Drainage Systems, Traps, and Vents
The Drainage Concept
A drainage system carries waste by gravity while protecting the trap seal with air. Every fixture except those specifically exempt (such as some direct-connected fixtures and certain interceptors) requires a trap. Each trap must have a vent, and the vent must connect so the trap arm does not become a combination waste and vent unless the code permits it. Understand the parts: fixture drain, trap arm, branch, stack, building drain, building sewer, and vent stack, stack vent, and vent terminal.
Trap Rules Candidates Must Recall
Traps must be self-cleaning, of the same nominal size as the fixture drain, and have a water seal of the depth specified by code (commonly 2 to 4 inches). Prohibited traps include S-traps, bell traps, crown vents, drum traps (except where allowed for specific fixtures), and double-trapping. A trap may serve one fixture only, except where code permits a single trap for a set of fixtures such as a combination sink and laundry tub with a continuous waste.
Venting Traps in Questions
Common exam traps: confusing a vent stack with a stack vent; forgetting that a vent terminal must extend above the roof by the code minimum and be located away from operable openings; and assuming a wet vent is always allowed. Wet venting is limited by fixture type and size, and a bathroom group wet vent has strict conditions. Also know that a vent must rise vertically before offsetting below the flood-level rim in most cases.
Worked Example
If a lavatory trap arm is 1-1/4 inch and the code limit is a developed length equal to the diameter times a stated factor, the arm must slope at the prescribed rate and the vent must connect within that limit; exceeding it makes the arm a wet vent and may require re-venting.