Flat Roof Calculator
Calculate flat roof area, drainage falls, and membrane quantities. Enter dimensions, specify fall ratio, and get material quantities. Free, no signup.
Roof Area Calculator
Calculate actual roof surface area from plan dimensions and pitch
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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.