Why we don't recommend GRP fibreglass flat roofs

If you’ve been quoted for a new GRP (fibreglass) flat roof, you need to read this before you sign anything. GRP gets marketed as a long-life, premium roofing system. In our 15+ years on the tools, the reality is something else.

We don’t fit GRP. Here’s why.

What GRP actually is

GRP stands for Glass Reinforced Plastic. It’s made by laying glass fibre matting over plywood decking, then saturating it with a liquid resin (the same family of resin used to build boat hulls). The resin sets rigid. A topcoat is rolled on for UV protection and colour.

That’s the appeal: a hard, smooth, monolithic surface. Looks great on day one.

Why GRP fails

The problem is that a rigid plastic shell over a flexible building has a fundamental mismatch. Buildings move. Timber decks expand and contract with the weather. Walls shift. Doors and windows close and reopen at the upstands.

GRP can’t move with them. So it does one of three things:

1. Cracks

The most common failure. Hairline cracks appear along stress lines — usually around upstands, at deck joints, and along the edges. Water finds the cracks. Once water is in, it sits between the GRP and the timber deck, accelerating rot.

2. Delaminates

The bond between resin and timber breaks down. You see it as bubbling on the surface. Eventually whole panels come away from the deck. Repairs in GRP are notoriously difficult because the resin doesn’t bond reliably to old, weathered GRP.

3. The topcoat wears off

The colour you see on a GRP roof is a thin resin topcoat. UV degrades it. Without the topcoat the glass-fibre weave shows through and water absorption accelerates.

How long does GRP actually last?

Manufacturer warranties on GRP are typically 10 years. In the field — on the roofs we see — failure between 10 and 15 years is the norm, sometimes sooner. We strip out a lot of GRP roofs that aren’t even old enough to drink yet.

For comparison, modern single ply membranes are tested to 40+ years and carry manufacturer warranties up to 25 years.

What we fit instead

Almost every roof we install is single ply membrane:

• A flexible, factory-made PVC, TPO or FPO membrane
• Heat-welded at the seams — no resin, no glue
• Moves with the building, doesn’t crack
• 40+ year life, 20-year manufacturer-backed warranty as standard
• The same system fitted on Wembley, the Emirates and the Google Data Centre

“Chris at Skyguard was brilliant from start to finish. He came out quickly to look at my leaking GRP roof and explained everything clearly. The work was done to a really high standard. I finally have peace of mind knowing it won’t be leaking again.” — Luke Greaves, Google review

See our single ply roofing guide for the full technical detail, or use the estimator to price up a single ply replacement on your roof.

What if my GRP roof is already failing?

This is most of what we get called out to. The honest answer:

• Localised damage on a young GRP roof (under 8 years) — sometimes worth a single-ply patch repair. We weld a small piece of single ply membrane over the damaged area to seal it.
• Multiple failure points or older GRP — replacement is usually the better call. Patching is throwing money at a roof that’s going to fail elsewhere within a year or two.

We’ll give you an honest answer at survey stage. If repair is the right call, we’ll quote it. If replacement is the only sensible option, we’ll tell you that too.

Bottom line

GRP fibreglass roofs look great until they don’t. They fail early, fail unpredictably, and are difficult to repair properly once they start to go.

If you’ve been quoted GRP and you want a second opinion, give us a call — or use the estimator and we’ll cost up a single ply alternative for the same roof.

We don’t fit GRP. And we wouldn’t put it on our own house.