Skylight Condensation & Leaks: Diagnose It, Then Fix It

Here’s the trade’s open secret: most “leaking” skylights aren’t leaking. Water on the glass or windowsill in winter is usually condensation — indoor moisture meeting cold glazing — and the fixes are ventilation and spec, not silicone. But some skylights really do leak, and the two problems get expensively confused in both directions. Sixty seconds of diagnosis first.

The diagnosis: match the water to the weather

It’s condensation if… the water appears on cold mornings after clear nights; it forms a film or droplets across the glass, pooling along the bottom edge; it’s worse in kitchens, bathrooms and bedrooms (breathing is humidity); and it happens with no rain in sight.

It’s a leak if… water arrives during or shortly after rain; it’s localised — one corner, one edge, a stain track down the reveal; it survives summer; or it appears after roof work, storms or heavy leaf-fall.

It’s a failed glazing unit if… the misting is between the panes where you can’t wipe it. That’s neither ventilation nor flashing — the sealed unit is done, and a replacement pane (not a whole window, if the frame’s sound) is the fix.

Fixing condensation

Condensation is a moisture-vs-glass-temperature equation; you attack both sides:

1. Ventilate the moisture out. Use the window’s trickle-vent position routinely (on centre-pivots, the first click of the control bar ventilates with the window locked); run extractor fans during and after cooking and showers; air bedrooms in the morning. Habit fixes are free and do most of the work.
2. Warm the glass. Old double glazing runs cold; upgraded low-energy or triple glazing keeps the inner pane warmer and condensation-free at the same humidity — the same upgrade that fixes rain noise. Background heat in cold snaps beats boom-and-bust heating for glass temperature too.
3. Check the cold bridges. If the reveal (the plastered surround) is where damp and mould appear, the insulation around the frame is thin or missing — a known weak point on older installs, and fixable from inside when redecorating.
4. Manage the interim. While causes get fixed, a dehumidifier keeps mornings dry and protects paintwork — treat it as the sticking plaster, not the cure.

Persistent condensation in a bathroom or kitchen under an old window is also the classic cue that the window itself is due — moisture-proof polyurethane-framed replacements exist precisely for those rooms.

Fixing an actual leak

Work down this list — it’s ordered by likelihood and cost:

1. Blocked drainage. Roof windows have drainage channels around the frame; leaves, moss and roof grit block them and the overflow shows up indoors. Clearing them is a ladder job, free, and fixes more “leaks” than any other single cause.
2. Perished gaskets. Rubber seals age; gasket kits exist for most models and swap without removing the window.
3. Flashing. The metal skirt tying window to roof covering is the most common real culprit — wrong kit for the roof material, poor detailing, or storm damage. Re-flashing is an installer job; the vetting advice here applies.
4. The surrounding roof. Cracked tiles, failed membrane or (on flat-roof units) a tired upstand tie-in can deliver water to the window that isn’t from the window. A good roofer diagnoses before quoting.
5. The window itself. Distorted frames and failed units on windows at the end of their 20–30 years — at which point replacement into the existing opening (£500–£800, per the cost guide) beats chasing repairs.

One warning against the tempting fix: don’t gun silicone around a “leaking” roof window. Sealed drainage channels can’t drain — mastic over the weep paths converts a minor leak into a trapped-water problem that rots the frame. If the fix isn’t obvious from the list above, pay for an hour of a roof-window specialist’s diagnosis before anyone opens a tube.