Falls and firrings
Flat roof fall: drop over a run at a chosen ratio
- where
- total fall over the run (mm)
- length of the run in the direction of fall (m)
- fall ratio, 1 in G
- gradient (%)
- angle of the fall (°)
Explain this equation
Works out how much a flat roof must drop over its length to achieve a chosen fall, such as 1 in 40. Use it when designing firrings or tapered insulation, or setting out a new deck.
A fall of 1 in G means the roof drops one unit for every G units along. L is in metres, so 1000 L / G gives the drop h in millimetres. The same ratio as a percentage is 100 / G, and arctan turns it into a (very small) angle.
It is a straight proportion: drop is length divided by the ratio number.
Sense check: 4 m at 1 in 40 drops 100 mm, a 2.5% gradient, about 1.4°. At 1 in 80 the same run drops only 50 mm.
Watch out: BS 6229 wants at least 1 in 80 finished fall everywhere, which is why designs aim for 1 in 40 to allow for deflection and building tolerances.
Works out how much a flat roof must drop over a run for a given fall ratio, so you can set out firrings, tapered insulation or screed.
Enter your values to see the result.
Good to know
BS 6229:2018 asks for a minimum finished fall of 1:80 at any point, including internal gutters, and recommends a design fall of 1:40 so that 1:80 is still achieved once the structure has deflected and site tolerances have been absorbed. Check cross-falls at valleys and around outlets, where the fall along the diagonal is shallower than the main fall.
Common mistake
Designing at exactly 1:80. Joist deflection and normal building tolerance will leave ponds, and a back-fall is not acceptable.
Limits
Straight, single-direction fall. Tapered insulation schemes with valleys and hips need the manufacturer’s layout.
Sources
- BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings – Code of practice (BSI)
- Bauder: BS 6229:2018 overview of changes (design 1:40 to achieve 1:80 finished; 150 mm upstands)
- LRWA Guidance Note 7: Falls (minimum finished fall 1:80 per BS 6229; 1:60 or 1:40 commonly designed)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.