What a flat roof costs in Chippenham — GRP fibreglass and EPDM rubber, priced by extension size — with the local factors behind Pewsham's felt-roof replacement wave.
In short: A flat roof in Chippenham costs £1,200–£2,500 for a standard extension in GRP fibreglass (25-year guarantee) or £900–£1,800 in EPDM rubber. Price depends on deck condition, drainage falls and size. GRP is the standard replacement for the failed felt roofs common across Pewsham and other 1990s–2000s estates.
A flat roof quote covers stripping the old covering, inspecting and repairing the deck, correcting drainage falls, and installing the new membrane with trims and upstands. GRP (glass-reinforced polyester) is a seamless liquid-applied fibreglass system with a 25-year life; EPDM is a single rubber sheet suited to larger or more complex roofs. Both massively outlast the felt they usually replace. The deck inspection matters most — a new membrane over rotten boards fails early, so we price for board replacement where the timber has gone soft.
We remove the old felt and inspect the deck for rot — the step most quotes skip and most failures come from.
We check and correct drainage falls so water runs off cleanly rather than ponding.
GRP is laid wet and cures seamless; EPDM is bonded as a single sheet. Trims and upstands are dressed in.
The finished roof carries a 25-year material guarantee (GRP) or 20-year (EPDM), plus our workmanship cover.
Pewsham is a 1995–2010 development estate where almost every property has a single-storey rear extension with a felt flat roof. Those felt roofs are now 25–35 years old — past their design life — and they are failing in waves across the estate. This is the single biggest source of flat-roof calls in Chippenham. The good news for Pewsham homeowners is that GRP replacement costs about the same as re-felting but lasts three times as long, so it is the obvious choice.
A standard Pewsham rear extension of 15–20m² runs £1,400–£2,200 in GRP including strip, deck check and new trims. Because the estate was built in a uniform phase, we know these roofs well: the falls were often laid shallow, and ponding water has accelerated the felt failure, so correcting the drainage is part of nearly every Pewsham job.
Across the rest of Chippenham the picture is more varied. 1980s extensions in Sheldon and Cocklebury often have chipboard or older plywood decks that have absorbed moisture through failing felt — on these we budget for partial or full board replacement, which adds £150–£400 to a typical job. Victorian and Edwardian properties in the town centre and Monkton Park sometimes have small lead or felt flat sections over bays and porches, where GRP or new code 5 lead is the appropriate replacement.
For larger flat roofs — garage ranges, wider extensions, or commercial-style flat sections — EPDM rubber is often the better call at £900–£1,800, because a single bonded sheet handles the larger area without the seams that let felt down. We assess deck, drainage and size on a free survey and recommend GRP or EPDM based on the specific roof, not a one-size default.
The headline price gap between flat-roof systems is smaller than the lifetime-cost gap, and that's the number that matters. Felt is cheapest up front but lasts only 10–15 years, so on a Chippenham extension you'll pay for it two or three times over the period a single GRP roof lasts. GRP fibreglass costs a little more than felt to install but carries a 25-year guarantee and is seamless, which is why it's our standard recommendation for the Pewsham extension market. EPDM rubber sits between the two on price and suits larger areas where a single bonded sheet avoids seams.
For a standard Pewsham rear extension of 15–20m², the felt-versus-GRP decision is genuinely easy: GRP costs perhaps £200–£400 more than a re-felt but lasts three times as long, so the annual cost is far lower. The only case where we'd suggest felt is a genuinely short-term fix — a roof coming off within a few years for an extension or conversion — where spending on a 25-year system doesn't make sense. We'll tell you honestly which situation you're in.
The single biggest driver of flat-roof failure — and of unexpected cost — is the deck beneath the covering. A new membrane laid over soft, water-damaged boards will fail early no matter how good the membrane, so a proper quote includes stripping back to inspect the deck and an allowance for board replacement where needed (£150–£400 on a typical Chippenham extension). A suspiciously cheap flat-roof quote often skips this inspection, laying the new covering over the old problem — and the roof fails again within a few years.
Drainage falls are the other hidden factor. Many of the 1980s–2000s extensions across Sheldon, Cocklebury and Pewsham were built with falls too shallow, so water ponds rather than draining, accelerating failure. Correcting the falls when we replace the covering — part of nearly every Pewsham job — costs a little more than laying flat over the existing profile, but it's the difference between a roof that drains and one that ponds and fails early. It's exactly the sort of thing a cheap quote leaves out and a proper one includes.
| GRP fibreglass — small extension (up to 15m²) | £1,200 – £1,800 |
| GRP fibreglass — standard extension (15–25m²) | £1,800 – £2,500 |
| EPDM rubber — standard extension | £900 – £1,800 |
| Garage roof (GRP or EPDM) | £800 – £1,600 |
| Deck / board replacement (add) | £150 – £400 |
| Larger / commercial flat roof | from £45/m² |
Indicative Chippenham prices for guidance. Every quote is free, itemised and fixed before work starts — no call-out fee.
“A single-ply or liquid membrane laid over a sound, correctly-falling deck will outlast built-up felt several times over — the deck and the falls, not the membrane brand, decide how long a flat roof lasts.” — NFRC flat roofing guidance
Speak to a local Chippenham roofer today. Honest advice, itemised quotes, 10-year guarantee.
📞 Call 01249 247 076“Our Pewsham extension roof had been patched twice. They stripped it, found two soft boards, replaced them and laid the fibreglass. Bone dry through the winter and a 25-year guarantee.”
“Garage roof in Sheldon was leaking for years. EPDM rubber, one sheet, done in a day. Should have done it sooner.”
“They talked me through GRP vs rubber for the extension in Cocklebury and recommended fibreglass for the size. Proper job, no ponding now they sorted the falls.”
A standard extension flat roof in Chippenham costs £1,200–£2,500 in GRP fibreglass (25-year guarantee) or £900–£1,800 in EPDM rubber. Deck repairs, if needed, add £150–£400. We survey free and quote a fixed price.
GRP fibreglass suits most domestic extensions — it's seamless, rigid and carries a 25-year guarantee. EPDM rubber is better for larger or more complex roofs where a single bonded sheet avoids seams. We recommend based on your specific roof size and shape.
Pewsham was built 1995–2010 and almost every property has a felt-covered rear extension. Those felt roofs are now 25–35 years old and past their design life, so they're failing across the estate. GRP replacement costs about the same as re-felting but lasts three times as long.
Only if the timber has gone soft. We strip the old covering and inspect the deck on every job — a new membrane over rotten boards fails early. Board replacement, where needed, adds £150–£400. Many decks are sound and need nothing.
A correctly installed GRP roof over a sound deck lasts 25–30 years and carries a 25-year material guarantee. The deck condition and drainage falls matter more than anything for longevity, which is why we always check both.
Yes. Ponding usually means the drainage falls were laid too shallow. When we replace the covering we correct the falls so water runs off cleanly — this is part of nearly every Pewsham job, where shallow falls are common.
A standard extension takes 1–2 days. GRP is laid wet and cures on site; EPDM bonds as a sheet. There's minimal disruption inside the property and we clear all waste on completion.
Trusted local flat roofing across Chippenham and Wiltshire. Free quotes, no call-out fee, 10-year guarantee.
Trusted local roof repairs across Chippenham and Wiltshire. Free quotes, no call-out fee, 10-year guarantee.
Trusted local guttering-fascias-cost-chippenham.html across Chippenham and Wiltshire. Free quotes, no call-out fee, 10-year guarantee.
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