Break Room and Second Toilet Room: Fixtures, Trap Arms, and Backflow
A 26-person office tenant improvement converts a storage room into a break room and adds a second single-occupant toilet room. The plan shows: an 18-by-20-inch bar sink with a 1½-inch trap and 14 feet of 1½-inch trap arm running to an existing 2-inch stack; a dishwasher whose drain hose discharges into the bar sink through an air gap fitting; a coffee brewer with a ⅜-inch supply hard-piped to the building water; and a new toilet room with a 1.28 gpf water closet and a lavatory, vented by a ¾-inch air admittance valve installed inside the wall cavity. The existing 2-inch vent stack rises through the roof about 4 feet from the new toilet room. There is no floor drain in the break room.
1. Evaluate the bar sink trap arm as designed. Identify every code problem and give the corrective options.
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Two separate failures on the same pipe:
- Length. The trap-to-vent distance for a 1½-inch trap is 6 feet under the code's trap-arm table. Fourteen feet is more than double the limit, which means the fixture is effectively unvented and the trap will siphon.
- Fall. A trap arm may not fall more than one pipe diameter before it is vented. At ¼ inch per foot over 14 feet the arm drops 3½ inches on a 1½-inch pipe - a self-siphoning arm regardless of length.
Corrections, in order of preference:
- Reroute the fixture drain so the trap arm reaches a vent within 6 feet, using the nearby 2-inch stack as the vent source.
- Add a vent on the arm within the length limit - a 1½-inch vent rising to the existing stack, or a properly sized and listed air admittance valve located so the trap arm stays within distance.
- Not acceptable: up-sizing the drain alone. A 2-inch trap buys only 8 feet; the arm is still too long.
Also confirm the arm's slope is at least ¼ inch per foot back to the stack and that the connection is by a wye or combination, not a sanitary tee on its back.
2. Is the air admittance valve shown for the new toilet room acceptable as drawn? What would you require, and what would you check about the shared vent?
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Not as drawn. An AAV buried inside a wall cavity is concealed and cannot be inspected, maintained, or replaced - nearly every code that permits AAVs requires them to remain accessible and to be installed per their listing and the AHJ's conditions. Concealment alone justifies rejection.
Require:
- Relocation to an accessible location, typically in a box or cabinet above the fixture served or in an accessible chase with a removable cover.
- A listed AAV sized for the fixture, serving only what its listing permits. One valve cannot be used to vent multiple fixtures unless the listing says so.
- Verification the local jurisdiction permits AAVs at all, and whether an engineer's or AHJ's approval is a precondition.
On the shared vent: check that the new toilet room's vent and the break room fixtures connect to the existing 2-inch stack without creating a wet vent or overloading it, that the stack size remains adequate for the combined load, that the fixture connections stay within their trap-arm limits, and that the vent termination remains 6 inches above the roof (more if the roof is used for other purposes).
3. Evaluate the water-supply and discharge connections for the dishwasher and coffee brewer. What protection is required, and what would you cite?
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Both are potable-water protection questions, and both are fail-as-drawn for different reasons.
Dishwasher. Discharging through an air gap fitting is the right method, but the air gap must be installed at or above the flood-level rim of the sink and the hose must be secured and not pass through a cabinet where it can be kinked or become a concealed connection. Confirm the fitting is a listed air gap, not just a high-loop. Where the air gap cannot be above the rim, an air-break arrangement to a properly trapped indirect waste receptor is the alternative.
Coffee brewer. A hard-piped ⅜-inch supply to beverage equipment is a direct connection and requires backflow protection - either an air gap or the listed device the manufacturer specifies (commonly a vacuum breaker or a reduced-pressure or double-check assembly, depending on the hazard). A bare pipe connected to the building water without protection is a cross-connection.
Also flag: the break room's lack of a floor drain is not a plumbing code violation by itself, but if a water heater or an ice machine is later placed in the room, the pan and drain requirements change the answer. Note that in the review comments so it does not surface at final.