Why GRP is the cowboy roofer's favourite product (and why that matters)

There’s a question we get asked all the time: “Why do so many roofers still fit GRP if it’s so bad?”

The honest answer isn’t flattering to the trade. GRP is the cowboy roofer’s favourite product — not because it’s the best system for British weather, but because anyone can buy it, anyone can fit it, and by the time it fails the installer has long gone.

This isn’t us being snobby about other roofers. It’s a structural problem with how GRP is sold and supplied in the UK — and it’s the single biggest reason we keep getting called out to strip GRP roofs that aren’t even old enough to drive yet.

Anyone can walk in and buy GRP

Go to your nearest builder’s merchant. Ask for a GRP roof kit. They’ll sell you one. No checks, no training certificate, no proof you know what a primer is or how to detail an upstand. Walk out with rolls of glass-fibre matting, a tub of resin, a topcoat tin and a brush.

You can fit a GRP roof at the weekend. People do. Then they leave a Google review saying the roofer was “quick and cheap” — right up until winter when the cracks appear.

You cannot walk in and buy single ply

Try the same thing with single ply. Walk into a merchant and ask for Sika Sarnafil, or Protan, or IKO Armourplan, or Renolit Alkorplan.

You’ll be turned away.

Manufacturers of single ply membranes (the proper PVC, TPO and FPO systems we install) do not sell to untrained contractors. To buy any of those systems — just to put them in a van — you have to:

• Attend the manufacturer’s technical training course
• Pass a competency assessment on welding, detailing and build-up
• Hold a valid manufacturer ID number that the merchant checks every time you order
• Renew that certification periodically as standards update

That’s how Sky Guard ended up certified across seven different manufacturer systems — Sika, Protan, IKO, BMI Icopal, Fatra, Alwitra and Renolit. Each one took its own training programme. Most contractors give up after one.

“We don’t fit GRP. It’s not in our van because we wouldn’t put it on our own house. And the manufacturers we’re trained with wouldn’t put their warranty on it either.” — Chris, Sky Guard

Why GRP’s “easy” supply chain is the problem

If anyone can buy and fit it, then most of the GRP roofs being installed in the UK right now are being fitted by people who shouldn’t be. Common installation mistakes we strip out every month:

• Wrong mix ratio on the catalyst — resin too soft (won’t cure properly) or too hot (cracks within months as it cures)
• Application in the wrong weather — GRP needs dry, mild conditions. We’ve seen it laid in a damp October because the customer pushed back the date
• No primer or wrong primer on the deck — the GRP delaminates within a year
• Insufficient overlap on the matting — failure points along every seam
• Wrong topcoat thickness — UV burns straight through within 5 years
• Detailing copied from a YouTube video — especially at upstands, parapets and rooflight surrounds, which is where 80% of failures start

Each of those is something we’d have to fix at a strip-out. Each one is invisible on day one. Each one shows up in winter two or three years later, by which point the “cheap” price has cost the homeowner the whole roof.

The warranty trap

Cowboy installers often offer a warranty. Sometimes 10 years. Sometimes 20.

The warranty is only worth as much as the company offering it. A one-man-band fitter who’s out of business by the time the leak appears can’t honour anything. There’s no manufacturer behind the warranty because GRP manufacturers don’t require accredited installers, so they don’t back the install in the way a single ply manufacturer does.

When we install single ply, the manufacturer registers the warranty directly with you. Sika, Protan, IKO — you get a document signed off by the manufacturer, valid for 20 or 25 years. If something fails within that period, the manufacturer will honour it even if we go out of business. That’s the whole point of the accredited-installer model.

The British climate makes it worse

Even a perfectly installed GRP roof has problems in UK weather. GRP is rigid. Buildings move. Decks expand and contract. UK temperatures swing from -5°C in February to +30°C in July. That’s a 35°C swing on a rigid plastic shell glued to a timber deck.

It cracks. Always. Eventually.

Now imagine that same GRP roof was fitted at the wrong temperature, with the wrong mix, by someone who learned the trade off YouTube. The clock starts ticking on a leak the day it’s laid.

Single ply — PVC, TPO, FPO membranes — is engineered to flex with the building. Heat-welded seams that are stronger than the membrane itself. Manufacturer-spec build-ups designed for UK temperatures. 40+ year membrane lifespan tested in conditions far harsher than South Yorkshire.

How to spot the cowboy

If you’re getting flat-roof quotes right now, ask the contractor these questions:

1. “Which manufacturer are you accredited with?” A single ply specialist will name the system without hesitating. A GRP-only fitter usually can’t.
2. “Can I see the manufacturer’s warranty document?” Not the contractor’s — the manufacturer’s.
3. “Where do you buy the materials from?” If it’s a builder’s merchant, walk away.
4. “Can I see three roofs you fitted more than 5 years ago?” A good roofer can. A cowboy can’t.
5. “What warm-roof insulation do you specify?” A proper roofer will quote a U-value. A cowboy will mumble about “some insulation”.

If they fail those questions, get another quote. If you’re anywhere in South Yorkshire, give us a call — we’ll come and survey it for free.

Bottom line

GRP isn’t a bad material in a lab. GRP is a bad material in the field, because the supply chain that puts it on UK homes is full of unaccredited installers using off-the-shelf kits in conditions the product was never designed for.

We don’t fit it. We strip out a lot of it. And we’d rather lose a job for being more expensive than win one for being a cowboy.

For an honest price on a properly fitted single ply roof, use the estimator — you’ll have a ballpark range in 60 seconds.

Related reading

• Why we don’t recommend GRP fibreglass flat roofs — the original full breakdown
• GRP vs Single Ply Roofing — which is better? — the side-by-side comparison
• Single Ply Roofing service guide — what we actually fit and why