Flat Roof Calculator
Calculate flat roof area, drainage falls, and membrane quantities. Enter dimensions, specify fall ratio, and get material quantities. Free, no signup.
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Flat roofs - did you know?
Flat roofs aren't actually flat
Building regs require a minimum 1:80 fall (1.25% slope, ~0.7°) for water drainage. Most flat roofs are 1:40 to 1:60 (1.4° to 2.4°). The fall directs water to outlets or gutters.
Membrane types and coverage
Bitumen felt: 2-3 layers, each covering full area (multiply by layers). EPDM rubber: single sheet, area + 150mm perimeter upturn. GRP fibreglass: resin + matting, ~1.5kg/m² resin per layer. TPO: single-ple, similar to EPDM.
Insulation on flat roofs
Warm deck: insulation above the deck (most common). Cold deck: insulation below (requires ventilation). Typical PIR insulation: 100-150mm for U-value compliance. Calculate by area × thickness.
Drainage and outlet sizing
Flat roof outlets sized by plan area and rainfall intensity. 1 outlet per 40-80 m² (UK typical). Check fall directs water to outlets - any ponding area is a failure point.
Formulas used
Flat roof area
area = length × width
Fall amount
fall = run × (1 ÷ ratio)
Membrane quantity
membrane = area × layers + perimeter × 0.15
Insulation volume
volume = area × thickness_m
Frequently asked questions
What fall does a flat roof need?
Minimum 1:80 (0.7°) per UK Building Regs. Recommended 1:40 (1.4°) to 1:60 (1.0°) for positive drainage.
How much membrane for a flat roof?
Area × number of layers + 150mm upstand perimeter. 3-layer felt on 50 m² roof: 50 × 3 = 150 m² + ~6 m² upstands = ~156 m².
What insulation thickness for flat roofs?
Typically 100-150mm PIR for U-value compliance. Check Part L for current requirements. Calculate volume = area × thickness.
Best free flat roof calculator?
One that handles area, falls, membrane layers, and insulation. This tool does all, free.