A dome rooflight yellows and goes brittle because ultraviolet light breaks down the plastic it is made from. Acrylic and polycarbonate both degrade under years of direct sun, losing clarity, colour and impact strength as the molecular chains that hold them together are cut by UV. A dome that was water-clear in 1992 is now amber, cloudy and crazed with fine surface cracks, it passes roughly half the light it once did, and it has become brittle enough to crack under a footstep or a hailstone. There is no coating or polish that reverses it, because the damage runs through the whole thickness of the material. The sensible end point is replacement, either with a modern flush glass unit or, where glass is not proportionate, a new multi-skin dome.
This page explains the degradation, what it costs you in daylight, warmth and safety, and how a like-for-like replacement is handled. When a dome has actually cracked and become a hazard, that is covered on a cracked dome rooflight. The wider job sits on flat roof skylight installation.
What UV does to the plastic
Acrylic and polycarbonate are long-chain polymers, and the bonds in those chains absorb ultraviolet energy. Every sunny day, a little of that energy breaks a bond. The process is called photo-oxidation, and over years it does three things that all show up in an old dome.
Yellowing
The broken chain-ends form new chemical groups that absorb blue light and reflect yellow, so the material takes on an amber cast. Polycarbonate yellows faster and more visibly than acrylic. It is gradual, so the people living under it rarely notice how dark the room has become until a new unit goes in next to the old one and the difference is obvious.
Crazing and clouding
As the surface degrades it develops a network of fine cracks, called crazing, and a milky haze. Both scatter light rather than passing it cleanly, which is why an old dome gives a room dull, flat brightness with no view of the sky through it. On a multi-wall polycarbonate dome the internal flutes also fill with condensation and dust over the years, and that cannot be cleaned out, so the clouding gets worse from the inside as well.
Embrittlement
This is the one that matters for safety. New polycarbonate is famously tough, which is why it was used for rooflights in the first place. UV degradation strips that toughness away. The material that could shrug off a football at installation becomes brittle enough that hairline cracks radiate from the fixing points, and the shell will crack if it is stood on, hit by hail or flexed by a gale. A brittle dome is a fall-through risk for anyone who goes onto the roof, which is one more reason nobody should.
What a degraded dome is costing you
An old dome is not just ugly. It is working against you on three fronts at once.
| 1990s triple-skin dome | Modern flush glass unit | |
|---|---|---|
| Whole-unit U-value | Around 2.8 to 3.5 W/m²K | Around 1.1 to 1.4 W/m²K |
| Light passed | Roughly half its original, and diffused | High and clear, with a view of the sky |
| Appearance from below | Amber, cloudy, no view | A clean rectangle of sky |
| Impact strength | Brittle, cracks at fixings | Toughened outer, laminated inner |
| Condensation | Runs inside the flutes, cannot be cleaned | Sealed cavity, warm-edge spacer |
U-values are typical figures for units of this type, not manufacturer data for a specific product. The comparison in full is on dome rooflight or flat glass.
The thermal figure is the one people underestimate. A dome at 3 W/m²K loses heat at more than twice the rate of a modern glass unit, so the room under it is cold in winter and the heating works harder to hold it. Over a Suffolk winter that is real money going straight through the roof, and it is money a replacement starts saving on day one.
Why it is a replacement, not a repair
You cannot restore a degraded dome. The yellowing, crazing and embrittlement run through the full thickness of the material, so polishing the surface removes a skin and exposes more of the same underneath. There is no coating that reverses photo-oxidation. The dome has reached the end of a service life that was never more than about 20 years, and it is now 30 or 40 years old.
The trade default is to run a bead of mastic round the frame and call it done. That does nothing for the plastic, nothing for the light, nothing for the heat loss and nothing for the wet timber that is usually in the kerb underneath by this age. It closes the visible route for twelve to eighteen months. We do not offer resealing or patching on any rooflight, because on a dome this far gone it is a payment plan for the replacement you are going to buy anyway. The reasoning in full is on reseal or replace a dome rooflight.
