Stack Sizing and Venting in a Converted Loft
A warehouse is converted to a restaurant on the ground floor and a dwelling unit above. The existing 4-inch cast-iron soil stack serves two water closets, one floor drain, and a new kitchen sink on the ground floor; the upper floor adds a lavatory, a shower, and a clothes washer. The building drain runs 40 feet at 1/8 inch per foot to a 6-inch public sewer. The contractor wants to keep the 4-inch stack, add an air admittance valve (AAV) at the washer standpipe, and skip a vent through the roof because the roof membrane is new. The inspector notes the stack carries more than 500 fixture units at its base and that the kitchen sink is on the same branch as the floor drain.
1. Assess the proposed venting strategy. Which elements are acceptable and which are not?
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Separate the request into its parts.
Keeping the 4-inch stack is a sizing question, not a venting question. A 4-inch stack can carry a large fixture-unit load, but the load must be checked against the table for the stack type (stack, stack with offset, stack with more than two branch intervals). If the computed load exceeds the table value for a 4-inch stack at that height, the stack is undersized regardless of venting. The stated figure of more than 500 fixture units at the base should trigger an actual calculation rather than an assumption.
The AAV at the washer standpipe is the weakest element. AAVs are permitted only where the code allows them, must be accessible, must be in the same room as the fixture served, and cannot substitute for a required vent through the roof where the code requires a building vent or where the AAV would serve as the only vent for the drainage system. A washing machine standpipe is a high-suds, high-discharge fixture; many codes specifically restrict AAV use there. The correct answer is: acceptable only if the local code permits AAVs at that location and the AAV is accessible and sized for the standpipe's drainage fixture units - otherwise it must be a true vent.
Eliminating the roof vent entirely is not acceptable if the building drainage system requires a vent stack or stack vent to open air. AAVs admit air but do not relieve positive pressure or vent the system to atmosphere; a system with no open vent cannot relieve sewer gas or equalize pressure.
2. The kitchen sink and floor drain share a branch. What is the concern and what would you require?
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The concern is cross-contamination and backflow within the drainage system, plus a venting problem.
A floor drain and a kitchen sink on the same branch means the sink's waste discharges past the floor drain's trap arm. If the branch is improperly vented or the drain backs up, waste can back up into the floor drain and, worse, the sink can be exposed to drain air from the floor drain's branch. Food-handling areas have stricter separation requirements: many codes require that the kitchen sink discharge through an indirect waste or that the floor drain be downstream and separately trapped and vented.
The correct response:
- Separately trap and vent the floor drain; do not rely on the sink's branch for its venting.
- Verify trap-arm distance from the fixture to its vent against the pipe diameter and slope; a long, flat branch will siphon the floor drain trap.
- Require an air gap or air break for any indirect connection from food equipment, and confirm the sink is not connected downstream of the floor drain in a way that lets it back up into the food area.
- Check the building drain slope: 40 feet at 1/8 inch per foot is 5 inches of fall, which is fine for a 6-inch drain, but the 4-inch stack-to-drain transition must be checked for adequate fall and for a proper fitting (no reducing fittings that create a flat spot).
The judgment being tested is that venting and drainage separation are not interchangeable: fixing the vent does not fix the contamination path, and fixing the contamination path does not fix the vent.