Basement Bathroom Added Off a 1920s Cast-Iron Stack
An owner of a 1926 Capitol Hill rowhouse wants to convert a basement storage room into a rentable suite. The basement floor is 7 ft below the curb. The existing 4 in. cast-iron soil stack is in a rear chase and continues through the roof; it serves two upper bathrooms. The new layout puts a water closet 8 ft from the stack, a lavatory 14 ft away, and a 36 by 48 in. shower 6 ft away. The owner bought a 2 in. air admittance valve and wants it to serve the lavatory so no wall has to be opened. The contractor proposes cutting the slab, connecting a new 3 in. branch to the stack with a sanitary tee, and relying on the AAV as the only vent for the new group. A 4 in. building drain runs under the slab to the public sewer; its invert elevation is unknown. The owner wants to avoid a sewage pump and asks whether the layout will pass inspection.
1. Evaluate the proposed venting arrangement. Will the AAV as the only vent pass, and what venting does the water closet need?
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The AAV cannot be the only vent for the group. An AAV is an individual or branch vent; it vents the fixture it serves and does not provide the open vent path required for the water closet. The closet is 8 ft from the stack, so the plan must check the closet's trap-arm length to its vent connection against the code table for the branch size. If the stack cannot serve as the closet vent within that limit, add a 2 in. vent on the closet branch, run it up to the existing stack above the highest fixture or through the roof. A wet vent is possible only if the lavatory drain is sized and arranged as a wet vent, individually vented, and the closet connects downstream within the allowed distance. Do not cut the slab until the vent route and fixture-unit load on the stack are checked.
2. The contractor says gravity drainage is fine because the basement floor is only 7 ft below the curb. What must be verified, and what backwater protection is required?
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Seven feet below the curb does not prove gravity drainage. The plumber must obtain the sewer invert elevation and compare it with the proposed building drain invert. At 1/4 in. per ft, a 20 ft run loses 5 in.; if the sewer is not low enough, the fixtures cannot drain by gravity and a lifting pump/ejector is required. Also, fixtures whose flood-level rims are below the curb level or below the next upstream manhole cover need backwater protection. Use an accessible backwater valve on the branch serving the basement group, and provide a cleanout. Check the DC amendments for the required type and location. Do not rely on a floor drain or a check valve at the fixture.