For flat roofing in Salisbury, GRP fibreglass (£70–£100/m², 30–40 year life) is our standard recommendation, EPDM rubber suits larger areas, and felt is the cheapest but shortest-lived. Correct fall and upstand height matter more than material. Free flat roof survey: 0172 223 9611.
If you have a flat-roofed extension, garage or bay window roof, you will at some point face the decision of which flat roofing system to use. The three main options — GRP fibreglass, EPDM rubber, and traditional felt — have very different performance profiles, lifespans and costs. This guide covers what you need to know to make the right decision for your Salisbury property.
The Three Main Flat Roofing Systems
GRP Fibreglass
GRP (glass reinforced plastic) fibreglass is the dominant premium flat roofing system in the UK residential market and our primary recommendation for most applications in Salisbury. It is applied as a liquid laminate that cures to form a seamless, waterproof membrane — no joints or seams to fail. A correctly installed GRP roof carries a 25-year manufacturer guarantee and typically lasts 30–40 years with minimal maintenance.
Best for: Extensions, bay roofs, garage roofs, any flat roof where longevity and performance are the priority.
Not ideal for: Very large areas (above approximately 150m²) where the cost advantage of EPDM becomes significant. Cannot be installed in temperatures below 5°C, which affects winter scheduling in Wiltshire.
EPDM Rubber
EPDM (ethylene propylene diene monomer) is a synthetic rubber membrane that has been used commercially for decades and is now widely used in residential applications. It is supplied in large sheets that are bonded to the roof deck, with sealed joins at any seams. A quality EPDM installation lasts 25–35 years and carries similar guarantee terms to GRP.
Best for: Larger flat roof areas where seamless GRP becomes more expensive, and for installations during colder periods when GRP lamination cannot be guaranteed.
Not ideal for: Highly visible small flat roofs where the rubber sheet appearance is less attractive than the hard GRP surface.
Traditional Felt (Built-Up Roofing)
Three-layer felt is the traditional flat roofing system. It remains in widespread use because it is the cheapest upfront option. However, its 10–15 year design life means it will need replacing 2–3 times over the same period that a GRP or EPDM roof serves without significant intervention. Over a 30-year horizon, felt is almost never the cheapest option.
Best for: Budget-constrained situations where upfront cost is the primary constraint and future replacement is accepted.
Not ideal for: New build extensions, garage conversions or any situation where you want a long-term solution.
BS 6229, the British Standard for flat roofing, specifies a minimum fall of 1:80 to the outlet — insufficient fall causing ponding is the single most common cause of premature flat roof failure.
— BS 6229:2018
Cost Comparison (Salisbury 2026)
| System | Per m² | 25m² extension | 50m² extension | Design life |
|---|---|---|---|---|
| GRP Fibreglass | £70–£100 | £1,750–£2,500 | £3,500–£5,000 | 25–40 years |
| EPDM Rubber | £60–£90 | £1,500–£2,250 | £3,000–£4,500 | 25–35 years |
| Felt (3-layer) | £40–£65 | £1,000–£1,625 | £2,000–£3,250 | 10–15 years |
The Critical Technical Factors
Regardless of which system you choose, these installation factors determine whether your flat roof lasts its design life or fails early:
Fall to Outlet
All flat roofs require a fall — a slight slope toward the drainage outlet — of at least 1:80. A roof that appears flat but has no designed fall will pond water, which accelerates membrane failure and can cause structural loading. Correct fall must be designed into the deck construction before the membrane is applied. This is the single most common installation error we see when replacing prematurely failed flat roofs.
Upstand Height
Where the flat roof membrane meets a vertical surface — a wall, parapet or step — the membrane must be turned up the vertical face (the upstand) to a minimum height of 150mm. Insufficient upstand height allows water to track behind the membrane at the junction. Most flat roof failures in Salisbury occur at upstands, not across the flat field.
Deck Condition
The membrane goes on top of the roof deck. If the existing deck (plywood, OSB, or old boarding) is soft, delaminating, or has previous water damage, the new membrane will not bond correctly and will fail early. A thorough deck inspection before installation is essential.
Not sure which system is right for your roof? We survey flat roofs free of charge across Salisbury, Amesbury, Andover and all surrounding areas and give you a no-obligation written quote.
📞 0172 223 9611Last reviewed: 21 July 2026 by Salisbury Roofing Experts