How to Fix a Leaking Skylight: Condensation, Seals, Flashing and Debris
First work out whether it is a leak at all. Then find where the water gets in, starting with the simplest causes.
- Advanced
- 1 to 4 hours
- Updated
The short answer
Before fixing a skylight leak, check whether it is condensation: drips on cold mornings, water running down the glass or shaft and no link to rain. For a real leak, check debris at the head, failed seals, cracked or fogged glass, then flashing faults such as missing laps, exposed nails or dried sealant. Clean and reseal minor faults. Replace the skylight and flashing if the unit or roof is old.
Skylights get blamed for a lot of leaks, and a large share of those are not leaks at all. In a cold, wet climate like the Fraser Valley, warm moist air from inside a house condenses on cold glass, frames and light shafts and drips down. It looks just like a leak, and it gets worse on cold mornings with no rain at all. Pulling the flashing apart to fix condensation wastes a day and does nothing.
When it is a real leak, the cause is usually one of a few things: debris damming water at the head of the skylight, a failed seal or cracked glass, or a flashing fault such as a missed lap, an exposed nail or sealant that has dried and cracked. This guide shows how to sort through these in order, cheapest and easiest first.
For how the flashing is meant to be built, see how to flash a skylight. For leaks elsewhere on the roof, see how to trace a leak from the attic.
Tools
- Flashlight
- Garden hose
- Caulking gun
- Hook blade utility knife
- Stiff brush
- Putty knife
- Tin snips
- Roofing hammer
Materials
- Sealant compatible with the skylight and flashing materials
- Self-adhered membrane patch or strip
- Replacement shingles and roofing nails
- Replacement flashing pieces or kit
- Desiccant or insulation for the light shaft, where needed
Safety first
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury. Wet roofs, moss and skylight glass are slippery.
- Never step on skylight glass or domes, even if they look solid.
- Do not chase a leak on the roof in the rain. Find it from the attic first, then test on a dry day with a hose.
Step by Step
Step 1: Decide whether it is condensation or a leak
Condensation forms on the coldest surface it can find, so it beads on the glass, runs down the frame and drips from the bottom corners. It often happens with no rain. It is worst in bathrooms, kitchens and rooms with lots of moisture, and in winter.
A true leak shows up during or after rain, often at the corners or along one side, and stains the drywall or shaft with brown rings. Wipe the glass and frame dry in the morning. If they are wet again within an hour on a dry day, it is condensation.
Tip: Look inside the attic with a flashlight during a rain. A wet roof deck and trimmer beside the skylight confirm a leak. A dry deck and a wet shaft interior suggest condensation.
Step 2: Reduce condensation if that is the problem
Check the light shaft. A well-insulated, air-sealed shaft has a warm surface and stays dry. An uninsulated shaft is cold and sweats. Add insulation and seal gaps at the ceiling and the skylight frame.
Use bathroom and kitchen fans that vent outside, see how to vent a bathroom fan through the roof. Venting units can be opened for a while to clear humidity. Double-glazed low-e glass helps keep the inner pane warm.
Step 3: Check for debris dams at the head and sides
On the uphill side of a skylight, debris collects and holds water against the flashing. Under Fraser Valley cedars and firs, needles pack the head and sides quickly. Clear them by hand or with a soft brush, never a pressure washer.
Look for moss lifting the shingle edges or the flashing. If this is a repeating problem, a small cricket above wide units helps split the flow. See how to flash a skylight.
Step 4: Inspect the seals and glass
Fog or moisture between the panes means the insulated glass seal has failed. That is not a roof leak but it is a sign the unit is near the end of its life. Cracked glass or a cracked plastic dome needs replacing.
On venting units, check the gasket around the opening sash. A hardened, flattened or torn gasket lets wind-driven rain in. Replacement gaskets are often available by model. Dried sealant on the frame can be cleaned off and redone with a compatible sealant.
Step 5: Look for flashing faults
Look at the bottom corners first, where most leaks start. Check that the sill piece laps over the shingles below, the sides are woven with the shingles and the head piece tucks under the shingles above. A piece that is reversed or missing is the fault.
