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

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) + base/laminating resin (~1.5 kg/m²) + topcoat (0.5 kg/m²).
• Felt: Three-layer built-up system. Standard cap-sheet rolls 1m × 8m (8 m² gross before side and end laps). Torch-applied.
• TPO: Thermoplastic rolls — we use a 1.5m × 20m example; widths vary by brand (~1.05-2.1m). 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 the Part L 2021 limiting flat-roof U-value of 0.16 W/m²K on a new dwelling (Approved Document L Vol 1, Table 4.1) or 0.15 W/m²K on an existing dwelling or extension (Table 4.2).

UK Building Regulations

Flat roofs must comply with Part L (insulation) and Part C (moisture resistance) of the Building Regulations. BS 6229:2018 requires a minimum 150mm upstand above the finished roof level at all abutments (relaxed to 75mm only at accessible door thresholds with the correct detailing). Warm roof construction (insulation above the deck) is the preferred method. A design fall of at least 1:40 is recommended so that, after deflection and tolerances, a minimum finished fall of 1:80 is achieved for positive drainage.