Is my skylight leaking, or is it condensation?
Clues that point to condensation
- Water appears on cold mornings or when the house is humid, with no rain involved.
- Beads cover the inside surface of the glass, sometimes fogging it completely.
- Drips fall from the lowest edge of the glass or collect in the small condensation channel some skylights have.
- It gets worse after showers, cooking, or a house full of people.
Clues that point to a leak
- Water appears only with rain, especially wind-driven rain from one direction.
- Stains start at the top or a corner of the skylight well, or at the drywall seam where the well meets the ceiling.
- In the attic, the framing around the skylight shaft is stained or soft.
Fog or moisture trapped between the panes is a third problem: the glass unit's seal has failed. That is not a roof leak, but it means the skylight is aging.
A simple test
On a dry day, run a garden hose gently over the roof above the skylight, starting below it and moving up in stages, while someone watches inside. Water that appears only when the hose reaches the upper flashing points to the head flashing or the shingles above. Keep the flow gentle; blasting water up under shingles can create a leak that does not happen in real rain.
Fixing each one
Condensation is a humidity problem: run bath and kitchen fans, keep air moving near the skylight, and consider a newer unit with better insulated glass. A leak is usually flashing, and on an older skylight it often makes sense to replace the unit and its flashing kit together, ideally along with the roof. See skylights and roof replacement.