Why drains need slope
Drain, waste and sewer pipes rely on gravity. Too little slope and solids settle and clog the pipe; too much and the water runs ahead of the solids, which can also leave them behind. Codes set a minimum slope for horizontal drains.
Common slopes
| Slope | Percent | Typical use |
|---|---|---|
| 1/8 in per ft | 1.04% | pipes 3 to 6 inches in diameter |
| 1/4 in per ft | 2.08% | pipes up to 2½ inches, the usual default |
| 1/2 in per ft | 4.17% | short runs, steeper sections |
The International Plumbing Code and the Uniform Plumbing Code both require 1/4 inch per foot for horizontal drains 2½ inches and smaller and allow 1/8 inch per foot for 3 to 6 inch pipes. Local amendments vary, so check with your building department.
The calculation
Drop = length × slope
Worked example
A 20 ft horizontal drain at 1/4 inch per foot:
- Drop: 20 × 0.25 = 5 inches.
- Slope as a percentage: 0.25 ÷ 12 ≈ 2.08%.
- Angle: arctan(0.0208) ≈ 1.19 degrees.
- If the upper end sits at 0, the lower end sits at −5 inches.
Laying out the slope
- Set the upper and lower ends first, then run a string line between them and measure down to the pipe at intervals.
- A 4 ft level with a shim at one end gives a quick check: a 1 inch shim under one end of a 4 ft level shows 1/4 inch per foot.
- Support pipes at the spacing required by code so they do not sag between hangers.
- Underground, compact the bedding well so the slope holds after backfilling.
Other pipes
- Storm drains and yard drainage often use 1% to 2% as a minimum.
- Gutters run much flatter, about 1/16 to 1/8 inch per foot toward the downspout.
- Condensate lines from air conditioners need at least 1/8 inch per foot.
- Accessible ramps are a different case: the ADA limits them to 1:12, or 8.33%.
When the drop is too large
If the fall over the run is more than the slope allows, keep the horizontal sections at the code slope and make up the difference with a vertical drop or a fitting at 45 degrees. Long runs at steep angles are a common cause of slow drains.