Gravity drains work by fall: the pipe drops steadily from start to finish. Too little fall and solids settle and block the pipe. Too much and water can run ahead and leave the solids behind.
Typical gradients
| Pipe | Typical fall |
|---|---|
| Waste pipes (basins, sinks) | 18–90 mm per metre |
| 100 mm drain | 1:40, or as flat as 1:80 with at least one WC connected |
| 150 mm drain | 1:60 to 1:150 depending on flow |
Confirm against Approved Document H and the pipe maker's guidance.
Setting out
- Fix the outlet level (where the pipe ends) and the inlet level (where it starts).
- Total fall = run length ÷ gradient ratio.
- Check that the fall available is enough. If it isn't, you need a pump or a different route.
- Set the bed with a level and spacer, or a laser, along the run.
The drain fall calculator gives the total drop and the fall per metre for any run.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the Building Regulations and Building Control for your project.