Our skylight is leaking (and fogging) — can it be repaired or does it need replacing?
We have a skylight/rooflight that's started leaking when it rains and sometimes fogs up inside.
- Can a leaking skylight be repaired (reseal/flashing) or does the whole thing need replacing?
- What usually causes skylight leaks here, and how do I stop water tracking in?
- Is the fogging a failed sealed unit?
Trying to fix it properly before it damages the ceiling. What's involved?
3 answers
Short answer: most skylight LEAKS are repairable and are NOT usually the glass failing — they're a FLASHING or SEAL problem where the skylight meets the roof. The commonest causes are failed/cracked perimeter sealant, degraded or poorly-installed flashing (the metal/membrane that weatherproofs the join to the roof), blocked drainage, or perished gaskets — all fixable by re-sealing, re-flashing and clearing the drainage. The FOGGING is a separate issue: condensation INSIDE the glass means the sealed double-glazed unit has failed and that glass UNIT is replaced (the frame/skylight can stay). So: leak = usually reseal/re-flash (repair); internal fogging = replace the glass unit; only a badly corroded/broken skylight or a wrong original install needs the whole unit replaced.
Why skylights leak (usually a seal/flashing job, not the glass): - Failed perimeter sealant — the silicone/mastic around the skylight perishes in the UV and lets water in. Re-seal. - Flashing problems — the flashing (the weatherproofing where the skylight meets the roof) is degraded, lifted, or was poorly installed. This is the number-one cause; re-flash it properly. - Blocked drainage/weep paths so water pools and backs up. Clear them. - Perished gaskets on the frame. - (True leaks THROUGH the glass are rare; it's almost always the junction to the roof.)
Fixing the leak: - Reseal + re-flash + clear drainage — trace where the water actually enters (not always where it drips inside), and redo the sealant/flashing/gaskets properly. This fixes most skylight leaks without replacement.
The fogging (different problem): - Condensation between the panes = a failed sealed double-glazed unit — the glass unit's seal has gone, so it's swapped (the skylight frame stays). Not a leak, and not cleanable.
Repair or replace the whole thing? - Repair (reseal/re-flash): the usual answer for leaks. - Replace the glass unit: for internal fogging. - Replace the whole skylight: only if the frame's corroded/broken or it was badly installed originally (wrong upstand/flashing) and can't be made good.
Common mistake: smearing more silicone over the inside where it drips (the water enters elsewhere on the roof) instead of tracing and re-flashing the actual entry point — so it keeps leaking and rots the ceiling. Get a glass repair specialist to find the real entry, re-flash/reseal, and swap the unit if it's fogged. Anyone traced a skylight leak to the flashing rather than the glass?
Ours leaked and we assumed the glass had failed — turned out to be the flashing/sealant at the roof junction, and once that was redone properly it was dry. The water was actually getting in a foot away from where it dripped inside, which is why "just silicone the inside" never worked. The fogging on a different pane WAS a failed unit and that got swapped separately. So don't assume leak = new skylight; it's usually reseal/re-flash. Trace the real entry point on the roof. We nearly paid for a whole new skylight we didn't need.
The "water enters somewhere else and tracks to where it drips" point is so true — chasing the drip inside is why people fail to fix skylight leaks. It's nearly always the flashing/seal at the roof, and re-flashing sorts it. Fogging between the panes is the sealed unit gone, replace just the glass. Both cheaper than a new skylight, which you only need if the frame's actually shot. Get someone up on the roof to find the real entry rather than sealing the ceiling side. Fix it before it stains and rots the ceiling — that's the expensive bit if you leave it.
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