Is GRP better than felt for a flat roof?

For most domestic applications in Wiltshire, yes. GRP is seamless, UV stable, carries a 25-year material guarantee, and doesn't suffer the joint failures common in felt. The installed cost is similar — typically £1,200–£2,500 for a standard extension flat roof in either system. The difference is: GRP is a permanent solution, re-felting is a medium-term repair.

How long does a GRP flat roof last in Wiltshire?

GRP fibreglass with a 25-year material guarantee regularly exceeds 40 years in service when correctly installed. The critical factors are deck preparation, correct resin formulation for ambient temperature, and correct upstand detailing. Felt, by comparison, has a 15–25 year design life before joint failures, blistering and delamination become active.

Is EPDM rubber roofing better than GRP?

EPDM is better suited to large or complex roof shapes where significant thermal movement is a factor — typically roofs over 40m² or with complex geometry. For standard domestic extensions up to 25m², GRP is our first recommendation: it's stiffer, more resistant to foot traffic, and produces a neater finish on standard rectangular roof shapes.

How much does GRP flat roofing cost in Chippenham?

GRP fibreglass on a standard domestic extension up to 20m²: £1,200–£2,500 including strip, deck inspection, drainage correction and installation. Re-felting a similar roof costs £600–£1,400 — lower upfront but with a shorter service life and higher probability of requiring repeat work.

Can GRP be installed over existing felt?

No — GRP must be applied to a clean, dry deck. We always strip the existing felt before GRP application. Installing GRP over existing felt traps moisture against the deck, prevents the GRP from bonding correctly, and voids the material guarantee.

Does felt flat roofing ever make sense?

Modern high-performance torch-on felt (SBS-modified bitumen) is a legitimate flat roofing system with a 15–20 year lifespan when correctly installed. It's more appropriate than GRP where: the roof area is very large and cost is the primary constraint; the roof has a complex curved profile that GRP can't follow; or the client specifically requires felt for budget reasons. We don't dismiss felt — we advise on the right system for each situation.

Does flat roofing need planning permission in Wiltshire?

Like-for-like flat roof replacement doesn't require planning consent. New flat roof extensions on listed or conservation area properties may need prior approval. For most Chippenham domestic properties, re-roofing an existing flat roof is permitted development. We confirm planning status during the free survey.

GRP vs Felt Flat Roofing — Which Is Better?

The Short Answer

For the vast majority of domestic flat roofing in Chippenham and north Wiltshire — rear extension roofs, garage roofs, bay top roofs — GRP fibreglass is the better choice at a similar installed cost. It lasts significantly longer, is seamless, and doesn't suffer the progressive joint failure that ends every felt flat roof's useful life.

There are specific circumstances where felt or EPDM is the right answer — large and complex roof geometries, very large commercial flat roofs, and budget-constrained situations. But for the standard 15–25m² domestic extension flat roof in Pewsham, Hardenhuish, or Calne, GRP is our standard recommendation.

Head-to-Head Comparison

Factor	GRP Fibreglass	SBS Felt (Torch-On)	EPDM Rubber
Material guarantee	25 years	10–15 years	20 years
Realistic lifespan	40+ years	15–25 years	30–50 years
Seams/joints	Seamless	Multiple seams	Taped seams
Typical installed cost (20m²)	£1,200–£2,500	£600–£1,400	£900–£1,800
Foot traffic resistance	Excellent	Moderate	Good
Complex shapes	Good (to 40m²)	Excellent	Excellent
Temperature sensitivity	Must cure above 5°C	Good in cold	Good in cold
Maintenance	Minimal	5–10 year inspection	Minimal

Why Most Wiltshire Extensions Have Failing Felt

The 1990s–2000s housing boom in Chippenham — Pewsham, parts of Hardenhuish, Royal Wootton Bassett's development estates — produced thousands of single-storey rear extensions with felt flat roofs. In 2026, those roofs are 25–35 years old. Standard bitumen felt has a design life of 15–25 years. Most of these roofs are at or past their end of life.

The failure mode is predictable: felt blisters from solar heat, joints open between sheets, drainage falls change as the felt membrane moves, and ponding water accelerates the process. By the time water is coming through the kitchen ceiling, the membrane has usually been deteriorating for 3–5 years. Patching the surface temporarily extends the symptom while the underlying failure continues.

How GRP Works and Why It Lasts

GRP (Glass Reinforced Polyester) flat roofing is applied as a liquid and cures to a rigid, seamless surface — there are no joints to open, no sheets to delaminate, and no seams for water to track along. The process:

Existing felt stripped to the deck
Deck inspected — rotten or damaged boards replaced
Drainage falls corrected where necessary
GRP chopped strand mat applied to the deck surface
Resin laminated through the mat and allowed to cure
Top coat applied — UV stable, hard surface finish
Upstands formed and trimmed

The result is a single continuous surface with no internal joints. It's the same material used in boat hulls, swimming pools and water storage tanks — proven in wet environments for decades.

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When Felt or EPDM Is the Right Choice

Very Large Flat Roofs

GRP becomes more expensive per m² on very large roofs (40m²+) due to material and application time. EPDM rubber is more cost-effective at large scale and performs well on wide-span commercial and industrial flat roofs. For domestic extensions, GRP is almost always the right answer — most are under 25m².

Complex Curved Geometry

GRP can be worked around most standard domestic roof shapes including L-junctions, internal corners and box gutters. Highly complex curved profiles — unusual architectural features, circular bay tops — are better suited to EPDM, which is a flexible membrane that conforms to any shape.

Budget Constraints

Where upfront cost is the primary consideration and the homeowner understands the trade-off, high-quality torch-on felt is a legitimate choice. We recommend modern SBS-modified bitumen over traditional felt — it has a longer service life and better UV resistance. We don't pressure-sell GRP to clients who genuinely need to manage costs.

The Drainage Question — Often Missed

The most commonly overlooked element of any flat roof replacement is drainage fall correction. A flat roof that ponds water — even a small amount — will fail faster than one with correct falls, regardless of which membrane is used. When we survey a flat roof in Chippenham, we assess the existing falls and include drainage correction in every GRP installation quote as standard.

Many felt flat roofs in Wiltshire were built with insufficient falls in the original construction. Replacing the membrane over the same insufficient falls simply recreates the ponding problem. Correct falls at the deck level before membrane installation is a fundamental part of the job.

Wiltshire-Specific Considerations

The Avon Valley towns — Chippenham, Melksham, Bradford-on-Avon — have higher ambient moisture than the chalk ridge towns like Calne and Malmesbury. High moisture accelerates felt deterioration and makes the argument for GRP stronger. In Bradford-on-Avon's riverside properties, where moisture is consistently high, GRP or EPDM is the only sensible flat roofing specification — felt deteriorates significantly faster in this environment.

Conservation area properties in Corsham, Box, Lacock and Bradford-on-Avon may have planning considerations around visible flat roofs. GRP in a smooth finish is generally acceptable on non-visible rear surfaces. For visible flat roofs on listed buildings, a lead specification may be preferred. We advise on this during the free survey.

GRP vs Felt Flat Roofing