Do rooflights leak?

The glass is the part least likely to fail

A sealed glazed unit in an aluminium, timber or composite frame is a manufactured object, made to a tolerance in factory conditions and tested for resistance to water and to wind-driven rain before it is packed. Nothing about that process resembles a roof in February, which is rather the point.

Units do reach the end of their working life, and when they do the symptom is misting between the panes rather than water arriving in the room. That is the seal around the sealed unit letting moisture into the cavity. A genuine fault, but not a route for rainwater into the building.

So the honest starting position is that where water is coming in past a rooflight, the odds strongly favour something at the edge of it. That is better news than it sounds: the edge is workmanship, and workmanship can be specified, inspected and guaranteed.

Four places water actually gets in

The height of the upstand on a flat roof. The kerb the unit sits on has to lift the frame and the end of the waterproof covering above any water that stands on the roof, and water will stand on a flat roof at some point. Built too low, the most vulnerable junction in the building spends every winter wet.

The falls around it. A flat roof is laid so that water leaves it. Introduce a rigid rectangle into that plane without rethinking where the water then goes, and the unit becomes a dam holding water against its upslope side.

The flashing laps on a pitched roof. Water running down a slope should always meet an overlap rather than an edge. Laps that are too shallow let wind-driven rain climb back under them, and a flashing kit made for one covering profile will not weather a deeper one.

How the covering terminates. The waterproof covering is what keeps water out; the frame is a weathering layer sitting above it. Any detail that reverses those roles, so a bead of sealant becomes the thing holding back the weather, has a short life. The flat roof skylight installation and roof lantern installation pages set the build-up out in full.

Water on the inside that never came from outside

Water appearing around a rooflight in winter is very often condensation, and that is a different problem with a different answer. Warm moist air from the room meets a surface colder than its dew point and gives up its moisture onto it. It runs down the glass, gathers at the bottom of the reveal and shows as a wet mark that looks exactly like something coming in.

Two things separate them. Timing. Condensation is worst on cold, clear, still mornings and often absent in mild wet weather. Water entering from outside does the reverse: it follows rain, and it follows wind direction more closely than temperature. Position. Condensation appears on the frame and around the edges of the glass first, because those are the coldest surfaces, whereas ingress usually shows at one corner or down one side and marks the plaster around it.

The remedy is not on the roof. Either the cold surface is made warmer, through better glazing, a warm-edge spacer and an unbroken line of insulation around the opening, or the air is made drier. An uninsulated upstand is the commonest cause of all, because it is a cold ring running right round the unit.

Age, and what reaches the end of its life

The second honest answer is that nothing on a roof lasts forever, and a rooflight is an assembly of parts with different lifespans.

Gaskets harden and shrink. Older plastic domes go brittle and cloudy under ultraviolet light, and by the time one crazes the fixings around it have usually been working loose for years. Lead moves with every warm day and cold night, and lengths dressed too long to begin with will eventually lift. On flat roofs, felt and older single-ply systems age out at the upstand before they age out across the field, because that is where the material is most worked.

None of that is an argument against rooflights. It is the reason a unit past its service life comes out and a new one goes in, properly detailed into the covering around it. We carry out new installations and replacements, and a failed unit is replaced rather than nursed through another winter.

How to tell whether an installer understands the perimeter

You do not need to know roofing to work out whether the person quoting does. Ask about the edge, then listen for whether the answer is about the roof or about the product.

On a flat roof. How high will the upstand stand above the finished covering, will it be insulated as a continuation of the roof insulation, and where exactly does the waterproof covering terminate in relation to the frame.

On a pitched roof. Which flashing kit is being used for this particular covering profile, how far up the slope the head detail reaches, and how the underlay is cut and lapped so that it drains outwards over the flashing rather than behind it.

On either. What is holding water out if you ignore the sealant entirely. There should be a clear answer, and the answer should not be the sealant.

An installer who talks only about the unit, the glazing and the brand, and treats the roof around it as something to be worked out on the day, has told you where their attention will be.

Why we guarantee the workmanship for ten years

We install rooflights and nothing else, and have done for fifteen years. That matters here, because none of the details above are difficult so much as unforgiving. They have to be done in the right order, every time, by people who do them every week rather than a few times a year between other trades.

Our workmanship carries a ten-year guarantee, with the coverage and the exclusions on their own page rather than in small print. We handle the Building Control notification as part of the job. We cover Oxford and up to twenty five miles by road, and we install VELUX, Fakro, Keylite and Roto units alongside the flush conservation ranges.

If you are being quoted for a rooflight and the whole conversation has been about the glass, ask about the upstand.