Adding a Basement Bathroom in Manchester
A 1955 Cape Cod in Manchester has a 4-inch cast-iron building drain at 1/8 inch per foot and a 3-inch cast-iron stack that runs through the roof. The homeowner wants to add a basement bathroom with a 3-inch water closet, a 2-inch lavatory, and a 2-inch shower. The basement is unfinished, the floor is slab-on-grade, and the nearest existing vent is the 2-inch lavatory vent on the first floor, about 12 feet away horizontally and 8 feet above the basement floor. The proposed fixtures are within 6 feet of the stack. The contractor proposes to connect the new fixtures to the stack and use a 2-inch wet vent from the lavatory for the group. The ceiling has 2-by-10 joists and limited space. The town inspector is concerned about trap arm lengths, vent size, and frost closure at the roof terminal.
1. Can the proposed 2-inch lavatory drain serve as a wet vent for the water closet and shower? Explain the sizing decision.
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Not as proposed. The bathroom group is a water closet (3 fixture units), a shower (2 fixture units), and a lavatory (1 fixture unit), for a total of 6 fixture units. Under the wet-vent sizing table in the adopted plumbing code, a 2-inch wet vent is limited to 4 fixture units; 6 fixture units exceeds that limit. The lavatory drain therefore cannot wet-vent the entire group at 2 inches. The installer must either increase the wet vent to 3 inches, if the code table and layout allow it, or provide a separate vent for the water closet or shower. The connection also has to be arranged so that the wet vent is downstream of the fixtures it serves and the fixture drains connect in the correct order. The inspector should require a revised isometric showing fixture units, pipe sizes, and the vent connection before approving the rough-in.
2. Check the trap arm lengths and slope requirements for the three fixtures.
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At a 1/4-inch-per-foot slope, the maximum trap arm length from the trap weir to the vent is commonly 8 feet for a 2-inch trap arm and 16 feet for a 3-inch trap arm. The lavatory and shower are 2-inch traps and are within 6 feet of the stack, so their trap arms are acceptable if the slope is maintained and the vent connection is properly located. The water closet is a 3-inch trap arm and is also within 6 feet, so it is acceptable from a length standpoint. The installer must still maintain the required slope, use approved fittings, and keep the trap arm from turning more than the code allows before the vent. If the actual routing adds length or turns that exceed the limit, a separate vent is required. The inspector should verify the slope with a level and check that the drain does not become an S-trap or a running trap.
3. What about the roof vent terminal and the possible use of an air admittance valve?
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New Hampshire’s cold climate makes frost closure a real concern. The adopted code generally requires vent terminals in areas subject to frost closure to be increased in size, often to at least 4 inches at the terminal, so a 2-inch vent through the roof may not be acceptable without confirmation from the local inspector and the code amendment. The vent must also terminate at the required height above the roof and away from windows, doors, and air intakes. An air admittance valve may be permitted for an individual lavatory or shower in some installations, but it is not a substitute for a required vent for a water closet, and it cannot be used where the local code prohibits it. The safer approval path is a properly sized roof vent or a connection to the existing vent system, not an AAV as the sole vent for the bathroom group.