The short answer
A tired polycarbonate dome can be replaced with a new dome or a modern flat rooflight on the same upstand, provided the kerb beneath is sound and the junction with the roof membrane is renewed. A replacement rooflight in an existing home has to meet the Part L figure of 2.2 W/(m²·K), so a single-skin dome is rarely a like-for-like choice now.
Why old domes fail
A polycarbonate dome is a moulded plastic bubble over a hole in a flat roof. It is light and cheap to make, which is why it was so widely fitted. Over the years the sun yellows the plastic, the surface crazes into fine cracks, and the dome loses clarity and strength. A knock or a hailstorm can crack it through.
Condensation is the other common complaint. Many older domes are a single skin or a thin double skin, so the inner surface is cold and the room air condenses on it. Water collects in the dome and drips, or runs down the kerb into the lining. This is often mistaken for a leak, as our answer on a flat roof skylight leaking explains.
The fixings and seals age as well. Screws loosen as the plastic expands and contracts, and the seal between the dome and its frame hardens. Water then finds a way past, and the lining below stains.
Polycarbonate sheets used on conservatory roofs have a typical lifespan of about 10 years according to HomeBuilding, and the Eco Experts say about 10 to 15 years. Those figures refer to conservatory roofing sheets, not to domes, but they show that the plastic is a short-lived material compared with glass or metal.
What a modern rooflight must achieve
Replacing a rooflight in an existing home is building work, and Approved Document L sets a standard for it. In Table 4.2, a new or replacement rooflight has a limit of 2.2 W/(m²·K), a U-value that measures how fast heat passes through. Roof windows follow a separate figure, 1.4, and the two products should not be mixed up.
A single-skin dome will not reach 2.2. A multi-wall or double-skin dome, or a glazed flat rooflight with an insulated frame, can. The data sheet states the U-value of the whole unit, and that number is what to compare. AD L 2026 takes effect on 24 March 2027, and until then the 2021 edition applies, so the order date can matter.
The upstand has its own rule. A kerb made on site is capped at 0.35 W/(m²·K). An old kerb may be thin, uninsulated timber, and fitting a good rooflight on top of it wastes the improvement. Replacing the kerb at the same time is often the better job.
Our guide to the flat roof rooflight compares the available types, from domes and lanterns to flat glass.
The upstand and the roof membrane
The dome is only half of the job. It sits on an upstand, the raised kerb around the opening, and the roof membrane is turned up the face of that kerb and sealed. A new dome on a poor kerb leaks as soon as it goes on.
We check the kerb for rot, cracks, damp and height. A low kerb lets standing water reach the dome frame, and BS 6229:2025, the code for flat roofs with flexible coverings up to 10 degrees of pitch, keeps a minimum finished fall of 1:80, which means a roof should drain. If the roof ponds around the opening, the kerb sits in water and the junction is tested constantly.
Then the membrane. If it is EPDM, GRP, felt or lead, the new turn-up has to bond to the old material or replace it. A patch over tired membrane may fail within a few seasons. Where the roof covering is also at the end of its life, doing the rooflight and the covering together saves paying for access twice.
At the top of the kerb, where frame meets membrane, a trim or flashing sends water outward. We photograph it before the dome goes on, so the junction can be seen afterwards.
Choosing the replacement
The options run from a like-for-like dome to a flat glass rooflight. A new multi-wall dome is the cheaper route and keeps the existing opening. A flat rooflight looks cleaner from inside and lets in more light, and its glass is less prone to the yellowing that spoils plastic, though it is heavier and costs more.
The size of the opening may be fixed by the structure. If the roof was framed for a particular size, changing it means altering the timbers, as our answer on cutting rafters for a roof window explains. Staying with the existing size is simpler.
Planning is rarely a barrier for a like-for-like swap on a house, but flats have no permitted development rights, and a listed building or a conservation area needs checking. Ask your planning team if you plan to change the size or the appearance.
We measure the opening, check the kerb and the membrane, and photograph the existing dome and the loft side. The options come with their U-values, and whichever you pick is priced as one written total. See roof windows and skylights for the rest of the service.
