Is the skylight leaking, or is the flashing?
This is the whole question, and it decides whether you are looking at a repair or a replacement.
A skylight is a hole deliberately cut in a roof. What keeps water out is not really the glass. It is the step flashing, the head flashing above it, and the curb the unit sits on. All of that is metal and sealant, and metal and sealant age faster than glass does.
So when we get a skylight call, we are looking at the edges before we look at the unit. If the flashing has lifted, corroded, or was laid in the wrong order when the roof went on, water gets in around a skylight that is working perfectly well.
Replacing a skylight because the metal around it failed is an expensive way to fix a cheaper problem. It also does not always work, because the new unit goes back into the same bad detail.
Is it a leak at all?
A fair number of the skylight calls we take turn out to be condensation.
Warm moist indoor air rises, hits cold glass, and gives up its water. Over a bathroom or a kitchen it can run down the frame and drip, and from the floor it is indistinguishable from a leak.
The tell is timing. Condensation shows up on cold mornings, often with no rain for days. A real leak shows up during or just after rain, and usually with a stain that grows. If yours only appears in January, the answer may be ventilation rather than roofing, which is the same territory as attic ventilation.
What actually fails around a skylight
- Step flashing. The pieces that lace between courses of shingle up each side. When these are face-nailed or sealed instead of layered, they fail early.
- Head flashing. The piece across the top, which takes everything running down the slope above it. This is the one that matters most and the one most often short.
- The curb. The frame the unit sits on. Once water has been getting in, the wood here goes soft and nothing fastened to it will seal.
- The seal on the unit. On older double-glazed skylights the seal itself fails, which shows as fog between the panes rather than water in the room.
- Debris above it. Leaves and needles pile on the uphill side, hold water against the head flashing, and keep that spot wet long after the rest of the roof has dried.
Replacing a skylight with the roof
If we are replacing the roof and the skylight is anywhere near the end of its life, it should go at the same time.
The reason is simple. The flashing has to come off and be rebuilt either way, so the labor is already being spent.
Leaving an aging unit in the middle of a new roof means opening that roof again in a few years to do a job that could have been done once. We raise it during the roof replacement walkthrough rather than after.
Adding a skylight
New skylights are worth it in the right room and disappointing in the wrong one. What decides it is usually not the roof, it is what sits between the roof deck and the ceiling.
Where the ceiling follows the roofline, it is a straightforward job. Where there is an attic in between, the light has to come down a shaft, and the length and angle of that shaft decides how much light actually arrives. That is worth talking through before anything gets cut.
After a storm
Hail and wind find skylights first. The glass is tougher than people assume, but the flashing lifts and the seals take impact, and a limb that glances off a skylight can crack a curb without breaking anything you can see from the ground.
If a tree came down across the roof, we take the tree off, tarp what is open, and document the skylight with everything else on the roof inspection. Doing the tree and the roof with one crew is the reason storm work here does not sit waiting on a second company.