Skylight Replacement: When to Repair vs Replace
Repair a skylight if the glass is clear, the frame is solid and the leak is coming from worn sealant or lifted flashing. Replace it if there is mist between the panes, the glass is cracked, the frame has gone soft or the leak keeps coming back. Most skylights over 20 years old are worth replacing.
If your skylight is letting water in or looks cloudy, it can be hard to tell whether a quick fix will do. Here's what to look for, how long skylights usually last and roughly what each option costs, so you know what to ask your roofer.
How Can You Tell If a Skylight Needs Replacing?
Start with the glass. If there's a misty haze between the two panes, the seal inside the glazing has worn out and moisture has got in. You can't wipe it away from either side and resealing the outside won't help. At that point you need new glass or a new skylight.
Next, look for cracks. Even a small crack in double glazing lets water through. Older plastic domes are worth checking too, as they tend to turn yellow and go stiff after years in the sun.
Then think about how many times you've had it repaired. If a roofer has resealed it two or three times and water still gets in, more sealant won't solve it. The frame, or the raised wooden kerb it's fixed to, has usually had its day. A new skylight will save you paying for repeat visits.
When Is It Better to Repair a Skylight?
If the glass is clear and the frame feels solid, a repair is often all you need. When a skylight starts leaking, the cause is usually at the edges, where the sealant has worn or a strip of flashing has lifted. Leaves blocking the path rainwater takes off the glass can cause the same thing. A roofer can usually sort any of these in one visit.
Repairs are fairly cheap. Based on 2026 UK trade prices, resealing typically costs £50 to £150 and flashing or frame repairs come to around £175 to £400. Simple seasonal skylight maintenance checks twice a year help you spot small problems before they turn into bigger ones.
Before you call anyone, check whether the water is actually a leak. On cold mornings, steam from showers, cooking and drying clothes settles on the inside of the glass as condensation. That's normal and the skylight itself is fine.
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Share your kerb's internal measurements and our team will confirm the right size, specification and lead time before you order.
Explore Rooflight Collection →How Long Should a Skylight Last?
A good-quality flat roof skylight that was fitted properly should last 20 years or more. Fixed skylights don't open, so there are no hinges or motors to wear out. Looking after one is mostly a case of cleaning the glass, clearing leaves in spring and autumn and checking the sealant once a year.
If yours is older than that, it's worth thinking about a replacement even if it doesn't leak. Manufacturers built skylights 20 or more years ago to much older energy standards, so they let a lot of heat escape. A modern double-glazed skylight has a U-value as low as 1.0 W/m²K and triple glazing brings that down to 0.5 W/m²K. The lower the number, the warmer the room stays in winter.
Is It Cheaper to Repair or Replace a Skylight?
For a one-off issue, repair is cheaper. The costs soon add up on a skylight that keeps needing work, though. Three reseals at £150 each come to £450, more than many new skylights cost.
Flat roof skylights start at £165 ex VAT for a 500x500mm frameless unit and £392 ex VAT for a framed one. Every skylight from Rooflights and Skylights UK is made in Britain, UKCA certified and weather tested to BS6375, with a 10-year guarantee. Standard sizes go up to 1000x3000mm and are dispatched in 3 to 4 working days, with free delivery to UK mainland addresses.
You'll also need to pay for fitting. Industry guides put a fully installed flat roof skylight at roughly £1,200 to £2,600 in 2026. If the kerb underneath is in good shape, a straight swap usually takes a roofer half a day to a day, which keeps you towards the lower end.
What Should You Check Before Ordering a New Skylight?
Get the size right first. Measure the opening on the inside of the kerb, not the outside of the old frame. Suppliers size flat roof skylights by this internal measurement, so an opening of 600x900mm needs a 600x900mm skylight.
Ask your roofer to check the kerb as well. It needs to be level, solid, covered with the roof membrane and usually at least 150mm above the roof surface so rain runs away. If the wood has gone soft, rebuild it before the new skylight goes on. Leave the old one in place until your new one is on its way, so your roof stays covered the whole time.
A replacement is also a good chance to upgrade. Frameless rooflights give you more glass and a cleaner look from inside, while triple glazing keeps more heat in a room you use all year round.
Summary: Repair or Replace at a Glance
|
What you notice |
Repair or replace |
Rough cost |
|
Worn sealant |
Repair |
£50 to £150 |
|
Lifted flashing |
Repair |
£175 to £400 |
|
Mist between the panes |
New glass or new skylight |
£250 to £850 for glass |
|
Cracked glass |
Replace |
From £165 ex VAT plus fitting |
|
Soft frame or kerb |
Replace |
£1,200 to £2,600 fitted |
|
Still leaking after two repairs |
Replace |
£1,200 to £2,600 fitted |
Frequently Asked Questions
- Can you replace just the glass in a skylight?
- Do you need planning permission to replace a skylight?
- How long does it take to replace a skylight?
- Why is my skylight misty between the glass?
- What size skylight do I need to replace my old one?
- Can I replace a plastic dome skylight with a glass one?
- Can I upgrade to triple glazing when replacing a skylight?
- Can I fit a bigger skylight when replacing my old one?
Yes, as long as the frame is still solid and sealed to the roof. A roofer can fit a new glazed unit into the old frame, which typically costs £250 to £850. If the frame itself is worn, it makes more sense to replace the whole skylight.
Usually not, if the new skylight is the same size and in the same place. Listed buildings and homes in conservation areas are the exception, so check with your local council first. The new skylight still has to meet current Building Regulations for energy efficiency.
Fitting usually takes half a day to a full day if the kerb is in good condition. Standard sizes from Rooflights and Skylights UK are dispatched in 3 to 4 working days, so most homeowners have the job finished within a week of ordering.
Mist between the panes means the seal inside the double glazing has worn out and moisture has got in. You can't clean it away because it's trapped inside the unit. The fix is new glass or a new skylight.
Measure the opening on the inside of the kerb, not the outside of the old frame. Standard flat roof skylights from Rooflights and Skylights UK come in sizes from 500x500mm to 1000x3000mm. If your opening falls outside that range, Rooflights and Skylights UK can make a custom size in 7 to 10 working days.
Yes. A flat glass rooflight can sit on the existing kerb once your roofer confirms it is level and sound, giving you clearer views and better thermal performance than an old plastic dome.
Yes and a replacement is the ideal time to do it. Triple-glazed flat rooflights from Rooflights and Skylights UK achieve a U-value of 0.5 W/m²K, keeping the room below warmer through the winter months.
Yes. Your roofer can enlarge the opening and build a new kerb to suit a larger skylight, bringing even more natural light into the room. A builder or structural engineer should check the roof joists first, and Rooflights and Skylights UK can make a custom size if the new opening falls outside the standard range.