The short answer
A flat roof skylight usually leaks at its joints, not through the glass. The common faults are a membrane that has pulled away from the upstand, a failed sealant line, a cracked or crazed dome, water ponding against the kerb, or condensation that only looks like a leak. A roofer can tell these apart from the outside and the underside, and the repair depends on which one it is.
Where the water gets in
A flat roof rooflight stands on a low box called a kerb or upstand, built up from the deck so the glazing stands clear of the surface water. The waterproof membrane runs up the outside of that upstand and finishes under a capping or the lip of the rooflight. Each of those layers gives water a chance to creep behind.
The usual sequence is simple. Rain lands on the roof, runs to the lowest point and meets the upstand. If the membrane has lifted at its top edge, split in a corner or lost its seal under the capping, water tracks behind it and into the deck. It then reappears as a stain on the ceiling that may be a metre away from the rooflight itself, which is why people blame the wrong thing.
Corners are the weak spot. A membrane has to be folded round a square corner, and that fold is where cracks open first as the material ages and moves with the temperature. If the leak started after a cold snap or a hot spell, look there first.
Upstand and kerb faults
The upstand is a timber or insulated box in most cases, and it can fail in its own right. Timber kept damp over a long period softens at the base, and a softened kerb lets the rooflight shift and break its seal. Rot is hard to see from above, because the membrane hides it, but a spongy feel when you press the upstand gently from inside is a warning sign.
Height matters as well. An upstand set too low allows standing water to rise against the rooflight, and splashed rain can be driven over the top. Approved Document L treats a kerb supplied with the rooflight as part of one 2.2 W/m²K unit, but a kerb built on site has its own limit of 0.35 W/m²K, which is why an upstand has to contain real insulation. If it was built with a gap or without proper insulation, you can get condensation running down its inner face that is mistaken for a leak.
Why falls decide where the water sits
Rockwool, citing the 2018 edition of BS 6229, says the design fall should be 1:40 so that the finished drainage fall reaches 1:80. If the deck is flatter than that, water ponds around the upstand and tests every joint. Ponding does not always cause a leak, but it speeds up ageing at the edges of the membrane.
Membranes, seals and sealed units
What the roof is covered with changes both the fault and the fix. Felt roofs can blister or split at upstands, EPDM rubber can pull away from the adhesive at corners and GRP fibreglass can crack where the deck moves. Sealant used as a repair holds for a short time, then splits again if the movement continues. A line of mastic round a rooflight is a sign of a past fix, not a lasting one.
Then there is the rooflight itself. A polycarbonate dome can craze or yellow and develop hairline splits. A glazed unit can lose the edge seal that keeps the panes airtight, and then mist between the glass layers. That is not a rain leak but it is a defect, and our answer on replacing roof window glass on its own explains what can be done about it. Gaskets in an opening rooflight harden with age and let drips through the closed vent.
Typical lifespans give some context. Sources quote 20 to 30 years or more for EPDM, between 10 and 20 years for built-up felt (with large differences by workmanship), and flat roofs generally between 15 and 30 years. A rooflight that is older than its surrounding membrane may well be the part that needs replacing.
Is it a leak or condensation?
Many reports of a leaking flat skylight turn out to be moisture inside. Moist indoor air touches chilly glass or a cold frame, turns to water and trickles down. The drips tend to appear after a cold night, they sit around the frame, they are worse in bathrooms and kitchens, and they stop in dry weather. A true leak appears during or after rain, and it often shows up in an area with no direct link to the glass.
Two quick tests help. Wipe the inside surface dry and see how fast it returns, and notice whether the drip pattern follows the weather outside or the temperature inside. We cover condensation at pitched windows separately, and the logic carries across. Where a leak is real and the cause is unclear, a hosepipe test done in stages from the lowest part upwards can find the entry point, with someone inside watching the ceiling.
Two further checks save time. Look at whether the roof outlets are clear, since a blocked drain adds ponding, and look at what is above the rooflight, such as a gutter or a valley, that might be throwing water at it.
Repairing the rooflight joint, and the price
The repair follows the fault. A lifted membrane edge can be stuck back and sealed under a new capping. A split corner needs a new patch or a fabricated corner piece. A rotten kerb has to be rebuilt, and then the membrane taken up and relaid round it. A failed dome or sealed unit is replaced. If the wider membrane has little life left, patching the rooflight is only a stopgap and re-covering the roof is the honest answer.
For national guidance, Checkatrade's repair cost guide, dated June 2026, gives per square metre ranges excluding VAT and access: bitumen flat roof £100 to £140, EPDM £120 to £170 and GRP £130 to £180. A rooflight repair is usually a small area, so the final price depends more on access and the kerb condition than on area. Our own quote comes after a survey, with photographs of your roof, and covers scaffold, labour, materials and making good. We give a 10-year workmanship guarantee in writing.
To compare what could go in instead, see our guide to rooflights for flat roofs. If the noise in the rain troubles you more than the drips, read whether roof windows are noisy in the rain. The full range sits on our roof windows and skylights page.