Drainage Slope, Velocity & IPC Sizing
Tony is running a 3" drain 24 ft from the toilet flange to the stack. IPC requires 1/4" per foot of fall — too flat and solids stall, too steep and water races ahead of the waste.
- Torpedo level
- Tape measure
- Pipe hangers
- Calculator
- 1
Identify the slope rule
Pipes ≤ 2 1/2" need 1/4" per ft. Pipes 3–6" need 1/8" per ft. 8"+ need 1/16" per ft.
- 2
Compute total drop
Length × slope. 24 ft × 1/4" = 6" of fall over the run.
- 3
Mark the high & low ends
Chalk the start elevation. From there, measure DOWN 6" at the stack.
- 4
Verify with the level
A torpedo level on a 12" section should read ~1/4 bubble off plumb.
60-ft of 3" sanitary drain under floor joists with only 10" of vertical headroom. IPC 704.1 mandates 1/4" per ft slope for ≤3" pipe (1/8"/ft allowed for 4"+). Manning's equation governs flow velocity — too slow and solids settle, too fast (>10 fps) and the slug runs away. Compute total drop, percent grade, and verify the line fits.
Tap Show next step to reveal the math one piece at a time.
30 ft of 3″ drain at 1/4″/ft
Given: Run = 30 ft, slope = 0.25″/ft
- 1
Multiply run × slope
30 × 0.25 = 7.5″
Will it fit under the floor joists?
Given: 60 ft run, only 10″ headroom for drop
- 1
At 1/4″/ft
60 × 0.25 = 15″ — TOO MUCH