What is the correct flat roof buildup for an extension?

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.

What fall does a flat roof need?

Building Regulations require a minimum fall of 1:40 (25mm per metre) to ensure rainwater drains to outlets without ponding. This is the finished fall — design the structural fall at 1:40 minimum, as deflection and settlement may reduce the effective fall over time. Some designers specify 1:30 for margin.

What insulation thickness for a flat roof extension?

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.

What are upstand details and why are they important?

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.

What is the best flat roof membrane for an extension?

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).

TradeCalculator

Flat Roof Extension Calculator — Insulation, Membrane & Falls

Calculate flat roof extension material quantities including deck, VCL, PIR insulation, membrane, upstand details, fall and drainage outlet specifications.

Roof length

Roof width (drainage direction)

Deck type

Membrane type

Insulation thickness

How We Calculate This

This calculator estimates material quantities for a flat roof warm deck construction based on plan dimensions.

Material calculation

Membrane: roof area x 1.1 (overlaps) + upstand perimeter area
Insulation: roof area x 1.05 (waste cuts)
VCL: roof area x 1.1 (overlaps)
Firring strips: at 400mm centres across the width
Fall height = roof width / fall ratio (1:40 standard)

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.