Flat Roof Fall Ratio Calculator
Calculate the required height difference (fall) for your flat roof to ensure proper water drainage. Enter your roof length and desired fall ratio to get precise measurements.
Roof Details
Enter your roof dimensions and target gradient to calculate the required fall
Enter the total horizontal length of the roof in meters.
Enter the denominator of the ratio. For a 1:40 fall, enter 40. For 1:80, enter 80.
Calculation Results
Required fall height and slope breakdown
Enter your roof details above and click Calculate Required Fall to reveal the exact height difference needed for proper drainage.
Common Fall Ratios & Applications
Understanding standard fall ratios helps ensure your flat roof complies with building regulations and effectively prevents water ponding.
| Fall Ratio | Classification | Slope Angle | Typical Application |
|---|---|---|---|
| 1:40 | Recommended Design | 1.43° | Standard for most membranes (EPDM, felt) |
| 1:60 | Standard Fall | 0.95° | Good balance for drainage and aesthetics |
| 1:80 | Minimum Finished | 0.72° | Absolute minimum per BS 6229 |
| 1:100 | Shallow Fall | 0.57° | Large areas, requires precise installation |
Flat Roof Fall FAQ
Everything you need to know about flat roof gradients, building regulations, and ensuring proper water drainage.
A flat roof fall ratio (or slope ratio) represents the gradient of the roof, expressed as a ratio of vertical fall to horizontal length. For example, a 1:40 fall means that for every 40 units of horizontal length, the roof drops by 1 unit vertically. This gradient is essential for ensuring water drains off the roof effectively.
Despite the name, ‘flat’ roofs are never completely level. They require a slight slope (fall) to allow rainwater to drain towards gutters or outlets. Without a sufficient fall, water will pool (pond) on the roof, which can lead to structural damage, leaks, degraded roofing materials, and excessive weight loading.
According to British Standard BS 6229, the minimum recommended ‘finished fall’ (the actual slope after construction) should be at least 1:80. However, to account for structural deflection and workmanship tolerances, a ‘design fall’ of 1:40 is recommended for most roof coverings to ensure the finished fall never drops below 1:80.
To calculate the required fall, divide the total horizontal length of the roof by the denominator of your fall ratio. For example, if your roof is 10 meters (10,000 mm) long and you want a 1:40 fall, the calculation is 10,000 mm ÷ 40 = 250 mm. The roof must drop 250 mm over that 10-meter span.
If a flat roof lacks sufficient fall, water will pond on the surface. Ponding water accelerates the degradation of roofing materials (like felt or EPDM), increases the risk of leaks at seams and joints, adds significant structural weight, and can lead to algae growth and blocked drains during heavy rainfall.