A well-fitted flat roof rooflight should not leak, but one that has been fitted badly, or has aged without anyone looking at it, can. When a flat roof skylight lets water in, the cause is rarely the glass. It is nearly always the junction where the kerb meets the roof covering, a worn seal or a blocked drain nearby. Each of those can be found, and each is avoided at the fitting stage.
Where water actually gets in
A flat roof sheds water slowly, so any weakness has time to show itself. When a flat roof skylight leaks, the water has usually come through one of four routes.
The kerb and covering joint
The rooflight sits on a raised kerb, and the roof covering turns up the outside of that kerb. If the upturn is too low, badly bonded or cut short, rain can run behind it. This is the commonest source, and it is a roofing detail, not a glass problem.
The perimeter seals
Where a lid meets its frame, a rubber gasket or bead of sealant keeps water out. These age with sun and temperature swings. A seal that has hardened or shrunk lets in a thin trickle in driving rain.
The glass unit
A sealed unit can fail at its edge. You see it as misting between the panes, not as a drip, and it is a thermal fault before it is a water one. A cracked outer pane is different: that is a break to deal with at once.
Condensation that looks like a leak
Warm, damp air meets a cold surface and forms drops on the underside of the frame. People often take it for a leak. A wet frame with a dry ceiling around it, and no mark on the kerb, points to condensation.
Why the kerb decides the outcome
A flat rooflight needs a raised kerb because a flat roof holds standing water for a while after rain. A kerb lifts the joint clear of that water. Its height and the way the covering is dressed up it are chosen together, to suit the membrane system on your roof. We cover this in detail in flat rooflight kerbs and upstands.
A rooflight bought as a cheap bolt-on, set straight onto a roof with a short upstand, is where trouble begins. The water does not need to find much of a gap.
Reading the signs
| Sign | Likely source | First check |
|---|---|---|
| Stain on ceiling after heavy rain | Kerb and covering joint | Upturn height and bond |
| Drops under the frame on cold mornings | Condensation | Ventilation and glass quality |
| Misting between the panes | Failed sealed unit | Edge seal of the unit |
| Damp patch at the corner of the light | Corner detail or ponding | Fall and drainage |
Fall, drainage and ponding
A flat roof with no fall holds water around the kerb. Pools sitting against a joint stress it every time it rains. Part of the survey is checking whether water leaves the roof freely or stands. If a roof is ponding badly, that is a covering question, not a rooflight question, and we say so plainly. Our answer on when the roof needs renewing as well explains how that decision is made.
How we fit one so it stays dry
At survey we look at the existing covering, whether felt, EPDM rubber or GRP fibreglass, because the kerb is built to work with it. The new upstand is dressed with the same system or a compatible one, bonded and lapped so water is shed outward. Where the opening is new, the rafters or joists are trimmed and the structure signed off, because a new opening is structural work. Where the rooflight is a replacement, we say at survey whether the old kerb is sound enough to reuse.
Our page on felt, EPDM and GRP around a rooflight goes through the three coverings and how each meets the kerb.
What wears, and how it is replaced
This is the life plan. The glass unit, the perimeter seals and, on an opening light, the actuator are the parts that wear. On a well-made unit, a sealed glass unit can be lifted out from above and replaced without disturbing the kerb or the membrane. The seals are replaced the same way. We check the seals at around year 5 and look at the kerb joint at the same visit. If the unit is bonded in, the whole light may have to come off its kerb, and we tell you that before you choose. It is worth asking any firm quoting for a replacement how the glass would be changed in ten or twenty years, because the answer shows how the light is built.
What to do if yours is leaking now
Note when it happens: during heavy rain, only in wind, or on cold dry mornings. Take photographs of the ceiling and the frame, and keep a bowl under any drip. Check too whether a gutter, outlet or drain close to the light is blocked, since water backing up from there can mimic a faulty rooflight. A garage or back extension roof is easy to forget, and leaves collect on it. What we offer is a survey of the rooflight and a fixed written quote to replace the unit or the whole light if that is what the roof needs. Some leaks are in the covering alone, and we will say if that is the case. Our flat roof rooflights page covers the full installation process, and if the light is still in good order but you want to know about opening, see fixed or opening flat rooflight.