Should You Reseal or Replace an Old Dome Rooflight?
Reseal or replace an old dome rooflight? Replace it. Running a bead of mastic round a failing dome buys twelve to eighteen months and hides the real problem, which by the time water shows inside is usually wet timber in the kerb underneath. Resealing does nothing for crazed, yellowed polycarbonate, nothing for perished gaskets, and nothing for a soft kerb. Run that cycle three times over five years and you have spent a real fraction of a replacement while the decay carries on. The tests below tell you which you have: a sound unit worth leaving, or a degraded one where a reseal is a payment plan for the replacement you will buy anyway.
We do not offer resealing or patching on any rooflight, and this page explains why in terms you can check yourself, from the ground or a window. The degradation itself is on yellowed and brittle dome rooflights, and the wider job on flat roof skylight installation.
Why resealing is temporary
Sealant is a gasket. It excludes air and closes a visible gap. It is not a waterproofing layer, and it is not a structural bond, so it cannot fix any of the things that actually fail on an old dome.
- It does not touch the polycarbonate. A yellowed, crazed, brittle shell is degraded through its full thickness. Sealing round its frame leaves it just as cloudy and just as fragile.
- It does not restore the gaskets. The gaskets and mastic bed between the dome frame and the kerb have hardened and shrunk back, particularly at the corners. A fresh bead over the top does not renew what is underneath.
- It does not dry the timber. By the time water shows inside, the kerb has usually been damp for years, and it carries on decaying under the new sealant, out of sight.
- It moves the leak, it does not stop it. Close the visible route and water finds another. The staining pauses for a season while the damage continues.
A reseal typically buys twelve to eighteen months. Do it three times over five years and the total cost is a real slice of a replacement, with a cold, dark, brittle unit and a rotting kerb still there at the end of it.
The condition tests that point to replacement
You can make most of this call yourself, from inside and from a window. Nobody should walk a flat roof to check a dome, least of all a brittle one that will not carry weight.
| Test | Points to replacement if |
|---|---|
| Clarity | The dome is amber, cloudy or crazed, and the room is dim at midday |
| Age | The dome is 20 years old or more, past a service life that was never longer |
| Brittleness | Hairline cracks radiate from the fixing points |
| Internal condensation | Condensation and dust sit inside the multi-wall flutes and cannot be cleaned out |
| Gaskets | The mastic bed has shrunk back and gaps show at the corners |
| Staining inside | Brown tide marks on the reveals, especially at one corner |
| The reveals | Even damp and mould on all four sides, a cold-bridge sign of a bare kerb below |
If several of these are yes, the dome is at end of life and the kerb below it is very likely wet, and a reseal is money spent delaying the inevitable. The even-damp-on-all-four-reveals test is worth understanding, because it points at an uninsulated kerb rather than a leak, and it is covered on cold bridging and damp around flat rooflights.
The one case where the unit is genuinely sound
Not every dome needs replacing today. A relatively young dome, still clear and not brittle, with sound gaskets and a dry kerb, has life left and there is no reason to touch it. If yours passes all the tests above, leave it. The point is not that every dome is finished. It is that a dome showing the signs above is finished, and resealing it does not change that. When it is genuinely uncertain, the honest answer comes when the unit is lifted and the kerb is probed, which is why we survey on that basis. Guidance on the timing is on when to replace a dome rooflight.
Patching buys a season. Replacement ends it. On a failed dome a reseal is a payment plan for the replacement you are going to buy anyway, and it leaves the kerb rotting underneath. We would rather say that plainly than take the money for a mastic bead.
What replacement actually involves
The dome coming off is the moment the kerb is finally visible. On a unit this age the timber is frequently soft on the top edge or at a corner. We survey on the expectation the kerb may need rebuilding, quote it as a provisional item at £350 to £900, and photograph what we find before going further. Where the kerb is sound we build the new covering detail onto it and set the new unit. Where it is not, we rebuild it insulated to 150mm, or 200mm on the coast. On a like-for-like swap into a sound kerb it is a one-day job at £1,150 to £1,650 up to 900 by 900mm. Prices are on flat roof rooflight installation cost.
Suffolk and north Essex specifics
The 1980s and 1990s dome, on a bare uninsulated kerb, is everywhere across Ipswich, Kesgrave, Rushmere and Chantry, and they are failing together because they went in together and none was built to last beyond 20 years. On the coast at Felixstowe and Harwich the kerb rot tends to be worse, because salt-driven weather and standing water keep the timber wetter, so a reseal there is even less honest. Estuarine areas like Woodbridge sit between the two.
Common questions
Should I reseal or replace my old dome rooflight?
Replace it if it is yellowed, crazed, brittle, over 20 years old, or the reveals are damp. Resealing buys twelve to eighteen months, does nothing for the degraded plastic or perished gaskets, and hides wet timber in the kerb that carries on rotting. On a failed dome a reseal is a payment plan for the replacement you will buy anyway.
How long does resealing an old dome last?
Typically twelve to eighteen months, after which the water finds another route. It does not address the crazed plastic, the shrunk gaskets or the wet kerb, so the underlying decay continues. Repeated over five years the total cost approaches a replacement, with a cold, dark, brittle unit and a rotting kerb still in place.
How do I know if the kerb underneath is rotten?
You often cannot until the dome is lifted, which is why we survey on the expectation it may need rebuilding. Signs pointing that way include brown staining at a corner, even damp on all four reveals, and a unit visibly sitting low in the roof. On a 1980s or 1990s dome the timber is frequently soft, and we photograph what we find before going further.
Get an honest assessment
Send us the rough size of the dome, its age if you know it, and any staining or damp you can photograph inside. We will survey it, tell you honestly whether it has life left or is finished, and give you one written fixed price for replacement with the kerb rebuild flagged as a provisional item. Every installation carries a 10-year workmanship guarantee.
Request a fixed quote in Ipswich, Suffolk or north Essex.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.