Installation of Sanitary Drainage, Waste, and Vent (DWV) Piping Systems
Sanitary drainage, waste, and vent piping
This domain tests gravity drainage layout, venting, trap protection, cleanouts, and testing. The exam expects you to keep the system self-cleaning and the trap seals intact.
Core rules
- Drain by gravity with a uniform slope, commonly 1:50 to 1:100 (1–2%) depending on pipe size and the code; never allow backfall.
- Use sanitary fittings: wyes and bends for drainage, sanitary tees on vertical stacks, and long-radius bends where needed.
- Every fixture trap needs a vent; the trap arm length and fall are limited so the trap is not sucked dry.
- Maintain the trap seal (typically 50–100 mm) and avoid double trapping, S-traps, and bell traps.
- Provide cleanouts at the base of stacks, changes of direction, and long runs; keep them accessible.
- Test the DWV system with water or air before closing; check joints, traps, and vent termination.
Worked example
A 100 mm DWV branch runs 8 m at 1:50. Fall = 8,000 / 50 = 160 mm. If the upstream invert is 99.50 m, the downstream invert is 99.50 − 0.16 = 99.34 m. The reasoning uses the slope ratio and subtracts because flow is downhill. The candidate then checks that the fixture trap arm is within its allowed length and that a vent is provided.
Traps
Installing a run with backfall, using a tee instead of a wye on a horizontal drain, omitting a vent, burying a cleanout, and measuring slope from the pipe outside rather than the invert all fail.