How to Repair a Skylight Leak: 4 Steps to a Dry Ceiling
The fastest way to repair skylight leak problems is to find the true source first and fix only that. Reseal the frame if the sealant has failed. Replace or re-lap the flashing if water is getting under it. Replace the whole unit if the glass is fogged or cracked, or if the curb is rotted. Most leaks start where the skylight meets the roof, not in the glass.
The hard part is that skylight leaks mislead you. Water runs along rafters and drywall and shows up as a stain several feet from the real entry point. Condensation can look like a leak. So many homeowners smear caulk around the whole frame, and the next storm proves it did nothing. In Puget Sound, where rain can last for months, that mistake gets expensive fast.
This guide walks you through four steps in the order we use them at Legacy Exteriors. You will trace the leak, reseal the frame, repair the flashing, and decide whether the skylight is worth saving. We install and repair skylights across the Kirkland, Bellevue, and Seattle areas, so the advice below reflects what actually fails on Pacific Northwest roofs.
Why skylights leak and what to prepare
A skylight leaks when water finds a path past its seals or flashing, and the cause is usually worn sealant, damaged flashing, or debris that makes water pool. Before you touch anything, you need to know the likely causes, rule out condensation, and gather the right gear.
The usual suspects
Skylights are designed to shed water, so a leak means one layer of that system has failed. These are the common failure points, roughly in order of how often we find them:
Failed frame sealant or gasket. Sun and temperature swings dry out sealant until it cracks and pulls away.
Lifted, bent, or corroded flashing. The metal around the unit is what actually directs water down the roof.
Debris dams. Fir needles, leaves, and moss pile up behind the upslope side and hold water against the flashing.
Cracked glazing. Hail, a falling branch, or age can crack the glass or acrylic dome.
Installation flaws. Poor overlap, missing flashing pieces, or exposed nails fail years later.
Most skylight leaks start where the unit meets the roof, not in the glass.
Rule out condensation first
Condensation is the most common false alarm. If drips appear on cold mornings, the glass is wet only on the inside, and the ceiling stays dry during rain, you have a humidity problem, not a leak. Running a bathroom fan, venting a kitchen, or using a dehumidifier usually solves it.
A true leak follows the weather. It shows up during or after rain, often a heavy one, and leaves rings or stains on drywall or the light well. Moisture trapped between two panes of glass is a separate problem, and Step 4 covers it.
Safety and supplies
Roof work comes before any repair. Work only on a dry day, use a ladder that extends well above the eave, and wear a harness tied to a roof anchor on anything steeper than a gentle pitch. Never step or sit on the skylight. It will not hold you. If your roof is steep, two stories up, or slick with moss, hire a pro for the roof portion.
Gather these before you climb:
Ladder, harness, and roof anchor
Garden hose and a flashlight
Plastic putty knife and utility knife
Isopropyl alcohol and clean rags
Exterior sealant rated for roofing and compatible with your skylight material (check the label)
Caulk gun, flat pry bar, and roofing nails
A flashing kit made for your skylight model, if the flashing is damaged
Step 1. Find the real source of the leak
To repair a skylight leak that stays fixed, trace the water to its entry point before you apply anything. Start inside, then confirm outside with a hose test. A stain on the ceiling only tells you where water came out, not where it got in.
Trace the water from inside
Look at the ceiling, the light well, and the frame. Check the attic side if you can reach it. Use a flashlight to find wet insulation, dark rafters, or rusted nail tips around the shaft. Water flows downhill along framing, so the entry point sits higher than the stain.
Mark the stain edges with a pencil and date them. If the stain grows after the next rain, you have confirmed an active leak. Put a bucket under it and a tarp on the floor while you work.
Run a hose test
A hose test is the only reliable way to separate flashing leaks from seal leaks. You need a helper inside watching with a flashlight. Do it on a dry day.
Start at the lowest point of the skylight and soak that area for about 10 minutes.
Move up the sides, soaking each section separately for 10 minutes.
Finish at the top, above the skylight, last.
Ask your helper to call out the moment any water appears.
Note the exact spot you were wetting when it did.
Always work from low to high. If you start at the top, water runs down and floods every section at once, and you learn nothing.
Wet the roof one section at a time, from the bottom up, or the test proves nothing.
Don't blast water uphill under the shingles with a nozzle. That forces water where rain never would and creates false results. Use a gentle shower pattern.
Match the symptom to the fix
Once you know where the water enters, this table tells you which step comes next.
What you see | Likely cause | Go to |
|---|---|---|
Drip along the bottom of the frame | Failed frame sealant or gasket | Step 2 |
Stain at the top corners, worse in heavy rain | Debris dam or failed head flashing | Clear debris, then Step 3 |
Stain along the sides after steady rain | Lifted or corroded step flashing | Step 3 |
Water at a visible crack in glass or dome | Cracked glazing | Step 4 |
Moisture or haze between the panes | Failed insulated glass seal | Step 4 |
Wet glass on cold mornings, dry ceiling in rain | Condensation | Ventilate, no repair |
Step 2. Clean and reseal the skylight
If the hose test points to the frame, you can fix a leaking skylight with fresh sealant in about an hour. The job works only on a clean, dry surface where the old failed sealant has been removed. Slapping new caulk over old is the most common reason this repair fails.
