Rooflights

Rooflights and Skylights on Flat Roofs

A rooflight is a hole cut in the one part of a building whose whole job is to be continuous. Whether it leaks has little to do with the glass and almost everything to do with the box it sits on and how the waterproofing is taken up and over it. That is our half of the job, and the half usually got wrong.

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Causes

Why rooflights leak

Almost always for one of four reasons, and each needs a different answer. Getting that wrong is how a rooflight gets replaced twice.

The kerb is too low. The raised frame the unit sits on does not lift the waterproofing far enough above the roof surface, so anything that slows the drainage puts water at the level of the detail. Rain that ponds for an afternoon reaches a joint a dry summer never would.
The upstand detail was never right. An upstand is the waterproofing turned up the side of the kerb, and it has to finish under the frame or under a flashing (a strip that sheds water off a joint). Cut short, held on with sealant, or turned up without reinforcement, and the corners go first: three planes meet there.
The unit itself has aged. Older polycarbonate domes go brittle and craze, gaskets harden and shrink, and a frame bedded on mastic thirty years ago is bedded on nothing now.
It is condensation, not a leak. Warm damp air on cold glass produces water that runs down the inside and drips in exactly the pattern a leak does. The tell is the weather: leaks come with rain, condensation with still cold nights and a busy kitchen below.

The work

What we actually do at a rooflight

Kerbs and detailing

The kerb decides the outcome, so it is the part we build or rebuild. A rotten timber upstand comes out, a low kerb is raised, a sound one is made good. The waterproofing then goes up the upstand and terminates where it belongs, corners reinforced rather than folded and hoped for. Where the geometry is tight or there is timber close by, the detail is finished in cold-applied liquid waterproofing: one cured piece, no joint, no flame near the frame.

Replacing a tired unit

A crazed dome or a failed double-glazed unit usually goes back on the kerb already there, once that kerb has been checked and made good. Where the opening is a non-standard size, which on a London extension it very often is, the kerb is adapted to a unit that can actually be bought rather than the building waiting for something bespoke. The waterproofing is reinstated in the same visit.

New openings

Forming a new opening during roof works

If a room below is dark and the roof above it is due work anyway, the two jobs belong together. During a refurbishment, a felt overlay or a full replacement the covering is already open, the access is already up, and a new kerb goes into the waterproofing sequence instead of being cut into a finished roof six months later. That order costs less and produces a better detail, because nothing is retrofitted around a cured membrane.

The kerb is set out first and the unit ordered against the measured opening, never the other way round, which avoids the usual outcome of a frame that nearly fits and a detail built to compensate.

Scope

What we do not do, said before the job rather than after

We are roofers. We build and repair kerbs, detail upstands, waterproof around openings and fit units into kerbs we have prepared. We are not structural engineers and we are not glaziers.

So where an opening needs a trimmer, a beam or any alteration to the structure holding the roof up, that is designed and signed off by somebody qualified, and the roofing follows their detail. Where a job needs specialist glazing, a walk-on unit engineered for loading or an opening mechanism, it goes to a firm that does that work. Both are normal on this kind of job, and both are named on site while the scope is still being agreed rather than discovered afterwards. In reverse: where another trade leads, we take the roofing package and leave the rest alone.

Questions

Frequently asked questions

Do you supply the rooflight itself?

We can order a unit to suit the opening, and often do, but the unit is a bought product. What we are responsible for is the kerb it sits on and the waterproofing that ties it into the roof. Where a job needs structural alteration or specialist glazing, that is a different trade and we say so on site rather than after the invoice.

Why does my rooflight only leak in certain weather?

Because two different things look identical from below. Wind-driven rain finds a low kerb or a tired seal and gets in from outside; condensation forms on cold glass in still, cold weather and runs down inside. The first is a waterproofing fault and we fix it. The second is a ventilation and heat problem, and sealing it harder makes it worse.

How high should the kerb be?

Higher than most of the ones we find. The upstand has to hold the waterproofing well clear of anything that can pond, drift or splash back, and a kerb built flush with the covering has no margin once an outlet (a roof drain) slows down. Where one is too low, raising it is part of the work rather than an extra.

It only drips in cold weather. Is that still a leak?

Probably not, and it is the commonest wasted call-out on this kind of work. Water that appears on still cold nights, and stops when the weather turns merely wet, is almost always condensation forming on cold glass and running down inside. Sealing the outside harder does not touch it and can make it worse by trapping more moisture. What helps is ventilation and heat in the room below. We will say which one you have before anybody buys materials, because the two need opposite answers.

Can a new opening be formed while the roof is being refurbished?

That is the sensible time to do it. The covering is coming off or being worked over anyway, access is already up, and the new kerb goes into the same waterproofing sequence instead of being cut into a finished roof later. Anything involving structural timbers is confirmed with whoever is responsible for the structure first.

Rooflight dripping, or a dark room under a tired roof?

Tell us what happens and in what weather, and send a photograph of the frame from below if you can. That usually separates a leak from condensation before anybody comes out.