The answer
A skylight should be replaced when it shows signs of failing, not on a fixed date. Misting between the panes, a stained or brittle frame, water at the edges or a sash that no longer closes tightly all say the unit is finished. A well-fitted rooflight can serve for a very long time, so check it every year.
Why there is no set date
If you ask how frequently a skylight needs replacing, you might expect a number of years. The honest answer is that a skylight is not a boiler or a car tyre. Its life depends on the glass, the frame, the seals, the flashings round the edge and the roof it sits in. One part can fail while the rest is sound.
The south-facing slope of a loft takes more sun than a north-facing one. A kitchen rooflight sees steam and cooking grease. A bathroom unit sees damp air every morning. Two skylights fitted on the same day can age at very different speeds. So the useful question is not how old the unit is, but what condition it is in. The rest of this answer is about reading that condition.
If you are weighing up a whole new opening rather than a swap, the main skylight and rooflight page sets out the full range, and the daylight guide helps you choose the size.
Misting is the clearest warning
A rooflight is a sealed glass unit: two or three panes with a gap between them, held apart by a spacer and sealed round the edge. When that seal fails, moist air gets into the gap. You see it as a cloudy film or beads of water between the panes, and no amount of wiping on either side will shift it.
The unit is blown. It still keeps the rain out, but it no longer insulates as it was built to, and the view of the sky turns milky. The glass itself is the part that has failed, so the cure is a new sealed unit or, where the frame has also gone, a new rooflight.
Condensation on the room-side surface is a different thing. That is damp air meeting a cold pane, and it points to a humid room or poor ventilation rather than a failed unit. A quick test: if the water is on a surface you can reach and wipe, the seal is probably fine.
What to look for in the frame, seals and flashings
Go up a ladder to the inside, or stand beneath the unit with a torch, and look at the following.
- Stains or tide marks on the plaster round the opening, which mean water is getting past the flashing or the frame.
- Timber that has turned dark, soft or flaky, or paint that has lifted in sheets.
- Seals that have gone hard, split or shrunk away from the glass.
- A sash that sticks, drops, will not stay open or will not close tight against its seal.
- Drafts or a whistle in wind, which says the seal no longer meets the frame.
- Older plastic or polycarbonate glazing that has gone yellow, cloudy, brittle or cracked, which is a sign that sunlight has worn the material out.
A single stain is worth watching. Several of these together say the unit has come to the end of its working life. Flashings are the metal or lead trim that carries water off the roof at the edge of the opening, and they wear on a different timetable to the glass. If they were fitted badly, or fitted to a pattern that is no longer made, a new flashing kit and a new unit go in together.
Is the old unit holding back the room?
Sometimes a skylight is not failing at all, but it is no longer the right one. Rules for new work have moved on since older units were fitted. Approved Document L, Table 4.2, sets the standard for replacement glazing in an existing home: roof windows should reach 1.4 W/m²K or WER band B, and rooflights 2.2 W/m²K. An older single-glazed or early double-glazed unit will fall short of both.
When you replace, the new unit has to meet that standard, and the fitting meets current Building Regulations. In practice you get a warmer room, less condensation and quieter rain on the glass.
Targets a replacement unit is built to
- 1.4 W/m²K
- for a roof window fitted to a home that already stands, or WER band B
- 2.2 W/m²K
- for a rooflight in an existing home
What replacing a skylight involves
Replacement is quicker than a new opening because the hole is already there. First comes the free survey. Someone measures the opening, looks at the flashings and the structure round it, and tells you what needs to go. You then get a fixed written quote that does not change on the day.
On the day, the old unit comes out, the new one is fitted, the flashings are renewed to suit, and we finish the inside, so no bare plaster is left behind. The fitting itself is a one or two day job. Our team files the Building Control notification when the work requires one, and the 10-year workmanship guarantee also comes in writing.
If you want to know how long the job runs end to end, our answer on fitting time goes through the day. If the opening itself is the issue, see whether your roof suits a rooflight in the first place.
A yearly check that saves a bad winter
Pick one dry afternoon each year, ideally in autumn before the wet weather. Run through this short list.
Your yearly skylight check
- Is there any cloudiness or water between the panes?
- Are there stains on the plaster or paintwork round the opening?
- Does the sash open, shut and lock without force?
- Are the seals soft and whole, with no splits?
- Is the glass or frame clear of moss, leaves and debris?
Related questions
Can I replace just the glass in my skylight?
Where the frame is sound and only the sealed unit has misted, a new sealed unit can go into the existing frame. If the frame, seals or flashings are also worn, replacing the whole rooflight gives a better result. The free survey tells you which case you have.
Will a new skylight fit the old opening?
The same opening can suit the new unit, because rooflights are made to measure and roof windows come in standard sizes. We measure at the survey and tell you if the opening needs any change. The roof type and pitch also play a part.
Do I need planning permission to replace a skylight?
A like-for-like swap is a different question from a new opening. The Milton Keynes planning guide sets out the route, and the council can advise on anything particular to your street.
