Why Skylights Leak: It's the Flashing, Not the Glass

The glass is the last thing to fail

When we get a call about a dripping skylight, the homeowner has usually already stared at the glass looking for a crack. We understand the instinct, but a modern tempered or laminated skylight pane is one of the most durable parts of your entire roof. It handles hail, branches, and decades of Carlsbad sun without so much as a hairline fracture. What fails is everything around it.

A skylight is not a window sitting in a hole. It is a self-contained unit that has to be married to the roof with layers of flashing, underlayment, and sealant. Those layers are what keep water out. When any one of them lifts, corrodes, or was installed wrong in the first place, water finds the path of least resistance and travels sideways and down until it reappears on your ceiling, often a foot or more from the actual entry point.

The flashing is where roofs and skylights argue

Flashing is the metal that laps water away from the seams where the skylight curb meets the roof plane. On a properly installed unit you'll find a step flashing up the sides, a saddle or head flashing at the top to divert runoff around the unit, and an apron at the bottom. Each piece has to lap over the one below it in the direction water flows. Get that sequence wrong and you've built a funnel instead of a diverter.

Out here the flashing takes a beating from two directions. The coastal sun bakes the sealant beading and the exposed fastener heads, drying them out and shrinking them until they pull away. Then the marine air, salt-laden and damp most mornings, works on any bare or galvanized metal and starts the corrosion. We see aluminum and copper flashing hold up fine, but cheaper galvanized step flashing near the coast can rust through in a decade.

How to tell flashing from glass as the source

There's a simple field test we use before we ever pull a unit. If the water only appears during wind-driven rain from a particular direction, that points to flashing on the exposed side. If it drips steadily in any storm, or you see condensation-style droplets on the interior frame in cool weather, that changes the diagnosis entirely and often points to a sealing or condensation issue instead.

We also look at where the stain lands. Glass or seal leaks tend to show right at the light shaft. Flashing leaks show up offset from the skylight because the water traveled along a rafter before it dropped. When we hose-test a suspect skylight, we start low and work up, isolating one flashing piece at a time so we know exactly what failed.

Why the roof around the skylight matters as much as the unit

A skylight is only as watertight as the field of roof it sits in. On tile roofs, which are our specialty, the pan flashing has to be woven correctly under the courses above and over the courses below, and the mortar or foam closures have to be intact. When a previous crew set the tile back over a fresh skylight without re-cutting the field properly, water dams up behind the head of the unit. That's not a skylight failure, that's a roofing failure that shows up at the skylight.

This is why we don't treat skylight leaks as an isolated repair. We inspect the surrounding underlayment, the tile condition, and the drainage path uphill. If the deck below has stayed wet for a while, there may be rot in the framing of the light well that also needs addressing before we reseal anything.

The fix depends on the diagnosis, not a tube of caulk

The worst thing you can do to a leaking skylight is smear roofing tar or silicone over the top and call it done. Sealant is a supporting player, not the waterproofing system. When someone caulks over failed flashing, they trap moisture, hide the corrosion, and buy maybe one dry season before it comes back worse and the deck underneath has softened.

Our approach is to lift what needs lifting, replace the flashing metal in the correct lapping order, renew the underlayment where it's compromised, and reset the tile or shingles so the water sheds naturally. Only then does sealant go on, and only at the manufacturer-specified joints. Done right, a reflashed skylight outlasts the next reroof.

Key takeaways

• Roughly nine of ten skylight leaks we find trace to flashing, not the glass.
• Coastal sun dries out sealant and salt air corrodes cheaper galvanized flashing.
• Where the ceiling stain lands tells you whether water traveled along a rafter.
• Caulking over failed flashing traps moisture and hides worsening damage.
• A proper repair replaces flashing in the correct lapping order, then reseals.

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