The three places it can be coming in
The Department of Energy's building science guidance names three ways an existing skylight fails, and the Building America Solution Center adds the joint between the skylight and the roof:
- Flashing at the roof. On a shingle roof that's the sill flashing along the bottom, step flashing woven into each course of shingles up the sides, and head flashing across the top, all lapped so water runs out and down the roof. A lap that's short or bent can stay dry in a drizzle and let water in when wind drives rain up under it. Why so many roof leaks start at the flashing covers how those laps fail.
- Frame joints. Water gets through where the frame pieces meet.
- Gaskets on a skylight that opens. Weather stripping between the sash and the frame can be missing, torn or worn flat. Make sure the unit was fully closed when it leaked.
- The glass itself. Fog trapped between the two panes means the insulated glass seal has failed. That's a reason to replace the unit, but it isn't usually the cause of a drip into the room.
Rule out condensation first
Droplets on the room side of the glass that show up on cold days, after showers or while cooking, with no rain, are condensation, not a leak. Warm, humid air hits the cold glass and the water runs down to the frame, where it can look just like a leak. EPA recommends keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. A cheap humidity meter will tell you where you are. Why condensation forms on the inside of a window explains the same effect on regular windows.
If the wet spot is on the drywall shaft between the ceiling and the skylight rather than at the frame, ask whether the shaft walls are insulated and air sealed. The Building America Solution Center says shafts that pass through an uninsulated attic should be insulated to the same level as exterior walls. Cold shaft walls collect condensation that exterior caulk won't fix.
Keep a record a roofer can use
- For each leak: the date, how hard it rained, the wind direction if you know it, and how long after the rain started the dripping began and stopped.
- One wide photo showing where the skylight is in the room, then close-ups of the first wet spot: the glass, a frame corner or the shaft wall.
- The skylight's make and model if a label is visible, the year the roof was last replaced, and any earlier sealant patches. Tell the roofer about those patches even if they stopped the drip for a while.
Take photos from the floor or a steady ladder indoors. Don't go on the roof, lift shingles or pry at flashing.
Don't recreate the storm with a hose
The Building America guidance warns that too much volume or pressure, or spraying upward at joints that are normally sheltered, can cause leaks that don't happen in normal weather and can soak interior finishes. Leave it to the roofer.
What to ask the roofer
- Which joint failed, and can you show me a photo of it?
- Is this a flashing repair, a problem with the skylight unit, or both? Can you put those as separate lines in the written proposal?
- Do you service this make of skylight, or do I need a skylight specialist for the frame, gasket or glass?
- Will the repair rebuild the flashing laps, or only add sealant? A bead of caulk over a lap that doesn't drain usually fails again.
- How will you check the repair afterward?
Philadelphia permits for skylight work
If the fix turns into a new roof covering, Philadelphia requires an L&I permit, and many re-roofing jobs can use the no-plans EZ permit. The City's EZ Re-Roofing standard, revised July 2026, excludes skylight installations and other new roof penetrations, and changes to the framing that supports a skylight. So a plan to add, move or enlarge a skylight, or rebuild what holds it up, needs a regular building permit. It also says deteriorated flashing can't be reused and that flashing has to be rebuilt to the manufacturer's instructions. Ask the roofer which permit the job needs and who is pulling it. When a Philadelphia rowhome roof replacement needs a permit covers fees and the historic-district step.
While you wait for the repair
Put a bucket and towels under the drip and dry anything wet as quickly as you can. EPA says to dry wet materials within 24 to 48 hours and not to paint over or seal materials that are still wet. Stay out from under a sagging ceiling, keep water away from light fixtures and outlets, and call an electrician if water reaches them.