Flat Roof Membrane Calculator — EPDM, GRP, Felt & TPO

EPDM rubber (such as Firestone or ClassicBond) is the most popular choice for domestic flat roofs in the UK due to its 50+ year lifespan, ease of installation and excellent weather resistance. GRP (fibreglass) is also excellent and creates a seamless, hard-wearing surface. Torch-on felt is traditional but has a shorter lifespan (15-20 years). TPO is increasingly used for commercial applications. For pitched roofs above 10°, see our Roof Tile Calculator instead. Use our Flat Roof Fall Calculator to work out the correct fall.

EPDM membrane typically costs £8-£15 per m² for the membrane alone, depending on the sheet size and supplier. A complete EPDM kit (membrane, adhesive, trim, primers) for a typical 15m² flat roof costs around £300-£500. This makes it one of the most cost-effective long-term flat roof solutions, given its 50+ year expected lifespan.

Upstands are the vertical sections where the membrane turns up against walls, parapets or kerbs at the edge of a flat roof. BS 6229:2018 requires the waterproof layer to be turned up to at least 150mm above the adjacent finished roof level at all abutments to prevent water ingress (this can be relaxed to 75mm only at accessible door thresholds with specific detailing). The membrane must extend up the full height of the upstand and be properly dressed and sealed. Our calculator includes upstand area in the total membrane calculation.

Yes — Building Regulations Part L 2021 set a limiting (worst-acceptable) U-value for flat roofs of 0.16 W/m²K on a new dwelling (Approved Document L Vol 1, Table 4.1, 'all roof types') and 0.15 W/m²K for a new or replacement roof on an existing dwelling or extension (Table 4.2). This typically means 100-150mm of PIR insulation board (such as Kingspan or Celotex) on top of the deck, beneath the membrane (warm roof construction). A warm roof is always preferred over a cold roof for flat roof construction.

For a standard GRP flat roof using 450g/m² chopped strand mat, UK manufacturer data sheets give roughly 1.35 kg of base/laminating resin per m² — allow about 1.5 kg/m² to cover spillage and detailing — plus 0.5 kg of topcoat per m². A 20m² roof therefore needs about 30 kg of resin and 10 kg of topcoat. Add 10% wastage on top, and allow more resin if you use 600g mat or have a lot of detail/upstands. GRP requires warm, dry conditions (above 10°C) for application. For a dedicated GRP calculation, see our GRP Flat Roof Calculator.