottom to top, so each layer sheds water onto the one below it:
- Apron (sill) flashing first, along the lower edge, overlapping the roofing material below it and fastened to the curb, not the roof deck itself.
- Step flashing along both sides, individual L-shaped pieces woven between each course of shingles, working from bottom to top, nailed high enough on each piece that the next course covers the fasteners.
- Head (saddle) flashing last, at the top, with its upper edge sliding under the next full course of shingles above the skylight, directing water around and past the opening entirely.
The uppermost step-flashing pieces slide underneath the head flashing, creating a continuous, overlapping water path with no single component expected to seal on its own.
Where Leaks Actually Start
- The head flashing junction. This is where water volume concentrates after running down from higher on the roof, making it the single most common failure point if the bend or fit isn’t precise.
- Step flashing nailed through the wrong layer. Fasteners should go into the curb, not through the roofing material in a way that creates a new penetration path.
- Underlayment gaps around the curb. Wrapping the curb with self-adhering membrane before the metal flashing goes on adds a backup layer that catches what the flashing alone might miss.
- Aging sealant used as the primary defense rather than as a secondary measure. Properly lapped flashing shouldn’t rely on sealant to actually keep water out; sealant alone degrades with UV exposure and isn’t a substitute for correct flashing geometry.
Placement Matters Too
Positioning a skylight away from plumbing stacks or other roof penetrations reduces the number of flashing systems competing for the same small area, which lowers the overall leak risk at that section of roof.
How Installers Verify a Watertight Seal
A proper final check runs water over the installation with a garden hose while someone checks the interior for any sign of moisture, rather than assuming the flashing sequence alone guarantees a dry result.
Frequently Asked Questions About Skylight Leaks
Can I install a deck-mounted skylight on a low-slope roof?
No, not under current code. The 2024 IRC requires curb-mounted skylights below a 3:12 roof pitch, since a deck-mounted unit on a flatter roof sits directly in the path of standing water.
Why do skylights leak even when the unit itself isn’t damaged?
Almost always because of a flashing failure, not the glass or frame. The head flashing junction at the top of the skylight is the single most common failure point, since that’s where the most water volume concentrates.
Is a curb-mounted or deck-mounted skylight easier to replace?
Curb-mounted. Since only the curb-level flashing needs to be disturbed, replacement is considerably simpler than a deck-mounted unit, which requires stripping back roofing material and completely re-flashing.
Should sealant be the main thing keeping a skylight from leaking?
No. Properly lapped and sequenced flashing should keep water out on its own; sealant is a secondary measure, and relying on it as the primary defense is a sign of a flashing installation that isn’t geometrically correct.
Get a Skylight Installed With Proper Flashing Sequence
As a local roofing company serving Scottsdale, AZ, Tip Top Roofing Service installs and re-flashes skylights with the correct curb or deck-mount approach for your roof’s actual pitch, not a one-size-fits-all install.
📞 Call Tip Top Roofing Service today at (480) 877-1643
📧 Email: info@tiptoproofingservice.com
🌐 Website: https://tiptoproofingservice.com/
