Can you overlay a cracked rooflight?

Most commercial rooflights fall into two families. On industrial cladding, factories, warehouses, retail sheds, they are profiled sheets of glass-reinforced plastic (GRP) or polycarbonate, bedded into the metal roof sheeting on the same profile so the corrugations line up. On flat and low-pitch roofs they are moulded domes or upstand-mounted units sitting on a builder’s kerb. An overlay takes either of those and fastens a fresh skin straight over the top: a new profiled sheet screwed down onto the old run, or a new dome bonded and clamped over the cracked one.

You will see it sold as a retrofit or an over-roofing measure, and the pitch is always the same. Nobody has to open the roof, nobody works below the covering, and the building stays weathertight through the afternoon. That is a genuine attraction when the roof is high, occupied and awkward to close off. It is worth being clear about what you are buying, though, because an overlay and a replacement are not the same product with a different price. One buries the problem. The other removes it.

A crack rarely fails on its own. By the time a rooflight has split, the plastic has usually gone brittle across the whole sheet, the fixing holes have opened up under years of thermal movement, and the sealant line at the edges has hardened. An overlay touches none of that. The old panel stays where it is, and everything that was going wrong with it carries on going wrong out of sight.

Then you have the cavity you have just built. Two skins with an unventilated gap between them will trap moisture, and any water that has already tracked past the crack has nowhere to dry to. What follows is misting, algae and staining across a surface you can no longer reach to clean. From the floor it reads as a permanently dirty rooflight, because effectively it is one, sealed behind a second layer.

There is a structural side as well. A new sheet plus its fixings and any trapped water adds load and wind uplift to a junction that was specified for a single skin. On a large industrial roof that is worth a proper look before anyone starts drilling, because the failure you least want is a cracked rooflight and its overlay both letting go together in a gale. If a panel has cracked from a one-off impact and the run around it is otherwise sound, keeping the building watertight in the short term is a job for a roofing contractor. Bringing the light quality back is a different question, and that is the one worth thinking about properly.

Old commercial rooflights degrade in a way that matters most where the sun is strongest, and Southend faces south across the Thames Estuary with an open southern horizon and reflected light off the water on top. GRP yellows and clouds as it ages. It keeps letting solar heat through while scattering the daylight into flat, harsh glare, so a warehouse or a rooflit office ends up hot and badly lit at the same time. Overlaying a second sheet of plastic over the first does very little for either. You now have two layers of ageing material between the sky and the floor, cutting the useful daylight further while the heat keeps arriving.

This is the part an overlay quietly gets wrong. The number that governs summer heat is the G-value, the share of the sun’s energy that passes through the glazing and lands in the room as warmth. Clear plastic and clear double glazing sit high, around 0.6, which is fine facing north and punishing facing south. You cannot dial that number in with an overlay, because you are stuck with whatever the old sheet already is. Lift the cracked unit out and fit a specified replacement, and the G-value becomes a choice: solar-control glazing down around 0.3 for the south-facing pitch, ordinary clear where the roof faces away from the sun and the free winter warmth is welcome. Choosing that figure deliberately is the whole basis of our energy-efficient skylight installation work, and it is exactly the lever an overlay throws away.

Replacing a failed rooflight is a replacement, and that is the work we do on commercial roofs. Rather than cover the crack, the cracked unit comes out, the upstand or the cladding junction gets checked while it is open, and a new unit goes in, sealed and fastened to current standards. That is the moment the specification is genuinely open. You can change from clouded GRP to clear or solar-control glazing, set the daylight and the heat where you want them, and know the seal and the fixings underneath are new rather than a decade past their best.

It is the more involved job, and it needs the area closed off safely while the roof is open, so it is planned rather than done in an hour. What you get back is a rooflight that performs like a new one instead of a tired panel hidden behind a fresh face. Our commercial skylight installation page sets out how that runs across profiled and flat commercial roofs, and where glazed units make sense against a like-for-like polycarbonate rooflight swap.

Start by working out whether you are looking at one cracked panel or a whole run reaching the end of its life. A single impact crack in an otherwise young roof is a narrow problem. A cracked, yellowed sheet among others that are clouding over is a sign the run is due for replacement, and an overlay only delays that while making the daylight worse in the meantime. Note which way the roof faces, measure the openings, and treat the crack as the prompt to put the specification right rather than paper over it.

If you would like us to look at it, tell us the roof type, the rough size of the rooflights and roughly how many are involved, and we will come back with a specification and a price. Ring 01702 898232 or request a quote, and we will assess the whole roof rather than just the panel that has cracked. Our skylight installation costs page gives you a sense of the numbers before you call.