Basement Bathroom Addition on an Existing Three-Inch Drain
A 1958 two-story house in Fargo has a full basement. The owner wants a basement bathroom directly below the first-floor bath: a water closet, a lavatory, and a 36 in by 48 in shower. The existing 3 in cast-iron building drain under the slab runs 22 ft at 1/4 in per ft to a 4 in PVC building sewer. The first-floor 3 in stack is within 6 ft of the new fixtures and has a 2 in vent through the roof. The basement floor is 18 in below the invert of the building sewer at the foundation wall, so the new shower and lavatory drains would be below the sewer. The owner proposes connecting the new group to the 3 in building drain with a 3 in PVC wye and using a 1.5 in trap arm for the lavatory and a 2 in trap arm for the shower, each about 5 ft long. No lift station is planned. The owner has pulled a homeowner plumbing permit and wants to start rough-in on Saturday.
1. Can the new bathroom group gravity-drain to the existing building drain? Explain the fixture-unit and elevation reasoning, and state what must change.
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No. The controlling issue is elevation, not fixture-unit capacity. The basement floor is 18 in below the sewer invert at the foundation wall. A 3 in drain needs 1/4 in/ft slope; even a short 6 ft run plus the wye and fittings consumes roughly 1.5 in of fall, and the shower trap weir and lavatory trap outlet would sit below the building drain and below the sewer. The proposed 3 in wye into the building drain would create a low point that holds waste and cannot be made to drain by gravity. Fixture-unit load is secondary: a private water closet (3 FU), lavatory (1 FU), and shower (2 FU) total about 6 FU, which a 3 in building drain can carry, but capacity does not cure a reverse slope. The fix is a listed sewage ejector/lift station with a vented basin, check valve, and full-size discharge, or a gravity route that stays above the sewer invert. Verify pump basin size and discharge with the adopted code and manufacturer; do not connect the new fixtures until the elevation is resolved.
2. Evaluate the proposed venting. Can the new fixtures use the existing 3 in stack as a wet vent or common vent? What venting arrangement would you require?
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The existing 3 in stack is a drainage stack, not an unlimited vent. A new basement group can be vented by a new 2 in vent that rises to the existing vent system or through the roof; it must connect at least 6 in above the flood-level rim of the highest fixture served by that vent, and the trap arms must stay within the adopted trap-arm length table. The lavatory 1.5 in trap arm at 5 ft is near the common 6 ft limit; the shower 2 in at 5 ft is within the common 8 ft limit, but confirm local amendments. Wet-venting the water closet to the lavatory is not automatic: the lavatory must be individually vented and the wet vent must be sized and arranged per code. An air-admittance valve may be allowed for individual fixtures under some amendments, but it cannot substitute for a vent through the roof for a water closet or for the building's vent system unless the AHJ specifically permits it. Provide a positive vent and test it.
3. What permit, inspection, and safety issues should the plumber address before starting?
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Before work: verify that the homeowner permit covers the scope and that a licensed plumber is allowed to perform it; submit a revised plan showing the lift station, vent, and electrical connection; obtain rough-in and final inspections. The ejector needs a dedicated circuit and alarm; the discharge must have a check valve and gate valve, and the basin must be sealed and vented. Check the existing 3 in building drain for condition and capacity, and confirm the new wye is not in a prohibited location. Protect the basement from sewage backup during tie-in, use a backwater valve if required, and follow trench and confined-space rules. Do not start Saturday without inspection approval.