Flat Roofs
Flat roofs on garages, extensions and dormers, laid on a deck that has been checked first.
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GRP fibreglass flat roofs in Windsor, Berkshire and South West London, laid over a sound new deck on extensions, dormers, garages and balconies. Book a free roof inspection and you get photos of your roof and a fixed written quote before any work starts.
GRP stands for glass reinforced plastic, the same material used to build boat hulls. On a roof it is laid wet: a coat of resin, a layer of glass fibre matting rolled into it, more resin, then a coloured topcoat. It cures into one hard, continuous skin across the whole roof, with no laps or seams for water to work into.
That makes GRP the strongest choice for some roofs more than others:
It is applied cold, with no naked flame, which suits roofs close to timber cladding or windows. And with pre-formed trims at the edges and drips, the finished roof looks sharp from the garden.
GRP is only as good as the deck it is bonded to. That is where every GRP roof we lay starts.
The old covering and deck come off and the joists are checked. Rotten timbers are cut out and replaced.
A new OSB deck is laid and fixed, with the falls set so water runs to the outlet instead of ponding.
Edge trims fixed, then resin and glass mat laid wet across the deck and rolled out to bond with no air trapped.
Once the laminate cures, a coloured topcoat seals and protects it from the weather and from UV.
Because GRP is rigid, it has no give. If the boards underneath are soft, damp or moving, the laminate cracks along the joints between them, and those cracks are where the leaks start. Laying GRP over an old, tired deck is the commonest reason a fibreglass roof fails early.
So we do not do it. Every GRP roof we quote starts with a new, dry deck: OSB boards laid tight, fixed down and set to the right falls. Any joist that has softened is replaced in treated timber first. On a roof being rebuilt from scratch, it is also the right moment to add insulation between the joists.
The photos here show that stage on our Chatham flat roof replacement, where the old deck was saturated and several joists had rotted. That roof was finished in torch-on felt, but the strip-out, the timber repairs and the new SterlingOSB Zero deck are the same groundwork every GRP roof needs.
Our flat roof cost guide puts a single garage roof in GRP fibreglass at £1,500 to £2,600 for 2026, and an extension roof of 15 to 25 square metres at £2,200 to £4,000. The guide recommends GRP where the roof doubles as a balcony or is walked on. Our new roof cost guide gives GRP a guide rate of £90 to £130 per square metre and a lifespan of 25 to 30 years.
GRP costs more than felt and a little more than EPDM. You are paying for a rigid, walkable surface and a crisp finish around complex details. On a plain, rectangular garage roof that nobody walks on, EPDM rubber is the better buy, and we will tell you so.
The price moves with the size of the roof, how much deck and timber has to be replaced, and access. Every quote is written and fixed after the free inspection.
A full flat roof replacement on a rear extension in Chatham, Medway. Stripped to the joists, rotten timbers replaced, insulation fitted and a new SterlingOSB Zero deck laid. It was finished in a three-layer felt system; the groundwork underneath is exactly what a GRP roof is laid on.
Tell us about the roof, garage, extension, dormer or balcony, and we will book a free inspection and give you a fixed written quote. Prefer to talk first? Call us any time.
Tell us about the job, we usually reply within the hour.
Prefer to talk? Speak to Cornelius on 01753 330566 or
Expert Roofer Verified member“Seal Pro Roofing did a superb job on our roof replacement. They were punctual, tidy and kept us informed throughout. The workmanship was excellent and the price was fair. Very impressed with their customer service and would happily use them again in future.”
“Seal Pro Roofing delivered a great service. The team were knowledgeable and worked carefully to ensure everything was done properly. The roof looks solid and well finished. Very happy with the professionalism shown.”
Flat roofs on garages, extensions and dormers, laid on a deck that has been checked first.
Flat roofs →
A single rubber membrane with no seams across the main roof, laid with no naked flame.
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Splits, blisters, ponding and failed upstands repaired, with an honest answer on repair or replace.
Flat Roof Repair →Free written quotes. 24hr emergency callout. A local team rated 5.0 by 150+ homeowners on Bark.