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 |
|---|---|
| Up to 2½" (sinks, showers) | ¼" per foot (2%) |
| 3"–6" building drains | ⅛"–¼" per foot |
Confirm against your adopted plumbing code (IPC or UPC).
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 foot for any run.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.