Low-slope roof fall estimator

Flat Roof Fall Calculator

Convert flat-roof run and target fall into height drop, slope percent, angle, and membrane order area. Use it for low-slope roof layout and material planning before checking the roof system rules.

Units

Roof run to drain (ft)Fall rate (in / ft)Roof width (ft)Waste / laps (%)Roll coverage (ft²)Membrane cost ($/ft²)

Height drop

Slope grade

Rolls to buy

Rough cost

Low-slope roofs require product and drainage checks

This is geometry and material math only. Ponding rules, minimum slope, drains, scuppers, tapered insulation, deck deflection, membrane laps, wind uplift, and local code must be verified before building.

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Flat roof fall formula

Height drop = roof run × fall per foot

Slope grade % = drop ÷ run × 100

Sloped length = sqrt(run² + drop²)

Rolls = sloped area × (1 + waste %) ÷ roll coverage

Sample takeoff

A 24 ft flat roof run at 1/4 in per ft drops 6 in. Across a 40 ft width with 10% waste, plan about two 1,000 ft² membrane rolls.A 7.2 m flat roof run at 20 mm per m drops 144 mm. Across a 12 m width with 10% waste, plan about two 93 m² membrane rolls.

Flat Roof Fall Calculator FAQ

How much fall does a flat roof need?+

Minimum fall depends on roof system, drainage, deflection, and local code. This calculator converts the fall rate you choose into height drop and material area.Minimum fall depends on roof system, drainage, deflection, and local code. This calculator converts the fall rate you choose into height drop and material area.

How do I calculate roof fall?+

Multiply roof run by the fall rate. For example, 24 ft at 1/4 in per ft drops 6 in.Multiply roof run by the fall rate. For example, 24 ft at 1/4 in per ft drops 6 in.

Does this design tapered insulation?+

No. It gives geometry and area only. Tapered insulation layouts need product data, drain locations, crickets, and drainage design.No. It gives geometry and area only. Tapered insulation layouts need product data, drain locations, crickets, and drainage design.