What Is a GRP Fibreglass Flat Roof?

There are three working parts. The first is the resin, a liquid plastic that hardens by a chemical reaction once it is mixed with a catalyst. The second is the glass-fibre matting, a mat of short glass strands held together loosely, which gives the cured resin its strength. The third is the topcoat, a pigmented layer applied last that seals the surface and gives the roof its grey, black or sometimes green finish.

Is GRP the same as fibreglass roofing?

Yes. Fibreglass is the everyday name, and GRP is the trade name. Both mean the same laminate. The word "laminate" simply means layers bonded together, and on a roof the layers are matting and resin. Because the material is laid in place rather than rolled out, there are no factory seams for water to find.

Beneath the laminate sits the deck, normally plywood or a similar board, fixed to the joists. GRP does not span gaps by itself. It is a skin on a deck, and a poor deck makes a poor roof whatever the quality of the resin. Insulation, if the roof is a warm roof, sits under the deck or on top of it. The article on warm and cold flat roofs explains how the two layouts differ.

The skin is hard rather than flexible, which is both its strength and its limitation, as the later sections explain. The finish colour is pigment in the topcoat, so a scratch shows the same colour beneath, and a damaged patch can be rubbed back and recoated without a visible join in the layers.

Roof windows and rooflights set into a GRP roof are built into the laminate, so the person laying the roof needs the rooflight on site before the resin goes down.

The sequence matters more than the brand of resin. A sound job follows the same order every time.

What happens to the old roof first?

The old felt or covering is stripped, and the deck is inspected. Boards that are soft, stained or sagging come out. The deck must be dry. Resin laid on damp timber does not cure properly and can blister later. The fall, the slight slope that drains the roof, is set at this stage, with timber firrings (tapered strips fixed on top of the joists) if the deck is flat.

In what order do the layers go down?

Edge trims go on first, then the resin and matting are rolled out in sections, wetted through and pressed flat with a roller so that no air is trapped. A second coat of resin closes the surface. The topcoat is the last step and is laid when the laminate is hard enough to walk on. Each stage has to be done within the working life of the resin, which is why GRP is a job for dry weather.

Cold and damp slow the cure. Our guide on flat roofs in winter describes what cold laying does to any covering. A GRP roof laid in poor conditions may look fine for a season and then craze, which is why the roofer's forecast matters as much as the roofer's skill.

A tidy site matters here. Resin has a strong smell, and offcuts and roller sleeves must leave the site with the roofer. We tell the owner what happens each day.

On a GRP roof the trims do much of the waterproofing. A trim is a profile, in GRP, timber or metal, that finishes the roof at its edge. A drip trim carries a small kicked edge, so rainwater drops into the gutter instead of running behind the fascia. A GRP edge is formed by laminating up over a timber batten, and the laminate runs right to the lip. If the edge is short, water tracks behind it and rots the board below.

How does GRP handle a wall junction?

At a wall the laminate is turned up to form an upstand, a vertical lip, and sealed. A flashing, which is a waterproof strip bridging the junction, sits over the top. Because the laminate is hard, the upstand cannot flex as the wall and the roof move at different rates. That is why wall junctions need careful detailing, and why a cracked upstand is the classic GRP leak. The answer on what flat roof trims and upstands are names each part.

What about rooflights and pipes?

A rooflight kerb is laminated into the roof like an upstand, and a soil pipe or vent is sealed with a collar. The detail is more work than the open field of the roof, and a roofer who rushes it leaves the flaw.

Edges also decide how much insulation can go under the roof, because a thicker build-up raises the finished level and means taller upstands and deeper trims. That is why GRP detailing and insulation are planned together, and why a survey looks hard at the edges before it prices the middle of the roof. A trim that is bent in storage or cut short on site cannot be fixed once the laminate is down.

The three coverings behave differently, and no single one wins on every count. Felt is bitumen-based sheet, laid in layers and often bonded with a torch. EPDM is a synthetic rubber membrane, rolled out as a single sheet over the deck and bonded with adhesive. GRP is the cured laminate described above. The table gives the published lifespans, with each source named.

Which covering costs less to fit?

Cost guides differ. MyJobQuote gives EPDM "around £60 per square metre" (MyJobQuote, Oct 2026), and puts a felt roof at around £1,500 on average. For a replacement flat roof overall, the same guide shows £1,500 to £6,000, with an average of £2,400. These are national typical ranges; what you pay is set by roof size, access and the state of the deck. Prices are a poor guide to value on their own, because a cheap covering that is replaced twice costs more than one that lasts. For the deeper comparison, see the guide on what a flat roof replacement costs.

What is the practical difference?

GRP is hard, so it takes light foot traffic well and can be coloured. Felt is cheaper and easy to patch but ages faster. EPDM stays flexible, which suits roofs that move. Our post on EPDM, GRP or felt sets the choice out in more detail. Repairs differ as well: felt can be patched with a torch and a new piece, rubber with a glued patch or tape, and GRP by grinding back and relaying matting. Each repair needs a clean, dry surface, which rules out working in the rain whatever the covering.

A hard skin cannot flex. If the building moves, the laminate can crack at the weak line. That is the main reason GRP roofs fail, and it is the first thing to ask about before agreeing to the material. A rear extension on poor foundations or a deck that bounces underfoot is a poor candidate.

Why does a GRP roof crack?

The usual causes are deck movement, thin laminate at corners, and thermal movement between the roof and the wall. Hairline cracks tend to start at the edges and at joints in the boards beneath. Our answer on why a GRP roof cracks goes through the signs. Once a crack lets water under the laminate, it spreads along the deck.

Can a damaged GRP roof be repaired?

Yes, in many cases. A cracked area can be ground back, fresh matting and resin laid over it, and the topcoat renewed. A repair patch, however, rarely matches the original colour, and the deck beneath it has to be sound for the repair to hold. Timber that has sat wet for months will not hold a repair. Where the cracking runs right across the roof, a fresh covering beats a string of patches.

GRP also does not suit every shape of roof. Complex plans with many small steps and valleys are slow to lay, and the number of joints increases the risk of failure. A roofer who is honest about the limits of the material will say when felt or EPDM is the better choice for the roof in question. Resin also gives off fumes while it cures, so neighbours and open windows need thought on the day.

GRP suits small to medium roofs with a stable, dry deck and few awkward junctions. A garage, a porch, a dormer top or a rear extension that is not in poor condition are the sort of roof where GRP is often chosen. It is also chosen where the owner wants a hard, walk-on surface in a colour of their choice.

When would we suggest something else?

If the deck moves, if the building has settled, or if the roof is a complicated shape, we would look at EPDM, which tolerates movement. If the budget is tight and the roof is a shed or a store, felt may be the practical answer. The choice should follow what the survey finds. The reasoning comes with pictures of the deck and edges, so the decision rests on evidence rather than a sales pitch.

Does a new GRP roof need Building Regulations approval?

It can. A new waterproof covering on a flat roof is treated as renovation under Part L, and the insulation may have to be improved at the same time. Our guide on flat roof replacement and Building Regulations goes through the test in full. Somerset Building Control, on 0300 303 7790, answers questions about a particular address. Planning permission is a separate question again.

If you are weighing GRP against another covering for a garage or an extension, the flat roofing page shows what we lay on house roofs, and a survey gives you a written price for the covering that suits your deck.