Who this guide is for
- Homeowners adding or replacing a skylight
- People with a skylight that leaks at the edges
- Renovators integrating a roof window into the covering
- Anyone briefing a roofer or installer on the perimeter
Why the perimeter leaks, not the glass
Modern skylight units are well-sealed, so leaks almost always come from the flashing and curb where the unit meets the roof, not the glazing itself. Treating the perimeter as the leak risk focuses the planning where it matters. The glass is rarely the problem.
The curb and raising the unit
Many skylights sit on a curb, a raised frame that lifts the unit above the water sheeting down the roof, giving the flashing a height to work against. A flush or poorly raised unit is more exposed to flow. Plan the curb as part of keeping water out.
Flashing that sheds water away
Skylight flashing is built up around the four sides so water flowing down the roof is led around and below the unit, always shedding over the flashing, not behind it. The way it laps with the roof covering is critical. Plan the flashing to integrate with the specific covering.
- Head flashing diverts water above the skylight
- Side flashing channels water past the unit
- Apron flashing carries water below the unit
- All flashing must lap so water sheds outward
Integrating with the roof covering
The flashing detail depends on the roof covering, since a tiled, shingled or metal roof each laps differently. A skylight flashed for the wrong covering leaks. Plan the integration to match the actual roof.
Roof work and professionals
Cutting a roof opening, building the curb and flashing the perimeter are weather-critical and at height, so they belong to qualified roofers and skylight installers. A leaking skylight should be assessed, not patched blindly. Requirements vary by location and project, and this is not DIY work.
Skylight flashing planning checklist
- 1Treat the perimeter, not the glass, as the leak risk
- 2Plan whether the unit sits on a raised curb
- 3Plan head, side and apron flashing around the unit
- 4Confirm all flashing laps so water sheds outward
- 5Match the flashing detail to the roof covering
- 6Coordinate the opening with the roof structure
- 7Have a leaking skylight assessed rather than patched
- 8Use a qualified roofer or skylight installer
Common mistakes to avoid
- Blaming the glazing when the flashing is leaking
- Skipping or under-building the curb
- Flashing detailed so water can get behind the unit
- Using a flashing detail for the wrong roof covering
- Cutting a roof opening without considering structure
- Patching a skylight leak without finding the cause
When to involve a professional
- Use a qualified roofer or skylight installer for the perimeter
- Treat the curb and flashing as weather-critical, at-height work
- Confirm the opening with the roof structure before cutting
- Have a leaking skylight assessed rather than blindly patched
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
Why do skylights leak?
Almost always at the flashing and curb where the unit meets the roof, not at the glazing, which is usually well-sealed. That's why planning focuses on the leak-critical perimeter rather than the glass itself.
What is a skylight curb?
A curb is a raised frame that lifts the skylight above the water sheeting down the roof, giving the flashing a height to work against. A flush or poorly raised unit is more exposed to the flow, so the curb is part of keeping water out.
How does skylight flashing work?
It is built up around all four sides, head, side and apron, so water flowing down the roof is led around and below the unit, always shedding over the flashing rather than behind it. The laps must integrate with the specific roof covering.
Can I install or reflash a skylight myself?
No. Cutting a roof opening, building the curb and flashing the perimeter are weather-critical and at height, so they belong to qualified roofers and installers. A leaking skylight should be assessed rather than patched, and requirements vary by location and project.
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