GRP fibreglass roofing
In short
GRP is laid wet and cures into a single seamless laminate over the whole roof, including the upstands and trims — so there are no joints anywhere, not even at the edges. It suits complicated shapes and roofs that are on show, and it needs dry weather and a properly prepared deck to go down correctly.
Also called: Fibreglass roof · GRP roofing · Glass reinforced plastic roof · Fibreglass flat roof
Signs you need this
- A complicated flat roof shape with several upstands or penetrations
- A flat roof visible from windows above, where finish matters
- A failed felt or asphalt roof needing full replacement
- A balcony or walkable flat area needing a non-slip finish
- Cracking in an existing GRP roof over a board joint
GRP — glass reinforced plastic, the same material as a boat hull — is applied as chopped strand mat saturated with resin, which cures into one continuous rigid laminate. Unlike a sheet system it forms itself around whatever shape the roof is, so upstands, corners, kerbs and outlets are all part of the same seamless skin rather than separate details bonded on.
That is its real advantage. On a roof with a lot going on — a dormer cheek, a parapet, several penetrations, an awkward junction against a wall — GRP handles the geometry in a way that sheet materials have to be coaxed into. It also takes purpose-made GRP trims at the edges, which are bonded into the laminate itself, so the perimeter is a continuous part of the roof rather than a mechanical joint.
It finishes better than most alternatives, too. The topcoat is pigmented and comes in a range of colours, and it can be laid with an anti-slip aggregate for a walkable surface. On a flat roof overlooked from a bedroom window — which describes most domestic extensions — that matters more than people expect.
The demands are real, though. GRP needs a dry, clean, properly prepared deck of the right material: OSB3 board is standard, and it must be dry, because resin will not bond to damp timber. It needs reasonable temperatures and dry weather to cure, so it is not a job to force through in November. And it is applied by hand, so it is more dependent on the installer's care than a factory-made sheet is.
Its one genuine weakness is rigidity. A cured GRP laminate is stiff, and a deck that moves a lot underneath it — a springy, under-specified or poorly supported deck — can crack it, usually at a stress point over a joint in the boards. That is a deck problem rather than a material problem, and it is why the deck specification and the noggin support are not optional detail.
As with every flat roof, falls decide the outcome. A finished fall of at least 1:80 is the requirement and 1:40 is the sensible design target, because timber and workmanship both eat into it. GRP will not leak under standing water, but a ponding roof is still holding weight, growing algae and hiding what is going on underneath.
What the job covers
- Old covering stripped and the deck fully inspected
- New OSB3 deck laid where the existing one is unsuitable or damp
- Falls formed with firrings or tapered insulation
- Bonded GRP edge and drip trims, laminated into the roof
- Single seamless laminate over field, upstands and details
- Pigmented topcoat, with anti-slip aggregate where wanted
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your roofer has to honour it. On roofing it would also be close to meaningless: access is often the biggest single factor, and it varies enormously house to house. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
Can GRP be laid at any time of year?
No, and beware anyone who says otherwise. Resin needs dry conditions and reasonable temperatures to cure properly, and it will not bond to a damp deck. A GRP roof pushed through in wet or cold weather is the commonest reason one fails early. Winter jobs are better done with EPDM or postponed.
My fibreglass roof has cracked. Why?
Usually deck movement rather than the material. A cured laminate is rigid, so if the boards beneath it flex — under-supported, wrong board type, or joints without noggins — the laminate cracks over the moving joint. The repair is a proper laminated patch, but the deck needs sorting or it happens again.
Is it better than rubber?
It is better for complicated shapes, for roofs that are on view, and for anything that needs a hard walkable surface. EPDM is quicker, cheaper on a simple rectangle, more tolerant of movement, and can go down in weather that GRP cannot. Neither is universally better — the roof decides.
Can it go over the existing roof?
No. GRP requires a sound, dry, correctly specified deck, and laying it over old felt means bonding a rigid laminate to a moving, damp substrate. It will fail. The old covering comes off and the deck gets inspected, which is also the only way to find the rot that a failed flat roof usually has underneath it.
Need this doing in Macclesfield?
Tell us the job and it goes straight to our local roofer — one call back, usually the same day. Free quote, no obligation.