Skylight flashing: what it is and how to spot a failing one
The glass rarely leaks. The metal around it does. Here is how skylight flashing works and how to check it without going on the roof.
Skylight flashing is the set of metal pieces that wraps the frame of a roof window and sends water around it. The glass gets the blame for most skylight leaks. In practice the flashing is the more common cause.
How it is built
A skylight is a hole in the roof surrounded by a raised curb or frame. Water has to be steered around all four sides. Most kits use four pieces that overlap in a set order.
- A sill piece at the bottom, which lies over the shingles below.
- Side pieces that work like step flashing along each edge.
- A head piece at the top, tucked under the shingles above.
- A frame cap or cover that finishes the joint with the skylight itself.
The top piece goes under, the bottom piece goes over, and the sides weave in. Water on the slope above is pushed to the sides and then down past the window.
Why the corners are the weak point
Each corner is where two pieces and the frame meet. A small gap there is enough to let water in, and the gap is hard to see. Corners also get most of the roof's movement as it warms and cools. Sealant that was added at install is usually the first thing to crack.
How skylight flashing fails
- Sealant that has dried, split or pulled away from the frame.
- Side pieces that were bent flat instead of woven into the shingles.
- A head piece that sits over the shingles above and catches water behind it.
- Rust or white powdery corrosion on older metal.
- Debris piled against the upper edge, which holds water against the joint.
Leak or condensation?
Many "leaks" around a skylight are condensation on cold glass in winter. A real flashing leak usually shows up during or after rain, often in storms with wind. Drips from the glass in cold, dry weather point to condensation. We cover that in skylight condensation and in skylight leak.
What to check from the ground and the attic
From the yard, use binoculars. Look at the metal on all four sides for lifted edges, rust and cracked or smeared sealant. Check whether leaves and grit collect above the window.
Inside, look at the light shaft and the ceiling around it for stains. If you can see it from the attic hatch, shine a flashlight at the wood around the shaft. Dark staining, damp insulation or rust on nail tips are signs of a slow leak. Stay on the ladder or at the hatch and do not step between the joists.
Why we do not show the repair
Fixing skylight flashing means working on the slope right beside an opening in the roof. Shingles come up, metal comes out and the glass may need to be lifted. That is a fall risk. We leave the steps out on purpose.
What to ask a roofer
Ask whether the job is a reseal or new flashing. Ask whether the skylight itself will be taken out and reset. A proper flashing replacement often needs that, because the side metal runs under the frame. Ask for photos of the old metal and of the finished work from the ground.
A roofer who proposes only a bead of caulk around the frame is offering a short-term patch. It is worth a second opinion.
Frequently asked questions
What is skylight flashing?
It is the set of metal pieces, usually a sill, two sides and a head, that wraps the skylight frame and steers rain around it and over the shingles.
Why does my skylight leak when the glass is fine?
Most leaks come from the flashing, especially at the corners where pieces meet the frame. Dried or cracked sealant and badly overlapped metal are the usual causes.
How can I tell if it is a leak or condensation?
A flashing leak shows up in or after rain, often with wind. Drips on cold, dry days point to condensation on the glass.
Can I seal around a skylight myself?
We recommend against it. It means working on a sloped roof beside an opening. Caulk alone also hides the real problem. Have a licensed roofer inspect and repair it.
Does fixing skylight flashing mean removing the skylight?
Often. The side metal runs under the frame, so a full replacement usually means lifting the unit and resetting it. A roofer should explain which is needed.
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.
- NRCA. National Roofing Contractors Association
- US OSHA. 1926.501 — Duty to have fall protection (OSHA)