When Should an Old Dome Rooflight Be Replaced?

Replace a dome rooflight once it is past about 20 years old and showing any one of four things: the plastic has yellowed or gone cloudy, hairline crazing has appeared across the shell, the frame or shell flexes and cracks at the fixing points, or the same corner has leaked more than once. One of those is enough. Two makes it urgent.

That is the short version, and for most 1980s and 1990s domes on Ipswich extension roofs it is the whole answer. The rest of this page is the checklist, how to run it safely from the ground, and what changes the decision either way.

The checklist. Six questions, yes or no

Answer each one honestly. You do not need to go on the roof for any of them, and you should not.

1. Is the unit older than 20 years? If the extension was built before about 2005 and the dome has never been changed, the answer is almost certainly yes. Polycarbonate and acrylic domes were sold with a working life around 20 years and most were fitted between 1975 and 2005.
2. Has the plastic changed colour? Look up at it from inside on an overcast day. A sound dome reads as a pale, even, slightly milky white. A finished one reads yellow, amber or brown at the edges, and the change is usually strongest on the south-facing side.
3. Can you see fine crazing? Crazing is a network of hairline surface cracks, like the glaze on old pottery. From inside it looks like a faint frosted texture that was not there before. It means the polymer chains at the surface have degraded and the shell has lost impact strength.
4. Is the room noticeably darker than it used to be? A dome that has yellowed and crazed passes roughly half the light it did when new, and it scatters what is left. If you now switch a light on at midday in a room that used to be bright, the dome has told you.
5. Has water appeared inside more than once? Once can be a blocked outlet or a storm. Twice in the same place, in the same conditions, is a failed detail rather than bad luck.
6. Is there condensation inside the flutes? Multi-wall polycarbonate has hollow chambers running through it. When the edge seals go, moist air gets into those chambers and the condensation, dust and sometimes algae inside them cannot be cleaned out. There is no fix for this short of a new unit.

The one you cannot check yourself

There is a seventh trigger, and it is the most decisive of the lot: whether the shell or the frame flexes under load. A dome that has gone brittle will crack around the fixing points before it visibly breaks, and those hairline radial cracks at the screw heads are how most old domes actually let water in.

Do not go and find out. Never stand on a rooflight, never test one by pressing on it, and do not walk a flat roof to look, particularly one with a rooflight of unknown age in it. An old dome is a fragile roof element in the sense the term is used on site, and people fall through them. This is one an installer checks at survey, properly propped and boarded, and it is a routine part of what we look at before quoting.

From the ground, from an upstairs window, or with a photograph taken from a bedroom overlooking the roof, you can answer the other six questions perfectly well. Send us the photograph and we can usually tell you which decade the unit came from.

Age is a trigger, not a verdict

Two things push the timeline in either direction, and both are worth knowing before you spend anything.

Orientation and exposure shorten it. A dome facing open sky on an unshaded roof takes more ultraviolet than one shaded by a neighbouring wall or a mature tree, and it will yellow first. On the coast at Felixstowe, Harwich or Clacton the fixings go before the plastic does, because salt-laden air concentrates chloride at every screw head through repeated wetting and drying. A dome at 18 years on an open-sea site is often further gone than one at 25 years at Stowmarket. Our page on coastal rooflight failure covers what that does to fixings in detail.

Cost of access extends it. If the dome is over a garage with a tower-friendly roof and it is only mildly cloudy at 22 years, waiting a year costs nothing. If it is on a two-storey rear addition needing scaffold, and the roof covering is also within a few years of renewal, the arithmetic changes completely. Doing both at once shares the access cost, which is the argument set out on our page about replacing rooflights when a flat roof is re-covered.

What we do about it, and what we do not

We replace. We do not reseal, patch or mastic anything, and we are straightforward about why: a bead of sealant around a failing dome addresses none of the six checklist items, and the timber upstand underneath, which on a unit of this age is usually bare and uninsulated, has often been damp for years by the time water shows inside. The full argument, including what resealing actually costs over a five-year cycle, sits on reseal or replace a dome rooflight. If the material science behind the yellowing is what interests you, yellowed dome rooflight replacement goes into why polycarbonate degrades the way it does.

On a like-for-like dome replacement into a sound upstand up to 900 x 900mm, we publish £1,150 to £1,650. A flush glass unit into a new opening at 1.0 x 1.0m runs £2,200 to £3,000, and on a habitable room that is nearly always the better spend. The wider picture is on the flat roof skylight installation hub, and the choice between the two products is covered on dome versus flat glass rooflight.

Run the checklist. If you got two yeses, send us a photograph from an upstairs window and roughly where the property is, and request a fixed quote. We survey, check the upstand condition properly, and give you one written price that does not move.