When to replace
Replace a skylight when the glass has misted between the panes, the sash no longer closes cleanly, the glazing is single, the timber frame has softened, or the flashing is of an outdated pattern that no longer suits the roof. One failed sealed unit in a sound frame needs only new glass. A tired frame needs a whole new unit. A survey tells you which case you have.
The signs that point to replacement
Most skylights do not fail on one day. They give notice, and the notice is usually visible from the room below. If you are asking when to replace a skylight, start with this list and see how many apply to yours.
None of these signs is a reason to rush. Each one tells you something about which part has gone, and that decides the size of the job. The same questions apply when you ask when to replace a rooflight on a flat roof: the vocabulary changes, the signs do not.
- Fogging or a milky film trapped between the panes that never wipes away
- A sash or lid that sticks, drops, or will not close flush against its seals
- A single pane of glass with no sealed unit at all
- Timber that is dark, soft or flaking when pressed with a thumbnail
- Cracked or discoloured flashing, or a flashing pattern that no longer matches the tiles around it
- A frame that has bowed so that the gaps around it show daylight
Misting between the panes
A modern double or triple glazed unit is two or three panes fused around a spacer, with the gap between them sealed. When that seal fails, moist air gets in and the gap fogs. The fog is trapped, so cleaning the glass on either face makes no difference.
This is the commonest reason people start looking at replacement, and it is often the smallest job. If the frame is sound, the sealed unit can be swapped on its own and the frame stays where it is. Our page on whether misted glass will clear explains why it does not, and our service for misted and failed units sets out the glass-only route in full.
A sash that no longer closes cleanly
On a roof window, look at the gap between sash and frame when the window is shut. It should be even all the way round. A sash that rubs on one side, sits proud at a corner or needs a shove to latch has usually moved because the hinges, the frame or the structure around it have shifted.
On a flat rooflight with a lid, the same test applies to the lid and its upstand. If the closed lid shows a visible gap, or the gasket has flattened and stays flat, the unit is no longer doing its job. Worn seals and distorted frames suggest a whole new unit rather than a glass swap, because new glass in a bent frame inherits the problem.
Single glazing, rotten timber and outdated flashing
Single glazed skylights from older houses are the clearest case. They have no sealed unit to fail, which is why they never mist, and they also have none of the thermal performance a current unit gives. Replacing the whole unit is the sensible route, and that route is notifiable work, and the notifiable question has its own answer.
Timber frames that are soft at the bottom edge, where water sits, tell you the frame has run its course even if the glass is perfect. Press a thumbnail into the lower rail. If it marks easily, a new frame is needed, and a glass swap would be money spent on the wrong part.
Flashing matters because it is what joins the unit to the roof covering. Older flashings were made for older tiles and slates. A unit may be fine while the flashing around it was cut for a roof covering that has since been replaced. When the covering and the flashing no longer agree, the whole unit usually comes out so that a matching kit can go in.
How long a skylight should last
Manufacturers do not promise a single lifespan. What they publish are guarantee periods, which differ between the sealed glass, the frame and the flashing, and between brands. Those periods are a floor, not a forecast. Many units outlast them, and some do not reach them if the roof has moved or the unit has been fitted badly.
Rather than trust a number, ask for the documents that came with the unit. The data plate on the frame gives the model and size code, and the maker's published guarantee for that model can then be looked up. If the plate has been lost or painted over, we measure the unit and work out the model from that.
Glass only, or the whole unit
What decides it is the frame. Where the frame is straight and firm and only the glass has failed, new glass is enough. Where the frame is warped, soft, single glazed or sitting in the wrong flashing, the whole unit goes. The table sets the common cases side by side.
The whole-unit route changes the paperwork as well. Frame and glass together count as a controlled fitting in the Building Regulations, so Building Control is told in advance, and we handle that notification where the work is notifiable. Changing only the glass is outside that rule.
| What you see | Usually enough | Why |
|---|---|---|
| Fog between panes, sound frame | New sealed glass unit | The seal failed, the frame did not |
| Sash will not close cleanly | Whole new unit | Frame or hinges have moved |
| Single glazing | Whole new unit | No sealed unit to swap |
| Soft timber at the lower rail | Whole new unit | The frame is the failed part |
| Outdated flashing, good unit | Survey first | Depends on the roof covering |
What happens when you ask us
Ridgeway Skylights starts with a free survey. We identify the unit from its data plate or by measurement, test the frame, look at the flashing and the roof covering around it, and say plainly whether glass or the whole unit is the right call. Our quote arrives in writing, with a job plan that lists the new unit and its size, how Building Control hears of it, the delivery wait and our fitting date.
If the answer is the whole unit, our rooflight replacement guide walks through what happens on the roof. A ten-year workmanship guarantee stands behind the fitting, and the roof is closed up neatly before the team goes.