Undersized Vent on a Bathroom Group in a Renovated Duplex
A contractor is renovating the second floor of a two-family dwelling. The existing 3-inch soil stack serving a water closet, lavatory, and bathtub on the second floor terminates through the roof. The contractor wants to add a second bathroom group (water closet, lavatory, shower) on the same floor and connect it to the existing stack, keeping the existing 3-inch stack as the only vent for both groups. The building is 28 feet tall to the roof penetration, and the stack is 12 feet from the new fixtures. The plumbing inspector must decide whether the existing vent arrangement may serve the expanded load or whether a separate vent is required. The contractor argues that the 3-inch stack is "plenty big" because it already vents three fixtures.
1. Using the logic of vent sizing for a stack serving multiple fixture groups, explain how the inspector should evaluate the contractor's claim that the 3-inch stack is adequate for the added group.
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The inspector should not accept "big pipe equals adequate vent" as reasoning. Vent adequacy depends on the total drainage fixture units (dfu) connected and the developed length of the vent, not on the pipe's diameter alone.
Step 1: Count the load. A water closet is typically 3 dfu (or 4 in some tables), a lavatory 1 dfu, a bathtub 2 dfu, and a shower 2 dfu. The existing group is roughly 3 + 1 + 2 = 6 dfu; the added group is roughly 3 + 1 + 2 = 6 dfu, for about 12 dfu total on the stack.
Step 2: Compare to the vent capacity table. A 3-inch vent at the developed length in question (stack height plus offset, say 40 feet) can vent far more than 12 dfu, so the size is not the issue.
Step 3: Check the real constraints. The critical questions are whether the stack is a wet vent (carrying drainage from one group while venting another), whether the new fixtures connect above the highest fixture on the stack, and whether the vertical distance from the fixture trap to the vent connection exceeds the trap-arm limit. A stack that is wet-vented has stricter limits than a dry stack.
The inspector should approve only if the stack is dry above the highest connection, the developed length is within the table, and each new trap arm meets its distance limit. Otherwise, a separate vent or a relief vent is required. The contractor's argument fails because it addresses diameter but not venting configuration.
2. The contractor proposes connecting the new group's drain into the stack at a point 6 feet below the existing lavatory's trap. What venting concern does this raise, and what should the inspector require?
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Connecting a new fixture drain into a stack below an existing fixture's trap creates a wet-vent condition if the stack is being used to vent the upstream fixture. The inspector must determine whether the stack segment between the two connections carries drainage while also serving as the vent for the upper fixture.
If the stack is wet-vented, the code places limits on the number of fixtures and the dfu that may be wet-vented, and it prohibits certain fixtures (such as water closets) from being wet-vented by a lavatory or similar fixture. The lavatory's trap arm must also connect to the vent within the allowable distance, and the wet-vented portion must be sized per the wet-vent table.
If the connection would make the lavatory wet-vent the new group, or vice versa, the inspector should require a separate vent for the new group, or a relief vent, or a reconfiguration so the stack remains dry above the highest fixture connection. The inspector should also verify that the new group's trap arms meet their distance limits to the vent. Approval should be conditioned on a revised isometric showing vent connections, not on verbal assurance.