Leaking skylight: causes, fixes and water damage

Leak or condensation?

They look alike from below and need different fixes.

Condensation

• Appears in cold weather, often first thing in the morning
• Happens with no rain or melting snow
• Water or frost on the inside of the glass and frame
• Worse in bathrooms and kitchens, and when indoor humidity is high
• Drips from the lowest edge of the frame

A leak

• Appears during or shortly after rain, or when snow on the roof melts
• Stains on the drywall of the skylight shaft, often at the upper corners
• May show some distance below or beside the skylight, where water has run along the framing
• Unrelated to how humid the house is

If you are unsure, note the weather each time it happens.

Where skylights leak

Flashing. The metal that seals the skylight to the roof is the usual culprit. It may have been badly installed, sealed with roofing cement instead of proper step flashing, or it may have corroded, lifted or been disturbed during re-roofing.

The roof above. Water entering further up the roof runs down the underlay until it meets the skylight curb and comes in there. The skylight gets the blame for a shingle or flashing problem above it.

The glazing seal. The gasket between the glass and the frame hardens and shrinks with age.

A failed sealed unit. Fogging or moisture between the panes means the seal has gone. This affects insulation and visibility more than it lets water in, but it shows the unit is at the end of its life.

Cracked glazing or dome. Acrylic domes craze and crack with age and hail.

Opening skylights. Weatherstripping that is worn or dirty, or a unit left slightly open.

Debris. Leaves and needles collect above the skylight and dam water.

Ice dams. Heat escaping round a skylight melts snow, which refreezes below it and backs water up.

Low-slope roofs. Skylights on shallow pitches need a higher curb and are less forgiving.

What to do when it drips

1. Catch the water and move furniture and rugs.
2. If the ceiling near it is bulging, keep clear. Switch off power to that room’s lights before draining a bulge through a small hole.
3. Photograph the drip, the stains and the weather outside.
4. Do not go on a wet or icy roof.
5. If it is raining hard and the leak is serious, have the area tarped. See emergency roof tarping.
6. Dry the area and arrange an inspection.

Fixes that last, and fixes that do not

Caulking from the inside does nothing. The water is already in.

Smearing roofing cement or caulk round the outside may stop a leak for a season. It cracks, traps water and makes the proper repair harder.

Re-flashing is the real repair for most leaks: the shingles round the skylight are lifted, a self-adhering membrane is applied to the roof deck and up the curb, and new step flashing and head and sill flashing are installed, ideally the manufacturer’s kit for that model.

Replacing the skylight makes sense when the unit is old, the seal has failed, the frame is rotted, or the roof is being redone. Skylights are commonly replaced when a roof is replaced, because fitting new flashing to an old unit is asking for trouble later.

Roof repair above the skylight, if that is where the water enters.

Fixing condensation

• Run the bathroom fan during showers and for about 20 minutes after, and the range hood when cooking
• Lower the indoor humidity in winter, to around 30 percent in cold weather
• Keep blinds on the skylight open in cold weather so that warm air reaches the glass
• Check that the skylight shaft is insulated and sealed. An uninsulated shaft through a cold attic is a condensation machine.
• Consider a better-performing unit. Old single-glazed domes and failed sealed units are cold.

Many skylights have a small channel to collect a little condensation. Heavy, regular dripping means humidity is too high or the unit is too cold.

The damage to look for

• The shaft. Stained, soft or moldy drywall, and peeling paint.
• Insulation round the shaft and in the attic nearby, wet and compressed.
• Framing. Dark, damp or soft wood at the curb, the rafters and the roof deck round the opening.
• The ceiling and walls below, where water has travelled.
• Flooring under a long-standing drip.

A slow skylight leak can rot the roof deck and curb before much shows indoors. If the drywall at the top corners is soft, have the framing looked at.

Drying and repair

1. The source is fixed first, or at least covered.
2. Wet drywall in the shaft is removed where it is soft, stained through or moldy.
3. Wet insulation is taken out.
4. Framing is dried with air movers and a dehumidifier until moisture readings are normal.
5. Any mold is cleaned from the wood, and rotted wood is replaced.
6. The shaft is insulated, sealed against air leaks and closed up.
7. Stains are covered with a stain-blocking primer before painting.

Older textured ceilings and drywall compound may contain asbestos and should be tested before cutting.

Insurance

Water entering suddenly through storm damage, such as a skylight broken by hail or a fallen branch, is generally covered. A leak from worn flashing, a failed seal or poor installation is usually treated as maintenance, and resulting damage that built up gradually is commonly excluded. Condensation damage is not covered. If the interior damage came on suddenly, report it and let the adjuster assess it. Keep photographs and the roofer’s findings.

Maintenance

• Clear leaves and debris from above and round the skylight each autumn
• Look at the flashing and the seal when the roof is inspected
• Check the shaft drywall for new stains after heavy rain
• Clean the weatherstripping on opening units and make sure they close fully
• Plan to replace skylights when the roof is replaced

More: water damage restoration.