Drainage
Drain fall and downstream invert level
- where
- total fall (mm)
- length of run (m)
- gradient, 1 in G
- downstream invert level (m)
- upstream invert level (m)
- fall per metre (mm/m)
Explain this equation
This gives the total fall along a pipe laid at a chosen gradient, the invert level at the lower end and the fall per metre, with a check against Approved Document H.
L is the length of the run and G the gradient as 1 in G. Fl is the total fall in millimetres, IL1 and IL2 the invert levels at the top and bottom, and m the fall per metre.
A gradient of 1 in G means one unit down for every G along, so the fall is L/G; ×1000 turns metres into millimetres, and /1000 turns it back to subtract from the level in metres.
Sense check: 1 in 40 is 25 mm per metre and 1 in 80 is 12.5 mm per metre, so a 12 m run at 1:40 drops 300 mm.
Watch out: a bigger G means a flatter pipe. AD H gives minimum gradients; flatter than that and the drain may block.
Fall along a pipe run at a chosen gradient, the invert level at the lower end, and a check against the Approved Document H minimum gradient.
Enter your values to see the result.
Good to know
Approved Document H Table 6 (foul drains): peak flow under 1 l/s, 75 mm or 100 mm pipe at 1:40; peak flow over 1 l/s, 75 mm at 1:80, 100 mm at 1:80 (at least one WC connected), 150 mm at 1:150 (at least five WCs). A 100 mm foul drain from one house therefore needs 1:80 or steeper: AD H Table 5 gives 2.5 l/s peak for one dwelling. Rainwater drains of 75 mm and 100 mm at not less than 1:100, 150 mm at 1:150 and 225 mm at 1:225 (para 3.15). Foul drains serving a WC must be at least 100 mm.
Common mistake
Mixing up "1 in 40" and "1 in 80": 1:40 is twice as steep (25 mm/m) as 1:80 (12.5 mm/m). The minimum is the flattest allowed; anything steeper is fine for normal house drains.
Limits
Straight run at one gradient. The AD H minimums are for normal domestic drains; sewers for adoption and larger flows need a hydraulic design to BS EN 752 / the water company’s Design and Construction Guidance.
Sources
- Approved Document H: Drainage and waste disposal, 2015 edition (Table 6 minimum gradients for foul drains; Tables 11 and 12 access fittings, inspection chambers and manholes; para 3.15 rainwater drain gradients; paras 3.27–3.30 soakaways)
- LABC, Underground foul drainage: how to get it right (AD H minimum gradients and capacities, 75 mm minimum, 100 mm for a WC)
- BS EN 752:2017 Drain and sewer systems outside buildings. Sewer system management (hydraulic design basis)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.