Flat Roof Extension Calculator — Insulation, Membrane & Falls

A warm roof buildup (most common) from top to bottom is: waterproof membrane (EPDM/GRP/single ply), PIR insulation (150mm+ for Part L), vapour control layer (VCL), timber or concrete deck, and firring strips to create the fall. The insulation goes above the deck (warm side) to prevent condensation.

Flat roof falls are governed by BS 6229 (the code of practice for flat roofs), not by a specific Building Regulations figure. BS 6229:2018 recommends a DESIGN fall of 1:40 (25mm per metre) so that, after construction tolerances and structural deflection under load — which can consume up to half the design fall — a MINIMUM FINISHED fall of 1:80 is still achieved, preventing ponding. (The 2025 revision replaces the generic 1:40 rule with a structural-analysis-derived design fall, but still targets a 1:80 minimum finished fall on most roofs.) Designing at 1:40 remains the safe, widely specified default.

Part L requires a U-value of 0.15 W/m²K for extension flat roofs. With PIR insulation (lambda 0.022 W/mK), this requires approximately 150mm thickness. The exact requirement depends on the full buildup — use the insulation manufacturer's U-value calculator for precision.

An upstand is where the flat roof membrane turns up vertically at edges, parapets and where the roof meets walls. The minimum upstand height is 150mm above the finished roof level. Upstands prevent water ingress at vulnerable junctions. The membrane must be dressed up, clamped and sealed with appropriate trims or lead flashings.

EPDM rubber is excellent for simple rectangular roofs — single sheet, no joints, 50+ year life. GRP fibreglass is seamless and extremely durable, ideal for complex shapes with multiple upstands. Single ply (TPO/PVC) is becoming popular for its speed of installation and 30+ year life. Three-layer felt is the cheapest but has a shorter lifespan (15–20 years).