Look for nail heads in the open, rust holes, cracked sealant and shingles pressing hard on the frame. Sealant that has been applied over old sealant in layers is a sign of repeated patching. See roofing sealants for what sealant can and cannot do.
Step 6: Water test to confirm the source
Start at the bottom of the skylight and wet one side, then the other, then the head, each for several minutes. Wait a few minutes between sections. The section where water appears inside is the source.
Do not spray upward under the shingles, which gives false results. Hose downward, the way rain falls.
Step 7: Make a repair that matches the fault
Small fixes: clear debris, replace a cracked shingle, cover an exposed nail with a dab of compatible sealant and patch a small gap with a membrane strip lapped correctly. Clean old sealant out before adding new.
If pieces are missing, reversed or corroded, the correct fix is new flashing, woven in properly. Caulking over a flashing fault buys a season at most.
Watch out: Do not seal the bottom of a skylight frame shut with caulking. Water that gets in has to be able to drain out.
Step 8: Decide whether to replace the skylight
A skylight that has been repaired several times, has failed seals and sits in a roof that is near the end of its life is a candidate for replacement, done properly as described in how to replace a skylight during a reroof.
For a new installation, see how to install a deck-mounted skylight.
Condensation Versus Leak: Quick Tests
- Timing: condensation peaks on cold mornings. Leaks appear during or after rain.
- Location: condensation drips from the bottom of the glass and frame. Leaks often show at corners or on one side.
- Attic check: a dry roof deck around the skylight points away from a leak.
- Stains: a leak leaves brown rings on drywall. Condensation leaves clean water, and black spotting if it continues.
Fraser Valley Notes
Long wet winters mean skylights spend months cold and damp, and needles and moss collect above them. A yearly look at the head and sides, and keeping indoor moisture down, prevents many call-outs. A damp attic can add to the problem, see how attic ventilation works.
Common Mistakes
- Treating condensation as a leak. The flashing gets torn apart and the drips continue.
- Caulking over the problem. Layers of sealant hide the fault and fail again.
- Sealing the bottom of the frame. Trapped water has no way out.
- Ignoring debris at the head. It dams water and feeds the same leak every winter.
- Hosing upward under the shingles. It creates false leaks and confuses the diagnosis.
- Repairing a failing skylight on a failing roof. The money is better spent replacing both together.
When to Call a Professional
Call a roofer if the leak continues after cleaning and sealing, the flashing is missing or corroded, the skylight is on a steep or high roof, or the framing around it is stained or soft. A leak that has been running for a while can have rotted the deck and framing.
Also call one if the glass is cracked or fogged. The unit should be replaced, and replacement and flashing are best done together.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Skylights service or the towns we cover.
How to Fix a Leaking Skylight: Questions
Is my skylight leaking or is it condensation?
Condensation appears on cold mornings, beads on the glass and frame and drips from the bottom, often with no rain. A leak shows up during or after rain, often at a corner or one side, and stains drywall brown. A dry roof deck around the skylight in the attic, during rain, points to condensation.
Why does my skylight leak when it rains?
Common causes are debris damming water at the head, failed seals or cracked glass, and flashing faults such as missing laps, exposed nails or cracked sealant. Wind-driven rain can also exploit worn gaskets on venting units. A hose test, started at the bottom of the skylight, shows which area is letting water in.
Can I fix a leaking skylight with caulk?
Only as a short-term measure, and only in the right place. Caulk can seal a small gap or cover a nail head, but it cracks and fails after a few seasons. A flashing that is missing, reversed or corroded needs to be replaced with lapped metal. Caulking the bottom of a frame can trap water.
How do I stop skylight condensation?
Reduce indoor moisture with exhaust fans that vent outside, and keep the glass and shaft warm. Insulate and air seal the light shaft so warm air cannot reach cold surfaces, and choose double-glazed low-e glass when replacing. Opening a venting skylight for a while also clears humid air.
When should a leaking skylight be replaced?
Replace it when the glass is fogged between the panes or cracked, the frame is corroded, the flashing has been patched repeatedly or the roof is near the end of its life. Doing it with a reroof is best, since the flashing is then built into the new roof from the start rather than patched into the old one.
Related Guides
All Skylights guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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