Skylight problems

Water at a skylight has three common explanations. Flashing failure: the metal pan, step flashing or head flashing was installed wrong, was reused when it shouldn't have been, or has corroded. Seal failure in the unit: the glazing seal has gone, which typically shows as fogging or streaks trapped between the panes. Condensation: warm humid indoor air hitting cold glass, which drips and looks exactly like a leak to a worried homeowner.

The condensation test is simple and free. Condensation appears on the inside face of the glass and the frame, is worst on cold mornings and in humid rooms like bathrooms and kitchens, and gets better when you run an exhaust fan. A real leak tracks with rain, not with temperature, and often shows as a stain on the drywall shaft or ceiling away from the glass.

VELUX lists yellow staining around a skylight, cracks of any size, dripping at the sides, and persistent between-pane condensation as signs a unit needs professional attention, and suggests an inspection around the 30-year mark. Any one of those on a unit that old usually points at replacement rather than repair.

The dangerous version of this problem is the leak you can't see. Water that gets past flashing runs down the rafter and soaks sheathing and insulation before it ever reaches your ceiling. If you have a stain, assume the wet area is larger than the stain and have someone qualified look in the attic — from inside, not from the roof.

How to choose

Diagnose before you buy anything

Rain-driven stain, or cold-morning drip? The answer decides whether you need a roofer or a bath fan.

Ask for photos of the flashing

Any contractor who has been on your roof can show you what they found. "It's shot" without a picture is not a diagnosis.

Treat fogging between panes as a unit problem

Moisture inside the glazing means the seal has failed. Sealant on the outside will not fix it.

Check the shaft, not just the ceiling

Stains often show at the bottom corners of the light shaft first. Look there with a flashlight before the drywall bubbles.

Don't let anyone caulk their way out

Sealant smeared over old flashing is the classic bad repair. It looks fixed, holds one season, and hides rot in the meantime.