GRP Fiberglass Flat Roofing: What It Is, How Long It Lasts and What It Costs in Sheffield

AJR Roofing — Sheffield & South Yorkshire

If you have a flat roof on an extension, garage, dormer or bay window, you’ve probably heard of GRP. It’s one of the most popular flat roofing systems in the UK right now — and for good reason. But what exactly is GRP fiberglass roofing, and is it the right choice for your property?

This guide covers everything you need to know: how GRP works, how it compares to older systems, how long it lasts, what it typically costs in Sheffield, and when to call a roofer.

What Is GRP Fiberglass Flat Roofing?

GRP stands for Glass Reinforced Plastic. It’s a composite material made by layering fibreglass matting over a flat deck and sealing it with liquid resin. Once cured, it forms a single, seamless, waterproof surface — with no joins or seams where water can get in.

It’s the same material used to make boat hulls, water tanks and car body panels. That should give you a sense of how tough it is.

Unlike traditional felt flat roofing, GRP doesn’t crack, shrink or bubble in extreme temperatures. And unlike EPDM rubber, it’s completely rigid — so it won’t lift or peel over time.

How is GRP installed?

The process is straightforward when done by an experienced roofer:

1. The existing roof deck is prepared or replaced (usually with 18mm WBP plywood)
2. Fibreglass matting is laid over the deck
3. Catalysed resin is applied with rollers and worked into the matting
4. Once cured (usually within a few hours), a topcoat is applied for UV protection and colour
5. Trims and upstands are finished in GRP to seal the edges

The whole installation is typically completed in a single day for a standard garage or extension roof.

How Does GRP Compare to Other Flat Roof Systems?

There are three main flat roof systems used in the UK today. Here’s how they compare:

GRP is generally the preferred choice for smaller residential flat roofs — especially extensions and garages — because of its seamless finish, long lifespan and very low maintenance requirements.

How Long Does a GRP Flat Roof Last?

A properly installed GRP flat roof should last 25–30 years. Some manufacturers claim 40+ years for their systems, and there are GRP roofs installed in the 1990s still performing well today.

The main factors that affect lifespan are:

• Installation quality — GRP needs to be mixed and applied correctly. Poor technique leads to delamination and cracking.
• Deck preparation — the plywood deck must be dry, flat and properly fixed before GRP is applied.
• Upstand height — upstands around edges and abutments should be at least 150mm to prevent water ingress.
• Drainage — flat roofs need adequate fall (at least 1:80) so water doesn’t pond.

Routine maintenance is minimal. A clean once or twice a year to remove debris is usually all that’s needed.

What Does GRP Flat Roofing Cost in Sheffield?

Prices vary depending on roof size, accessibility, deck condition and whether the existing structure needs repair. As a general guide for Sheffield:

These figures include materials, labour and standard edge trims. They assume the existing deck is in reasonable condition. If the plywood needs replacing, add roughly £15–£25 per m².

Always get at least two quotes, and check that the installer is experienced with GRP — it’s a specialist system and poor installation can void any guarantee.

Does GRP add value to my property?

A well-installed GRP roof can improve your home’s appeal to buyers, simply because it removes the concern of a failing flat roof. Estate agents and surveyors often flag old felt flat roofs as a negative — a GRP replacement removes that flag entirely.

Is GRP Suitable for All Flat Roofs?

GRP is suitable for the vast majority of residential flat roofs. It works especially well on:

• Rear extensions and kitchen extensions
• Garage roofs
• Dormer roofs
• Bay window roofs
• Porch roofs
• Flat-roofed bungalows

It’s less suitable for very large commercial flat roofs (where EPDM or a warm roof build-up may be more cost-effective), or for roofs with complex penetrations (pipes, skylights) where flexibility may be an advantage.

If you’re not sure whether GRP is right for your roof, the best approach is to have an experienced roofer take a look and advise. At AJR Roofing, we’ll give you an honest assessment — not just tell you what you want to hear.

Signs Your Flat Roof Needs Replacing

Whether you have an old felt roof or an ageing GRP system, watch out for these warning signs:

• Water stains or damp patches on the ceiling below
• Bubbling, cracking or splitting on the roof surface
• Ponding water that doesn’t drain within 24–48 hours after rain
• Visible gaps or lifted edges around upstands and trims
• Mould or moss growth on the surface
• The roof is over 15 years old and hasn’t been inspected recently

If you spot any of these, it’s worth getting the roof checked sooner rather than later. A small repair now is nearly always cheaper than dealing with water damage inside later.

Why Choose AJR Roofing for GRP Flat Roofing in Sheffield?

We carry out GRP flat roof installations across Sheffield and South Yorkshire — from small bay window repairs to full garage and extension roof replacements. We use quality GRP systems with manufacturer guarantees, and we’ll always explain the work clearly before we start.

We’re fully insured and cover Sheffield, Rotherham, Doncaster, Chesterfield and surrounding South Yorkshire areas.

Frequently Asked Questions

How long does GRP flat roof installation take?

Most standard garage or extension roofs are completed in a single day. Larger or more complex roofs may take two days. Weather needs to be dry — GRP resin shouldn’t be applied in rain or very cold temperatures (below 5°C).

Can GRP be installed over an existing felt roof?

Usually, no. The existing felt and deck need to be stripped back to check the structure and ensure the new plywood deck is dry and sound. Installing over a damp deck is one of the most common causes of GRP failure.

Will GRP crack in cold weather?

Properly installed GRP is flexible enough to handle normal thermal movement. However, it can crack if the deck moves excessively — which is why proper deck fixings and a good fall are important.

Is GRP the same as fibreglass?

Yes — GRP (Glass Reinforced Plastic) and fibreglass are the same material. GRP is the more technical term used in the roofing industry.

Does GRP need planning permission?

In most cases, like-for-like flat roof replacements don’t need planning permission in Sheffield. If in doubt, check with Sheffield City Council or speak to your roofer before work begins.