Prep the surface
Clear all needles, moss, and grit from the frame and the roof around it. Then use a plastic putty knife to scrape out cracked or peeling sealant. Plastic protects the glass and finish from scratches. Cut stubborn strips with a utility knife, keeping the blade away from the glazing.
Wipe every joint with isopropyl alcohol and a clean rag, then let it dry fully. Sealant won't bond to dust, oil, or moisture. If rain is in the forecast, wait.
Apply the new sealant
Choose an exterior sealant rated for roofing and compatible with your frame material, whether that is aluminum, wood, vinyl, or acrylic. Silicone stays flexible, and polyurethane bonds firmly. The label should say which surfaces it suits.
Cut the nozzle to a small opening so you control the bead.
Run a continuous bead into the joint between the glazing and the frame.
Seal the joints where the frame meets the curb or flashing.
Tool the bead with a gloved finger or a plastic tool to press it into the joint.
Leave the lowest edge and any weep slots open so trapped water can drain.
Seal the joints, not the whole skylight, and leave the drainage paths open.
Cure and recheck
Follow the cure time on the tube, then repeat the hose test on the same section. If the leak is gone, wait for the next real rain before calling it fixed.
Skip the trick of coating the glass and frame in heavy sealant and fabric mesh. It might hold for a season on a commercial dome, but on a home it hides the real problem and makes later repairs harder. If a storm is coming and you need an emergency patch, waterproof tape will do, but treat it as temporary and schedule the real repair.
Step 3. Repair or replace the flashing
When water gets past a sound seal, the flashing is almost always at fault, and fixing it is the proper way to fix a skylight leak that sealant can't stop. Flashing is the metal collar that sheds water around the unit. Sealant slows water down, but flashing is what moves it off the roof.
Inspect the flashing
On a standard shingled roof, a deck-mounted skylight has three zones. The sill flashing sits at the bottom edge, step flashing runs up each side in overlapping pieces, and head flashing caps the top. A curb-mounted unit has a raised frame wrapped with base flashing and counterflashing.
Look for bent or lifted pieces, rust, gaps between the metal and the frame, and exposed nail heads. Check that the shingles still overlap the step flashing and the head flashing tucks under the upslope shingles. Pressing a lifted section with a gloved hand tells you if the fasteners have let go.
Replace the damaged pieces
Use a flashing kit made for your skylight model. Generic metal rarely fits the frame correctly. Work in mild weather, because cold asphalt shingles crack when you bend them.
Gently lift the shingles around the damaged area with a flat pry bar. Don't tear them.
Remove the bad flashing pieces and the old fasteners.
Clean the deck and check for soft or dark wood.
Install new pieces from the bottom up so each upper piece overlaps the one below.
Lay shingles back over the side and head flashing, and under the sill.
Seal any exposed nail heads with a dab of roofing sealant.
Sealant slows water down. Flashing is what actually sends it off the roof.
Soft or spongy decking under the flashing means rot, and rot calls for a carpenter, not more sealant.
When flashing work is beyond DIY
Call a licensed roofer if the roof is steep, if the covering is tile or metal, or if the leak persists after you have replaced the flashing. A roofer with skylight experience can also rebuild a rotted curb and re-integrate the flashing into the roofing correctly. If you are asking who to call about a leaking skylight, that is the person to look for, not a general handyman.
Step 4. Decide when repair is not enough
Repair makes sense when the damage is limited to sealant or flashing and the unit is otherwise sound. If the skylight itself has failed, any repair skylight leak job is only a delay. Look for the signs below before you spend more time or money on an old unit.
Signs the skylight needs replacing
Sign | Verdict |
|---|---|
Fog or moisture between the panes | Replace. The sealed glass unit has failed and cannot be repaired. |
Cracked glass or crazed, yellowed acrylic | Replace the glazing or the unit. |
Rotted or soft curb, frame, or decking | Replace, and repair the structure underneath. |
Leak returns after a proper reseal and reflash | Replace. The problem is deeper than the surface. |
Unit is well past roughly 15 to 20 years old | Likely replace, especially if the roof is also aging. |
Sealant or flashing damage only, unit sound | Repair. |
If the roof itself is near the end of its life, replace the skylight at the same time. It costs less than flashing a new roof around an old unit, and you will not have to pull the new shingles up later.
Cost, insurance, and interior damage
Compare the repair estimate with the price of a new unit installed. If the repair eats up a large share of the replacement cost, and the skylight is old, replace it. Homeowners insurance generally covers sudden damage, such as a storm or fallen limb, and often excludes wear and tear. Check your policy and photograph everything before you start.
Finally, deal with what the leak did inside. Dry wet insulation and drywall within 24 to 48 hours, since that is when mold tends to take hold. Cut out and replace drywall that stays soft or keeps staining.
Keep the water out for good
Fixing a leaking skylight comes down to a clear order of operations. Rule out condensation, trace the water with a hose test, reseal failed frame joints, repair flashing that no longer sheds water, and replace the unit when it is fogged, cracked, rotted, or simply worn out. Skip a step and you repair the wrong thing.
If the hose test leaves you unsure, or the roof is too steep or too high to work on safely, let a pro handle it. You can book a free roof inspection with Legacy Exteriors and we will find the actual source of your leak. Our locked-in price quote means you know the cost before work starts.
