Which flat roof membrane is best for the UK?

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.

How much does EPDM cost per m²?

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.

What are upstands and why do I need them?

Upstands are the vertical sections where the membrane turns up against walls, parapets or kerbs at the edge of a flat roof. Building Regulations require a minimum 150mm upstand height to prevent water ingress. 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.

Do I need insulation on a flat roof?

Yes — Building Regulations (Part L) require flat roofs to be insulated to achieve a U-value of 0.18 W/m²K or better. 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.

How much GRP resin and topcoat do I need per m²?

For a standard GRP flat roof, you need approximately 2.5 kg of laminating resin per m² (applied with 450g/m² chopped strand mat) and 0.5 kg of topcoat per m². A 20m² roof would need about 50 kg of resin and 10 kg of topcoat. Always add 10% wastage. GRP requires warm, dry conditions (above 10°C) for application.

Flat Roof Membrane Calculator — EPDM, GRP, Felt and TPO Quantities

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

Calculate membrane quantities for flat roofs including EPDM rubber, GRP fibreglass, torch-on felt and TPO. Includes upstands and insulation.

Roof Length

Length of the flat roof

Roof Width

Width of the flat roof

Membrane Type

Upstand Height

Minimum 150mm per Building Regs

Include insulation boards

PIR insulation boards (1200 × 600mm) for warm roof build-up

Include wastage allowance

Adds 10% extra for trimming and overlaps

Wastage Percentage

10% is standard for flat roof membranes

Price Per Unit (optional)

Per EPDM sheet

How We Calculate This

Our flat roof membrane calculator determines the total membrane area needed for your flat roof, including the essential upstand allowances around the perimeter. It then calculates material quantities based on your chosen membrane type.

The formula

Membrane area = Roof area + (2 × Length × Upstand) + (2 × Width × Upstand)

Membrane types

EPDM: Sold in set sheet sizes (single piece for smaller roofs). No joints needed for roofs under ~50m².
GRP: Fibreglass matting (450g/m²) + laminating resin (2.5 kg/m²) + topcoat (0.5 kg/m²).
Felt: Three-layer built-up system. Standard rolls 1m × 8m. Torch-applied.
TPO: Thermoplastic rolls 1.5m × 20m. Hot-air welded seams.

Insulation

If including insulation, we calculate the number of standard PIR boards (1200mm × 600mm) needed to cover the roof area. The required thickness depends on the target U-value — typically 100-150mm to meet Part L of the Building Regulations (U-value 0.18 W/m²K).

UK Building Regulations

Flat roofs must comply with Part L (insulation) and Part C (moisture resistance) of the Building Regulations. A minimum 150mm upstand is required at all abutments. Warm roof construction (insulation above the deck) is the preferred method. Falls of at least 1:40 (with a finished fall of 1:80) are recommended for positive drainage.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

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

How We Calculate This

The formula

Membrane types

Insulation

UK Building Regulations

Frequently Asked Questions

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