Patching buys a season. Replacement ends it. A degraded dome is failing through its whole thickness. No surface treatment reverses that, and no sealant addresses the cold, dark, brittle unit or the damp kerb below it. Replacement is the point at which the problem stops coming back.
Choosing the replacement
Two honest options, and which one fits depends on the room and the budget.
Flush glass, for a habitable room
On a kitchen, dining room, living space or any room you actually spend time in, a modern flush glass unit is nearly always the right answer. It reads as a clean rectangle of sky, it holds around 1.1 to 1.4 W/m²K against the dome’s 3-plus, and the glass does not yellow. The outer pane is toughened, the inner is laminated so nothing can fall into the room, and on anything above about 1.2m across we specify solar control glass as standard. The full detail is on flat glass rooflight installation.
A new dome, where it is proportionate
On a garage, outbuilding, utility room or low-budget replacement, a glass unit can be more than the space warrants, and a new multi-skin dome is a perfectly sensible replacement. Modern polycarbonate is better stabilised against UV than the 1980s material, so it lasts longer than the one it replaces, and a dome sheds water by shape rather than by fall, which is genuinely useful on a poorly drained roof. We fit new domes where they are the right call and steer you to glass where they are not.
What the replacement job involves
The dome coming off is the moment the kerb underneath is finally visible, and on a unit this age the timber is frequently soft on the top edge or at a corner, because a bare uninsulated kerb of that era has usually been damp for years. We survey on the expectation that the kerb may need rebuilding, quote it as a provisional item, and photograph what we find before we go further.
- The old dome and its frame are lifted off and the kerb is exposed and probed.
- Where the timber is sound, we build the new covering detail onto it and set the new unit. Where it is not, the kerb is rebuilt, insulated, to 150mm above the finished roof, or 200mm on the coast.
- The falls are checked so water runs away from the unit rather than sitting against it.
- The new unit is bedded onto the dressed covering, levelled and mechanically fixed. The roof is watertight before we leave.
On a like-for-like swap into a sound kerb it is a one-day job. Where the kerb needs rebuilding it runs into a second day. Prices are published on our flat roof rooflight installation cost page, and if you are timing it against a wider roof recover, the sequencing is on recovering a flat roof and rooflight together.
Common questions about yellowed dome rooflights
Can a yellowed dome be cleaned or polished clear again?
No. The yellowing, crazing and haze run through the full thickness of the plastic, not just the surface, so polishing removes a skin and exposes more degraded material underneath. There is no coating that reverses UV degradation. Once a dome has gone amber and cloudy it is at the end of its life and the sensible answer is replacement.
How long does a polycarbonate dome last?
Older polycarbonate typically becomes noticeably cloudy at 12 to 15 years and structurally brittle by around 20. Most of the domes we replace are 30 to 40 years old, well past a service life that was never designed to be more than 20. Modern polycarbonate is better UV-stabilised and lasts longer, but a glass unit does not yellow at all.
Should I replace a dome with glass or another dome?
On a habitable room, glass, almost always. A flush glass unit is clearer, much warmer and does not yellow. On a garage, outbuilding or utility room where glass is not proportionate to the space, a new UV-stabilised dome is a sensible and economical replacement, and we fit them where they are the right answer.
Is a brittle old dome dangerous?
Yes, to anyone who goes onto the roof. UV degradation makes the shell brittle, so it can crack under a footstep or hail and it is a fall-through risk. That is one reason nobody should walk a flat roof with an old dome in it. If the dome has already cracked, treat it as urgent and see our page on a cracked dome rooflight.
Get an old dome replaced
Send us the rough size of the dome, whether the room below is habitable, and where the property sits. We will survey it, tell you honestly whether the kerb underneath is likely to need rebuilding, and give you one written fixed price for glass or a new dome, whichever suits the room and the budget. Every installation carries a 10-year workmanship guarantee, and the installers have 25 years fitting rooflights behind them.
Request a fixed quote to replace a yellowed dome rooflight 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.