How much slope does a 10 foot drain pipe need?
2½″ of drop (¼″ per foot)
The short answer
A 10 foot drain pipe that is 2½ inches or smaller needs at least ¼ inch of fall per foot, so it should drop 2½″ from one end to the other. Pipes from 3 to 6 inches may use ⅛ inch per foot, a drop of 1¼″. These are the minimums in the International Residential Code, section P3005.3.
How it is worked out
- Run length: 10 ft
- Pipes 2½″ or smaller: 10 × ¼″ = 2½″
- Pipes 3″ to 6″: 10 × ⅛″ = 1¼″
Other sizes and options
| Run length | ¼″ per foot | ⅛″ per foot |
|---|---|---|
| 5 ft | 1¼″ | ⅝″ |
| 10 ft | 2½″ | 1¼″ |
| 15 ft | 3¾″ | 1⅞″ |
| 20 ft | 5″ | 2½″ |
| 25 ft | 6¼″ | 3⅛″ |
| 30 ft | 7½″ | 3¾″ |
What can change the amount?
Too much slope can be a problem too, because water outruns the solids. Check your local code, which may adopt the IRC with changes, and use a level with a slope reading to set each hanger.
Plan your bathroomRoomveil opens with these measurements filled in. Change them to match your room, see the result on a photo, and get the exact amount. Free, no sign-up.Open in Roomveil
Worked out with common rules of thumb and code minimums, for planning. Check the product label, the maker's instructions and your local building code before you buy or build. More detail in the Bathroom layout guide.