Drainage Systems, Fixtures, and Fittings
Drainage Systems, Fixtures, and Fittings
This domain tests gravity drainage: slope, fitting selection, cleanouts, supports, and fixture connections. The exam asks you to recognize a code-compliant arrangement and to calculate fall.
Core rules
- Slope is 1/4 in/ft, about 2 percent, for 2-1/2 in and smaller pipe; 1/8 in/ft, about 1 percent, for 3 in and larger pipe, unless the code or manufacturer requires otherwise.
- Horizontal-to-vertical changes use a sanitary tee; vertical-to-horizontal and horizontal-to-horizontal changes use a wye or combination. Do not use a sanitary tee on its back.
- Each fixture gets a trap; no double trapping, no S-traps, and no trap below a fixture unless it is accessible.
- Cleanouts are required at the base of stacks, at changes of direction over 45 degrees, and at code intervals; cleanouts must remain accessible.
- Support pipe at intervals by size and material, allow for expansion in plastic and cast iron, and protect against shear at the foundation.
Worked example
A 3 in building drain runs 40 ft at 1/8 in/ft. Fall equals 40 times 0.125, or 5 in. If the same run were 2 in pipe, the required slope would be 1/4 in/ft, giving 10 in. The reasoning is that smaller pipes need more velocity to stay self-scouring.
Traps that cost marks
Using 1/4 in/ft on 4 in pipe when 1/8 is permitted can create excessive fall and poor venting; using too little slope causes solids to settle. Other losses include a sanitary tee on a horizontal branch, a missing cleanout at a 90 degree turn, and a fixture drain that is not aligned with the trap.