Gravity drains work by fall: the pipe drops steadily from start to finish, so water and solids flow downhill. Too little fall and solids settle and block the pipe. Too much and water can run ahead and leave the solids behind, especially on long runs. Getting the gradient right, and keeping it consistent, is the heart of drainage work.
Ways of stating a gradient
- As a ratio: 1:40 means a drop of 1 unit for every 40 units of length. It's the standard way of stating gradients in Canada, usually written as 1 in 50 or 1 in 100.
- As a drop per foot: ¼" per foot (about 1 in 50, or 2%) is the common way of saying it on site.
- As a percentage: drop ÷ length × 100. 1:40 is 2.5%.
Typical gradients
| Pipe | Typical fall |
|---|---|
| 3" and smaller | 1 in 50 (2%), about ¼" per foot |
| 4" and larger | 1 in 100 (1%), about ⅛" per foot |
Confirm against the plumbing code adopted in your province (based on the National Plumbing Code of Canada), and get a permit for new drains.
Setting out
- Fix the outlet level, where the pipe ends (the connection to the municipal sewer or septic system).
- Fix the inlet level, where it starts. The pipe must be deep enough there to pick up the fittings it serves.
- Total fall = run length ÷ gradient ratio. Check the fall you have between the two levels is at least that much.
- If it isn't, you need a pump, a different route, or a different connection point.
- Set the pipe bed at the right fall with a level and spacer block, a laser, or sight rails and a traveller on long runs.
A worked example
A 40 ft run of 3" building drain at ¼" per foot needs 10" of fall. If the house end is 18" below grade, the outlet end must be at least 28" deep.
Bedding and backfill
Pipes need even support along their whole length, or they sag and hold water. Lay them on a bed of granular material, surround them as the pipe maker specifies, and backfill in layers. Protect shallow pipes under driveways from traffic loads. Keep rock and large stones away from the pipe.
Access and inspection
Drains need access for clearing blockages: cleanouts at changes of direction, at the base of stacks and at set distances along long runs. Plan them in when you set out the run, not afterwards.
Testing
New drains are normally tested before they're covered, usually with an air or water test, and inspected by the plumbing inspector. Price the testing time, and don't backfill until the test is passed.
Common mistakes
- Not checking the outlet level before digging
- Inconsistent fall: dips and humps along the run
- Too steep a fall on long runs
- Poor bedding, leading to sagging joints
- Missing access points
The drain fall calculator gives the total drop and the fall per foot for any run, and the slope calculator converts between ratios, percentages and degrees.
Surface water and land drainage
Falls matter above ground too. Patios and paths should fall away from buildings, about ⅛–¼" per foot, and driveways need a fall toward a channel or permeable edge. French drains in yards are usually laid at a gentle fall, about 1:100 to 1:200, in a gravel-filled trench to an outlet or dry well. The same setting-out method applies: fix the outlet level first, then work back up the run.
Common questions
Can a drain be too steep?
On long runs, very steep falls can let water run ahead of solids, which then settle. If the ground falls steeply, drop the pipe down in steps with a drop fitting at a cleanout or manhole, rather than laying it at a steep angle all the way.
What if there isn't enough fall?
Look for a lower connection point, re-route the pipe, or use a sewage ejector or lift pump. Laying a drain flatter than the minimum invites blockages.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.