Home & Construction⏱ 5 min read
How to Calculate Materials for a Flat Roof
Flat roof projects fail more often from wrong material quantities than from installation errors. Here's how to calculate felt, EPDM rubber, insulation board, and drainage falls precisely.
Flat roofs require careful material calculations because under-ordering mid-project causes delays and potential batch-matching issues, while over-ordering can be expensive. Here's the complete guide for the most common flat roofing systems.
The Critical First Step: Roof Area
Roof area = Length x Width (measured at the roof surface, not floor plan)
For a simple flat roof:
8m x 5m roof surface = 40 m2
Add upstands (the vertical edges where roof meets walls):
If upstands are 150mm high on all 4 sides:
Perimeter = 2(8+5) = 26m
Upstand area = 26 x 0.15 = 3.9 m2
Total to cover = 40 + 3.9 = 43.9 m2
EPDM Rubber Membrane
EPDM comes in large single sheets — the key advantage is
fewer joins, which means fewer potential leak points.
Standard sheet widths: 3.05m, 6.1m, 12.2m
Add 150mm overhang on all edges that terminate (not upstands)
Example: 8m x 5m roof
Choose 6.1m wide sheet:
Length needed: 8m + 0.15m overhang = 8.15m
Width coverage: 6.1m > 5m + 0.15m (5.15m) ✓ (single sheet fits)
Order: 1 sheet, 6.1m wide x 8.5m long
(round up to available lengths — typically 10m, 15m, 20m)
Order: 6.1m x 10m sheet, cut to size on site
GRP Fibreglass Roofing
GRP is laid in two stages:
1. Fibreglass matting (chopped strand mat, CSM)
2. Topcoat resin
Coverage rates:
CSM matting: sold per m2, order area + 15% waste
Fibreglass resin (base coat): 1 litre covers ~2 m2
Topcoat resin: 1 litre covers ~5 m2
For 40 m2 roof:
CSM: 40 x 1.15 = 46 m2 of matting
Base resin: 40 / 2 = 20 litres
Topcoat: 40 / 5 = 8 litres
Corner tape/trim: perimeter length in metres
Insulation Board
UK Building Regs minimum for flat roof (2024):
U-value of 0.18 W/m2K or better (cold roof construction)
PIR insulation board thickness needed:
0.18 U-value = approximately 100mm PIR board
(manufacturer's tables give exact thickness for their products)
Boards typically 1.2m x 2.4m = 2.88 m2 each
For 40 m2: 40 / 2.88 = 13.9 boards → order 15 boards (7% spare)
Tape all joints with foil tape — every untaped joint is
a thermal bridge that significantly reduces insulation value.
Falls and Drainage
Minimum fall for a flat roof: 1:80 (approximately 1.25%)
Recommended: 1:40 (2.5%) for better drainage
Fall calculation:
Fall = Length of roof / gradient ratio
8m roof with 1:40 fall:
Height difference = 8 / 40 = 0.2m = 200mm
Tapered insulation boards are available to create the fall
without raising the roof structure — specify the fall
gradient when ordering tapered insulation.