Drainage
Drain gradient from two invert levels
- where
- fall (mm)
- upstream invert level (m)
- downstream invert level (m)
- gradient, 1 in G
- length between the levels (m)
- fall per metre (mm/m)
Explain this equation
This works out the gradient of a drain from the invert levels at each end, for checking an existing run or a proposed one against Approved Document H.
IL1 and IL2 are the upstream and downstream invert levels in metres. Fl is the fall in millimetres, L the length between the points, G the gradient as 1 in G and m the fall per metre.
The fall is the difference in level, turned into millimetres. Dividing the length (in mm) by the fall gives how many units along for each unit down, which is the 1 in G figure.
Sense check: a 300 mm drop over 18 m is 1 in 60, or about 17 mm per metre, which suits a 100 mm house drain.
Watch out: use invert levels, not cover levels or ground levels.
Works out the gradient of an existing or proposed drain from the invert levels at each end, and checks it against Approved Document H.
Enter your values to see the result.
Good to know
Take invert levels (the bottom inside of the pipe) from the manhole or chamber at each end, and the length along the pipe between them. A larger "1 in" number means a flatter pipe. See the drain fall calculator for the AD H Table 6 minimums.
Common mistake
Using cover levels or depths from ground instead of invert levels. Ground slopes are not pipe slopes.
Limits
Assumes a straight pipe at constant gradient between the two points.
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)
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.