The short answer
No single figure fits every rooflight, because one is really a handful of parts that wear at different speeds: the sealed glass, the gaskets and seals, the frame, and the flashing or kerb covering that sheds rain around it. A unit reaches the end of its life when its weakest part gives out, and the glass is not always the part that goes first. Where the house stands, which way the glass faces and how the roof was finished around it all move the answer.
- Free survey, fixed written quote
- 10-year workmanship guarantee
What gives way first
Start from the outside of the house and work in. The covering around the frame takes the weather first. On a pitched roof that means the flashing and the tiles or slates dressed up to the window. On a flat roof it means the kerb and the membrane lapped over it. Either can reach the end while the glass is still perfectly clear.
Next come the rubber gaskets and the sealant lines. These flex every day and harden with age, and a hardened gasket lets the sash sit a little loose in its frame. The sealed glass unit has its own edge seal, and when that fails the pane mists between the sheets. We cover that fault in why rooflights mist up between the panes.
The frame can outlast the other parts when it is looked after, and suffers when it is not. A timber frame in a steamy en suite needs its finish kept up. A painted aluminium frame can start to lift at a corner where water has lodged.
An opening rooflight adds moving parts: hinges, stays, a handle, or an electric actuator on a motorised unit. Anything that moves can stiffen or loosen, so a window that opens every day shows its age sooner than a fixed pane above a landing.
| Part | What wears it |
|---|---|
| Flashing or kerb covering | Rain, frost, and movement where it meets the tiles or the roof membrane |
| Gaskets and sealant lines | Daily flexing and sun on the exposed face |
| Sealed glass unit | A failing edge seal that lets moisture into the cavity |
| Frame | Damp at a low corner, a worn finish, or knocks from ladders |
| Hinges, stays and actuators | Use: the more often it opens, the more they work |
What an ageing rooflight shows you
An ageing unit usually gives signs before it stops working. Look at the lining under a roof window in daylight, on a wet day if you can. A tide-line on the plasterboard, bubbled paint or a dark patch on the timber reveal says water has been getting past the covering. A cloudy film that will not wipe off says the sealed unit has let go. A sash that now catches on the frame, or a window that will no longer stay open on its stay, points to worn hinges or a frame that has shifted.
Something else to notice is draught. A cold breath of air round the edge of a closed window on a still night means a gasket has stopped doing its job. On a loft landing that you only pass through, you may never feel it; in a loft study where you sit under the glass all day, you will.
None of these means the whole unit must go. A hardened gasket and a failed glass unit are different jobs from a failed flashing, and the survey sorts out which you have. Our page on whether a misted skylight is leaking goes through how to tell the two wet faults apart.
Myth
If a rooflight has never dripped, it is still in good order.
Fact
A sealed unit can fail with no drip at all, and a flashing can be losing its grip for a season before the first stain appears on the lining.
Timing a replacement
The cheapest moment to replace a rooflight is usually when something else is already happening in the room. If the loft bedroom is about to be redecorated, a new window beforehand saves painting a lining twice. If a flat roof is being re-covered, the kerb and the lantern or flat rooflight can be dealt with at the same visit, so the new covering is dressed to the new unit and not cut around the old one.
Where one unit has failed and its neighbours are the same age, it can make sense to look at them all at the same survey. They have seen the same weather for the same years, and the ones that still look fine may be nearer the end than you think. We can price one unit alone or the whole set, so you can compare doing one now with doing the set.
A replacement roof window has to meet the window limit for new and replacement glazing in an existing home, 1.4 W/m²K or Window Energy Rating band B at minimum. A new unit is therefore specified to a defined standard, and the Part L and rooflights guide explains the figures.
Getting a unit looked at
The survey is free and happens at the house. We check the glass, the frame, the lining, the flashing or kerb and the moving parts, and go onto the roof where it is safe to do so. You then receive a fixed written quote that names what is changing, from a sealed glass unit to a whole new rooflight.
If you already suspect the glass, a glass swap versus a full replacement is the place to weigh that up, and the wider misted rooflight replacement section sits above it. To book, send us a message with a photo of the window and the room it lights.
