Slab Bathroom Addition with a Shallow Building Drain
A one-story ranch has a 4-inch PVC building drain under a 4-inch slab. At the proposed tie-in for a new basement bathroom, the building-drain invert is only 13 inches below the finished slab. The new group-water closet, lavatory, and a 36-by-60-inch shower-is 30 feet upstream of the tie-in. The water-closet outlet is about 7 inches below the slab, and the contractor wants to run a 3-inch PVC branch at the usual 1/4 inch per foot. The owner does not want a sewage ejector or a raised floor. The existing 3-inch soil stack is 9 feet from the new bathroom and continues through the roof. The local inspector has flagged the planned slope as unbuildable and asked for a revised layout. The contractor also planned to put a backwater valve on the new branch only, because the basement floor is below the next upstream manhole cover.
1. Determine whether the bathroom can drain by gravity and state the branch size and slope you would use. Show the calculation and explain the field margin.
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Available fall is the difference between the building-drain invert and the water-closet outlet: 13 in − 7 in = 6 in. A 3-in branch at 1/4 in/ft over 30 ft needs 30 × 0.25 = 7.5 in before fittings, so it cannot make the tie-in. A 4-in branch may be laid at 1/8 in/ft where the adopted code permits that slope for 4-in and larger drainage pipe: 30 × 0.125 = 3.75 in. That leaves about 2.25 in for fitting loss, insulation, and field variance. Use 4-in PVC from the closet bend to the building-drain wye, keep the slope uniform, and use long-sweep fittings. Confirm the local code allows 1/8 in/ft and that the building drain has capacity for the added group. If field measurement leaves less than about 1 in of margin, do not force it; use a sewage ejector or raise the fixtures.
2. Assume the lavatory trap arm is 8 feet and the shower trap arm is 7 feet of 2-inch pipe. Can the lavatory wet-vent the water closet? What venting layout would you install?
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The lavatory trap arm is too long for 1.5-in pipe, which is commonly limited to 6 ft. Upsizing the lavatory drain to 2 in gives an 8-ft limit in many model codes, but the table must be checked. The lavatory cannot wet-vent the water closet through a 1.5-in drain. A horizontal wet vent serving a water closet must be at least 2 in, and the water closet must be the most downstream fixture on that wet-vented branch. The shower’s 7-ft, 2-in trap arm is within the usual 8-ft limit, but it is acceptable only if properly vented. Best layout: run a 2-in vent-and-drain from the lavatory through the shower connection to the stack, connect the water closet downstream, and keep the lavatory trap arm at 6 ft or less-or use 2 in for up to 8 ft. Do not rely on the lavatory as the only vent.
3. Evaluate the proposed branch-only backwater valve. What location and type would you require?
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A branch-only valve can protect the new fixtures from sewage backing up the branch, but it does not protect the building drain or any other fixture upstream. Because the basement floor is below the next upstream manhole cover, the usual requirement is a backwater valve on the building drain downstream of all fixtures that can be affected. Install a normally open, accessible valve with a removable cover and a manual test or cleanout, protected from freezing and debris. If the branch valve is the only practical location, confirm with the AHJ that it protects every fixture on that branch and that no other basement fixtures drain through a point downstream of it. Do not install a normally closed valve that would leave the bathroom unusable during normal flow.