GRP Fibreglass Roofing in South London

Four things go down, in order, and each does a different job.

GRP earns its keep on roofs that get walked on and roofs with a lot going on. Balconies, roof terraces, small rear extensions with a rooflight, dormer tops, bay roofs, anything with pipes and corners and an abutment on two sides. Because it is laid wet, it follows shapes that a sheet material has to be cut and joined around, and the finished surface is hard enough to take foot traffic without a paving layer over it.

It is the wrong choice in three situations, and we will say so rather than sell it to you. A large plain rectangle — a whole garage block, a flat over a shop — is better and cheaper in EPDM rubber, which goes down in one sheet. If you want to keep an existing sound covering rather than strip it, a liquid system overlays it without the tear-off. And on a Victorian valley gutter behind a parapet, the sort that runs down the middle of a butterfly roof in SE5 or SE22, we would generally line it in lead. That gutter moves with the building. Lead moves with it. Cured GRP does not move at all, and something has to give.

Fibreglass is rigid. That is its strength on a walkable roof and its weakness everywhere else, because it cannot absorb movement — it can only crack. So the deck under it has to be still. New OSB3 screwed to sound joists is still. Old boards that flex when you stand on them are not, and no amount of resin over the top will change that.

The other thing we cannot know from the ground is what is under your existing covering. Rot under an old felt roof is common and invisible until the felt is off, because felt fails at a seam and the water then tracks sideways through the boards. When we open a roof up and find soft timber, we photograph it and ring you before we go any further. Nobody should find out about a new deck from the final invoice.

Two more things get set at this stage. The fall, because standing water shortens the life of any flat roof and GRP is no exception — BS 6229 has flat roofs designed to a fall of 1:40 so that they finish at no less than 1:80 after everything has settled. And the insulation: building regulations treat a flat roof covering as a thermal element, so replacing most of one usually means bringing it up to around 0.18 W/m²K. In practice that means a warm deck — insulation board over a vapour control layer, then the OSB, then the GRP. It adds height at the edges and cost to the quote, and it stops the deck sweating from underneath.

Resin cures by chemical reaction, not by drying. It is mixed with a catalyst on the roof, and the amount of catalyst changes with the temperature — more when it is cold, less when it is warm — but either way the mix is workable for about fifteen minutes. So the roof is laid in bays, wet edge into wet edge, and once a bay is started it gets finished.

That is why we watch the forecast. The deck has to be dry, and the day has to be above about 5 °C for the resin to go off properly. A wet fortnight in February moves your date; it does not usually move your price. If you need a flat roof covered during a bad spell, EPDM will go down in weather that GRP will not, and we would rather tell you that than lay a roof that never cures properly.

The upside of a cold-applied system is that there is no gas torch on your roof. No naked flame near a timber fascia, a neighbour's fence or a loft full of insulation, and no hot-works risk to argue about with an insurer. On a terraced street where the roof is a metre from someone else's woodwork, that matters more than it sounds.

When we are called to a failed fibreglass roof, it is almost always one of a short list, and almost never the resin itself.

One genuinely good thing about fibreglass: it repairs into itself. Grind the damaged area back, key the surface around it, lay fresh mat and resin over the top and re-coat, and the repair becomes part of the original laminate rather than a patch sitting on it. A split at an upstand or a cracked board joint is usually half a day, not a new roof.

On lifespan, treat the advertising with some caution. Manufacturers quote thirty years and more; the independent certificates behind those systems are worded more carefully. Twenty-five to thirty years is what we would plan around for a properly built roof, with a topcoat re-coat somewhere in the middle of that if it faces south. A GRP roof that fails at five years did not fail because of the material.

If your existing fibreglass roof is crazed but sound, ask us about a re-coat before you ask us for a replacement. It is a fraction of the money and we would rather do the honest job than the big one.