Flat Roof Replacement Costs at a Glance

Roof SizeGRP FibreglassEPDM RubberSBS Felt
Small (up to 10m²)£800–£1,400£600–£1,100£450–£800
Standard extension (10–20m²)£1,200–£2,500£900–£1,800£700–£1,400
Large extension (20–40m²)£2,000–£4,000£1,600–£3,200£1,200–£2,500
Garage (standard)£1,000–£2,000£800–£1,600£600–£1,200
Per m² (larger roofs)£60–£120/m²£45–£90/m²£35–£70/m²

All prices include strip and disposal, deck inspection, drainage correction and installation. Board replacement is quoted separately if required — typically £30–£60 per m² for new OSB deck boards.

What Drives the Cost of Flat Roof Replacement?

Membrane Choice

The membrane specification is the primary cost driver. GRP is typically the most expensive but the most durable on standard domestic roofs. EPDM rubber costs slightly less than GRP and is better suited to large or complex geometries. Modern SBS torch-on felt is the budget option — legitimate for cost-constrained situations, shorter service life.

Deck Condition

Before any membrane is applied, we inspect the deck boards. Rotten, wet or structurally compromised boards must be replaced — applying a new membrane over a failed deck produces a short-lived result. Deck board replacement typically adds £300–£800 to the total on a standard extension. We find rotten deck boards on approximately one in four flat roof replacement surveys in Wiltshire — often not visible until the old membrane is stripped.

Drainage Falls

Flat roofs should not actually be flat — they need a fall of at least 1:80 (approximately 12mm per metre) to drain correctly. Many 1990s–2000s extensions in Chippenham and Pewsham were built with insufficient falls. We correct drainage falls as part of every GRP installation — this adds modest cost but is the difference between a roof that performs for 40 years and one that ponds water and fails in 15.

Upstand and Trim Detail

Where the flat roof meets vertical surfaces — house walls, parapet walls, door frames — an upstand must be formed that carries the membrane up the vertical face to prevent water tracking behind. Poorly formed upstands are the most common source of early flat roof failure. Good upstand detailing takes time and slightly increases labour cost, but it is non-negotiable in our specification.

The Real Cost of Re-Felting vs GRP

A re-felt costs £700–£1,400 for a standard extension. A GRP replacement costs £1,200–£2,500. The £500–£1,100 difference is real — but consider the timeline. A correctly installed GRP carries a 25-year material guarantee. Modern felt has a 15–25 year design life at best. If you re-felt a 1990s Pewsham extension today, you're likely replacing it again in 15–20 years. If you install GRP today, you won't touch it for 25–40 years.

Over a 30-year window, re-felting (done twice) costs £1,400–£2,800. GRP costs £1,200–£2,500 once. GRP is cheaper over the property's lifetime even though it's more expensive today.

Free Flat Roof Survey

We assess your existing roof and quote for GRP, EPDM or felt as appropriate. Free written quote.

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Wiltshire Properties: Where Flat Roofing Calls Come From

Pewsham and similar 1990s–2000s Chippenham development estates generate the highest volume of flat roof calls — original felt extensions now 25–35 years old, many showing active failure. We replace these with GRP as standard.

The Avon Valley towns — Chippenham, Melksham, Bradford-on-Avon — have higher moisture than chalk ridge towns, which accelerates felt deterioration. In Bradford-on-Avon, we regularly replace felt roofs that are only 15–20 years old because the valley moisture has significantly shortened the membrane's effective life.

Conservation area properties in Corsham, Box, Lacock and Bradford-on-Avon may have planning considerations around visible flat roofs. GRP on non-visible rear surfaces is generally acceptable. Visible flat roofs on listed buildings may require lead to match historic character. We advise on this during the free survey.

Questions to Ask Before Agreeing to a Flat Roof Quote