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Skylight leaking: the glass is usually not the problem

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Skylights get blamed for water that arrived somewhere else entirely, and about half the time nothing has leaked.

Water appears around a skylight and the skylight gets the blame. It is usually innocent. The glazing unit itself is a sealed, factory-made assembly and it rarely fails; what fails is everything around it, or nothing at all.

First: is it actually a leak?

Start here, because it changes everything. A skylight is a large area of cold surface set into a warm ceiling, which makes it the best condensing surface in the house. On a cold morning, in a bathroom or a kitchen, water running down the inside of the frame and dripping onto the floor is very often condensation, exactly like a cold drink sweating in summer.

How to tell:

  • Where is the water? Condensation appears on the inside face of the glass and the frame and runs down. A leak usually shows up at the edge, on the drywall of the shaft below, or as a stain on the ceiling beside the opening.
  • What is the weather? Condensation happens on cold days regardless of rain. A leak tracks rain.
  • Which room? Skylights over showers and stoves condense far more, because that is where the humidity is.
  • Is there a channel? Most modern skylights have a small condensation gutter built into the frame to catch exactly this and drain it away. When it fills or blocks, it overflows — which looks like a dramatic leak and is not one.

If it is condensation, the answer is ventilation and humidity in the room below, not anything on the roof.

The flashing kit

If it is a genuine leak, the flashing is the first suspect. A skylight is flashed with a set of pieces that step up the sides and lap correctly at head and sill, much like a chimney. Two things go wrong. The kit may have been improvised rather than used — a skylight flashed with sealant and a single piece of bent metal instead of the manufacturer's stepped kit is a leak with a delay built in. Or the kit is right and has aged, particularly at the head, which is the hardest detail on the whole assembly.

The kerb and what sits above it

A skylight either sits on a raised kerb or is mounted lower into the plane. Either way there is an uphill side, and that side is a dam. Everything coming down the slope — leaves, needles, granules, ice — arrives at the head of the skylight and has to go around it. When it stops going around and starts piling up, water backs up behind it and gets under the courses above the head flashing.

This is why skylight leaks are so often seasonal. A skylight under a tree leaks in autumn. A skylight in a cold climate leaks in a thaw, because the dam above it is ice.

If a skylight is high on a slope with a long run above it, or is wide, the amount of water arriving at its head is considerable, and some installations use a small diverter above for that reason.

Water that only looks like it came from the skylight

The shaft below a skylight is a slope, and water that entered the roof higher up can run down a rafter and arrive on that shaft. Because the shaft is the visible feature, the skylight takes the blame for a leak that started at the ridge or at a vent several feet away. As always, the way to settle it is to look in the attic alongside the shaft and trace where the wet run begins.

The sealant trap

A skylight with a thick ring of black sealant around it has almost certainly been leaking for years. Sealant is the wrong tool for the job: flashing works by geometry, by directing water downhill over a lap, and sealant works by adhesion, which fails as materials expand and contract at different rates through the seasons. Once it cracks it holds water against the joint instead of shedding it. A ring of old caulk is a symptom, not a repair, and the next person will have to remove it before doing the job properly.

What you can establish, and where it stops

From inside you can determine whether it tracks rain or tracks cold, which room it is in, whether the water is on the inside face of the glass, and whether there is a wet run in the attic beside the shaft that starts higher up. That is enough to know whether you need a roofer at all.

What you should not do is go up to look at the head flashing. A skylight is set into a slope, it is usually not near the edge, and the area right around it is glass. This is not a place to be standing on a ladder, and the diagnostic value of being there is low compared with what you can work out from below.

Frequently asked questions

Is water on the inside of my skylight a leak?

Often not. A skylight is a large cold surface in a warm ceiling, so it condenses readily, especially over a shower or a stove. Condensation forms on the inside face of the glass and frame and happens on cold days regardless of rain; a leak tracks the weather.

Why does my skylight only leak in autumn?

Because the uphill side of a skylight is a dam. Leaves and needles pile up at the head, water backs up behind them and gets under the courses above. The winter version of the same problem is ice rather than leaves.

The glass is fine. What is failing?

Almost always the flashing, and usually at the head, which is the hardest detail. The other common cause is that the manufacturer's stepped flashing kit was never used and somebody substituted bent metal and sealant.

There is a lot of black sealant around my skylight. Is that bad?

It means it has been leaking for a while and somebody has been treating the symptom. Flashing works by geometry, sealant by adhesion, and adhesion loses to seasonal movement. Cracked sealant traps water against the joint rather than shedding it.

Can I check the flashing myself?

We would not. A skylight sits mid-slope rather than at the edge, and the immediate surroundings are glass. Almost everything you need in order to decide what is happening can be worked out from inside the room and from the attic beside the shaft.

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.

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