Drainage, Waste, and Vent Systems
Drainage, Waste, and Vent Systems tests gravity drainage, trap seals, venting, slope, and sizing. Questions often give a pipe length or fixture load and ask for slope, trap-arm length, or vent size.
Core rules
- Building drains and sewers run at uniform slope: 1/4 inch per foot for 2 1/2-inch and smaller pipe, and 1/8 inch per foot for 3-inch and larger pipe.
- Drainage fixture units (DFU) convert fixtures to load; use the code tables to size drains and branches.
- Each fixture needs a trap, and the trap seal is normally 2 to 4 inches deep.
- Trap-arm length from the trap weir to the vent is limited: 1 1/4-inch = 5 ft, 1 1/2-inch = 6 ft, 2-inch = 8 ft, 3-inch = 12 ft, 4-inch = 16 ft. The arm slopes toward the drain.
- A vent terminal must extend at least 6 inches above the roof and above anticipated snow; 7 feet is required where the roof is used for purposes other than weather protection.
Worked example
A 3-inch building sewer runs 40 feet at 1/8 inch per foot. Total fall = 40 x 0.125 = 5 inches. A 2-inch lavatory branch at 1/4 inch per foot over 20 feet also falls 5 inches. If that lavatory trap arm were 10 feet from its vent, it would exceed the 8-foot limit for 2-inch pipe, so the trap seal could be siphoned. The fix is to move the vent closer or increase the trap-arm size only where the table permits.
Traps that cost marks
- Using 1/4 inch per foot on all pipe sizes.
- Measuring trap-arm length incorrectly or ignoring the vent limit.
- Reducing a vent below the required size.
- Connecting a vent so it also serves as a drain.
- Forgetting that horizontal drain slope must be continuous, without sags.