Skylight Leaks: Curb-Mounted vs Deck-Mounted
When a skylight leaks, the skylight is usually not the problem. The flashing around it is, or the glazing seal has gone, or what looks like a leak is condensation. Those are three different problems with three different answers, and telling them apart saves a lot of unnecessary work.
The mounting type matters too, because it changes both how the unit is flashed and what your options are when something fails.
The two mounting types
Curb-mounted
A raised frame, the curb, is built on the roof deck, and the skylight sits on top of it. The unit is lifted a few inches above the roof surface.
Advantages: the glazing sits above the water plane, so runoff has to climb the curb before it can reach the unit. Flashing is straightforward because you are flashing a box rather than a shallow frame. Better suited to low-slope roofs, where water moves slowly. And when the unit eventually fails, a replacement can often be dropped onto the existing curb without touching the roof covering.
Trade-offs: it stands proud of the roof, which some find visually intrusive. The curb itself is a timber structure that can rot if the flashing above it fails.
Deck-mounted
The unit fastens directly to the roof deck with an integrated flashing kit, sitting much closer to the roof plane.
Advantages: a lower, cleaner profile. Modern units come with manufacturer-engineered flashing kits designed for specific roof coverings, and when those kits are used correctly they perform very well.
Trade-offs: the glazing sits closer to the water, so the flashing detail carries more of the burden. Replacement is a bigger job because the surrounding roof covering has to come up. Not the right choice on genuinely low slopes.
Why most skylight leaks are flashing leaks
A skylight is a hole in the roof with a window in it, and the flashing is what makes that survivable. The pattern of failure is consistent:
- Head flashing at the top. The most critical piece, because it carries everything coming down the roof above the unit around it. A poorly detailed head flashing is the single most common cause.
- Step flashing at the sides, which must be woven into the shingle courses, exactly as at a chimney.
- Sealant used as a substitute for flashing. Same story as every other penetration: sealant has a service life of a few years, and a skylight relying on it is on a countdown.
- Mixed components. A manufacturer flashing kit is engineered as a set. Substituting site-made pieces into it usually undoes the design.
- No cricket above a wide unit, which lets water pile against the head flashing rather than divert around it.
Condensation, which is not a leak
This gets misdiagnosed constantly and it is worth learning to recognize, because chasing it as a leak wastes money.
A skylight is a cold surface in a warm room. Warm moist indoor air touching cold glass condenses, runs down the glazing, collects at the bottom of the frame and drips. It looks exactly like a leak.
How to tell:
- Condensation appears in cold weather regardless of rain. A leak correlates with rain. This is the clearest test.
- Condensation is clean water. A leak usually carries some staining or discolouration from whatever it has passed through.
- Condensation is worst in kitchens and bathrooms, where indoor humidity is highest.
- Condensation forms on the glass surface itself, and you can watch it happen. A leak appears at the frame edge or the surrounding ceiling.
The remedy is ventilation and humidity control, not roofing. Better attic ventilation helps as well, see attic intake ventilation.
When the unit itself has failed
Two genuine unit failures:
- Failed glazing seal. On a double-glazed unit, the seal between panes eventually fails and moisture gets into the cavity. You see permanent fogging or condensation trapped between the panes that no amount of cleaning touches. This cannot be repaired, only replaced.
- Failed frame seals or hardware on an opening skylight. Gaskets perish, and operating mechanisms wear.
Older units are also simply worse than modern ones. Acrylic domes from decades past yellow, craze and become brittle, and by the time they are leaking they have usually reached the end of a natural life rather than developed a fault.
Repair or replace
The rough rule we work to:
- Flashing failure, sound unit. Repair the flashing. Contained job.
- Fogging between panes. Replace the unit. On a curb-mounted skylight this is often straightforward.
- Old acrylic dome, any symptom. Replace, and consider converting to a curb-mounted modern unit.
- Roof being replaced anyway. Replace the skylight at the same time, even if it is currently fine. Putting a new roof around an aging skylight guarantees you will disturb that new roof within a few years. This is the most common regret we hear.
That last point is the practical one. If your roof is due and you have skylights over fifteen years old, doing both together costs far less than doing them separately.
Our skylight repair service covers flashing rebuilds and unit replacement, and a free roof inspection will establish which of the three problems you actually have. If a leak has been running for a while, the cost of deferred maintenance is worth reading.
Skylight questions
Most often the flashing rather than the skylight, and most often the head flashing at the top of the unit. Other causes are a failed glazing seal, sealant used in place of proper flashing, or condensation that is not a leak at all.
Condensation appears in cold weather whether or not it is raining, is clean water, forms on the glass surface itself, and is worst in kitchens and bathrooms. A leak correlates with rain, usually carries some staining, and shows at the frame edge or surrounding ceiling.
A curb-mounted skylight sits on a raised frame built on the deck, lifting the glazing above the water plane. A deck-mounted unit fastens close to the roof plane with an integrated flashing kit. Curb-mounted is better on low slopes and easier to replace later; deck-mounted has a lower profile.
No. Fogging between the panes means the seal between them has failed and moisture is inside the sealed cavity. It cannot be resealed. The unit needs replacing, which on a curb-mounted skylight is often a contained job.
If they are more than about fifteen years old, yes. Fitting a new roof around an aging skylight means disturbing that new roof within a few years to deal with the unit. Doing both together costs considerably less than doing them separately.
Any roof penetration is a risk that has to be managed, but a well-flashed modern unit on an appropriate roof pitch performs reliably for decades. The risk comes from poor flashing detail, sealant used as a substitute for flashing, and units left in place well past their service life.