How long does a rooflight last?

The parts that wear, and the parts that do not

A rooflight is not one object. It is an assembly, and its components age at different rates, which is why a single figure for the whole thing would mislead you even if we were willing to offer one.

The glass itself is the most durable part of it. What ages is the seal around the sealed unit, the perimeter bond holding two panes and the cavity between them together. When that gives up, moisture enters the cavity and the unit mists internally. Gaskets and weather seals harden, shrink and lose their compression, which is usually felt as a draught before it is seen as anything.

An opening unit has moving parts as well: hinges, friction stays, the operating gear and, on a powered unit, a motor, a control board and a chain. Anything that moves under load, in weather, is doing more work than anything that sits still. A fixed unit has less to go wrong for the simple reason that it does less.

Longevity is decided at the perimeter

The part of an installation most likely to end its working life early does not appear in the manufacturer’s catalogue at all. It is the junction between a factory-made frame and a roof somebody built by hand.

On a pitched roof that means the flashing kit chosen for the actual covering profile, the soakers, the laps above and below the unit, and how the underlay is cut so it drains outwards over the flashing rather than behind it. On a flat roof it means the height of the upstand, the falls around it, insulation carried up the kerb, and where exactly the waterproof covering terminates in relation to the frame. Where a bead of sealant has become the thing holding back the weather, that installation started ageing far earlier than the owner thinks.

Detailed properly, the perimeter simply keeps working, and the unit becomes the first thing to reach the end of its life rather than the roof around it.

What exposure does to the same unit

Two identical units fitted in the same week on two different roofs will not age at the same rate, and that is not a defect in either of them. Exposure is doing the work.

Ultraviolet light degrades polymers, so a south-facing slope with nothing shading it works harder on gaskets, seals and any plastic component than a sheltered northern one. Temperature swing matters too. A surface that heats through the day and cools sharply at night is expanding and contracting continuously, and every joint in the assembly moves with it. Lead behaves the same way, which is why lengths dressed too long to begin with eventually lift.

Then there is water and wind. An exposed roof takes driving rain at a low angle rather than the vertical fall a roof is designed around, and wind-driven rain finds shallow laps. A roof that holds snow or lies damp through the cold months is wet for longer each year than one that sheds and dries by lunchtime. None of that argues against glass in a roof. It argues for specifying and detailing for the roof actually in front of you.

Older plastic is its own case

A good deal of what we take out is not a modern glazed unit at all. It is a plastic dome or a twinwall sheet on a flat-roofed extension or a garage, and that material has a clearly recognisable way of ageing.

Polycarbonate and acrylic both yellow under ultraviolet light. The change is gradual enough that owners often do not register it until a new sheet is stood next to the old one and the difference in light becomes obvious. Alongside the discolouring, the material embrittles. A dome that flexed under a hand when it was new will craze and then crack, and by the time it does, the fixings around it have usually been working loose for a long while as the sheet expanded and contracted through every season.

That combination, cloudy and brittle, is the clearest end-of-life signal on any roof. Our page on plastic and polycarbonate installation sets out where the material still earns its place and where glass is the better answer.

The signs a unit is near the end

Most of them can be read from inside the room without anybody going up a ladder.

Misting between the panes. Moisture or haze sitting inside the sealed unit which cannot be wiped off either face. The seal has gone and the unit is finished, whatever condition the frame is in.

Draughts. A noticeable movement of cold air around the opening while the unit is shut, usually from gaskets that have hardened and stopped compressing against the frame.

Operation. A stiff handle, a stay that will not hold its position, a sash that has to be persuaded down on to its seal, or a powered unit that stalls or runs unevenly.

Movement in the frame. Visible distortion, a sash no longer sitting square, or a gap that varies along one edge.

Any one of those is worth looking at. Together they generally mean the unit has done its work, and what follows is a specification conversation about what replaces it.

Ten years on what we install

We install new rooflights and replacement units, and nothing else, and we have been doing that for fifteen years. When a unit reaches the end of its working life it comes out and a new one goes in, correctly detailed into the covering around it, rather than being kept going through another winter.

Our workmanship carries a ten-year guarantee, with the coverage and the exclusions written out on their own page rather than buried in small print. That guarantee sits on the part we control: the opening, the upstand or the flashing, the insulation line, the vapour control layer and the making good. The unit itself carries whatever the manufacturer offers with it, and we will tell you what that is for the product being specified rather than blur the two together.

We install VELUX, Fakro, Keylite and Roto units alongside flush conservation ranges, we handle the Building Control notification, and we work across Oxford and up to twenty five miles by road.