How long does a rooflight last?
Twenty to twenty five years is the honest figure for the sealed glass unit inside a domestic rooflight, and it is the number that usually decides the whole answer. The frame around that glass frequently outlives it. The perimeter detail beneath, if it was built properly, outlives both.
The range is wide because a rooflight is not a single component. It is a sealed glazing unit, a frame, a set of gaskets, sometimes an actuator, and a length of carefully built roof around the edge. Each of those runs on its own clock. The one that runs out first is the one you end up looking at from the sofa.
The clock that runs inside the glass
A sealed unit is two panes held apart by a spacer bar packed with desiccant, closed with a butyl primary seal and a polysulphide or silicone secondary seal. The cavity leaves the factory dry, and the desiccant absorbs the small volume of vapour that migrates through the seals year by year.
That desiccant has a fixed capacity, and when it is full the cavity starts to reach dew point on cold nights. Nothing has broken in the ordinary sense. The consumable simply ran out. Manufacturers of good units will typically warrant against that happening for ten years and quietly expect twice that in service.
Frames, and the material that sets the outer limit
Frame life diverges sharply by material, and it is worth knowing which one is over your head before predicting anything.
| Frame material | Typical service life | How it ends |
|---|---|---|
| Powder coated aluminium | Thirty years and beyond | Coating dulls long before the metal gives |
| Timber with a factory finish | Twenty five to thirty five years | Finish breaks down at the sash edge first |
| Modern uPVC | Twenty five years or so | Surface chalks, gaskets harden |
| Unstabilised acrylic dome | Twenty to twenty five years | Yellows, then goes brittle |
| Polyester coated steel trim | Twenty five years | Edge corrosion where it was cut on site |
Why a roof unit ages faster than the same glass in a wall
A double glazed window in a south wall and a rooflight in a flat roof are made to the same standard, and the rooflight will nearly always fail first. The reason is thermal cycling rather than any difference in quality.
A horizontal unit takes full solar load at midday in July, driving the outer pane well above air temperature, then radiates to a clear sky on a still January night and drops below it. That daily swing works the seal edge harder than any vertical plane ever does. Add standing rainwater sitting against the edge seal on a flat unit and the difference is easily five years of life.
Pitch, orientation and the units that go first
Given four rooflights of the same age on the same house, they will not fail together. The flat one goes first, then the south west facing slope, then east, then north.
The mechanism is the same in each case. A flat unit holds water at the edge and takes the highest summer sun angle straight down. A west facing slope catches low, hot late sun that heats one edge of the frame far more than the other, which sets up differential movement across the unit. A north facing slope in a Chelmsford terrace gets even light, little gain and a comparatively easy life.
The detail underneath, which decides more than the product
The single largest variable in how long a rooflight lasts is not printed on any brochure. It is the upstand height, the fall of the roof around it, the laps in the flashing and where the covering terminates.
Get those four right and a unit reaches its natural end quietly, misting one winter and coming out the next. Get them wrong and the unit is dragged down early: water tracks along the edge seal, timber under the frame softens, fixings lose their grip and the whole assembly starts moving under wind. That is why the workmanship on a Sunspire installation carries a ten year guarantee rather than the pitch resting on the brand of glass.
Domes on Essex flat roofs, and the age they have reached
Flat roofed rear extensions, garages, stairwells and bathrooms built across Harlow, Basildon and the Chelmsford suburbs in the nineteen seventies almost all carry a single or twin skin plastic dome. Those units were given a twenty to twenty five year life and a great many are now past forty.
Ultraviolet degradation is what finishes them. Acrylic yellows and loses impact strength, polycarbonate crazes into a fine web of surface cracks, and light transmission falls steadily the whole time. A dome that started near eighty per cent transmission can be down around half of that, so the room feels grey without anybody being able to name the change.
Reading the age of the unit you already have
Most units carry a data plate, usually etched into the spacer bar or printed on a label at the sash edge, and it often gives a manufacture date directly. That single photograph is worth more than any amount of guessing.
Where there is no plate, the frame tells you. A timber sash with a mid grey aluminium cladding and a plain air filled cavity is likely nineteen eighties or nineties. A slim white uPVC surround with a moulded corner is nineties or later. A cast aluminium dome kerb with visible screw fixings through the flange is typically pre nineteen ninety. Our page on identifying an existing rooflight goes through this in more detail.
Buying for a longer life, and what actually helps
Three specification choices measurably extend service life, and none of them is exotic. A warm edge spacer rather than an aluminium one reduces the thermal stress at the seal edge. A laminated outer pane resists thermal shock and hail better than monolithic glass. And a frame in powder coated aluminium over a timber core gives you the movement tolerance of timber with the weathering of metal.
The fourth choice is the installation itself. A generous upstand and a roof that drains cleanly around the kerb costs a little more once and buys years. If yours has reached the point where the glass has gone milky, the sensible next step is replacing the unit, which on a sound existing kerb is usually a single day’s work.
Related questions
- Do rooflights need planning permission?
- Do I need building regulations approval for a rooflight?
- Who notifies building control about a rooflight?
- How much does a rooflight cost?
- Do rooflights add value to a house?
- Is a rooflight cheaper than a dormer?
Everything on skylight & rooflight installation & replacement chelmsford →
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