Second-Floor Bathroom Branch: Slope, Trap Arm, and Wet Vent in a Tight Ceiling
A general contractor is remodeling a 1950s two-story house in Duluth. The second-floor bathroom is moving to the other side of the house, 24 feet (developed length) from the existing 3-inch cast-iron stack. The new branch will be 3-inch PVC and must cross a first-floor hallway ceiling framed with 2x10 joists; the finished ceiling will be 8 inches below the bottom of the joists. The owner wants to keep the ceiling height. The new bathroom group includes a 1.28-gpf water closet, a lavatory with a 1-1/2-inch trap, and a tub/shower with a 2-inch trap. The lavatory is 7 feet (developed length) from the point where it can be vented to the existing stack; the tub is 5 feet from the same vent. The contractor proposes to run the 3-inch branch at 1/4 inch per foot, use the lavatory as a wet vent for the toilet, and connect the lavatory trap arm to the stack without a separate vent.
1. Calculate the fall required at 1/4 inch per foot and at 1/8 inch per foot over the 24-foot run. Does the 3-inch branch fit in the available 8 inches, and what do you tell the contractor?
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At 1/4 inch per foot: 24 ft x 0.25 in/ft = 6 inches of fall. Add the 3-inch pipe diameter and you need about 9 inches of vertical space, which exceeds the 8 inches available. At 1/8 inch per foot: 24 ft x 0.125 in/ft = 3 inches of fall. Add the pipe diameter and you need about 6 inches, which fits with roughly 2 inches to spare for fittings, insulation, and framing variation.
The code permits horizontal drainage piping 3 inches and larger to be installed at 1/8 inch per foot, provided the slope is uniform and approved. It does not permit a 3-inch line at less than 1/8 inch per foot, and it does not permit a smaller pipe to be substituted just to gain slope without checking fixture-unit capacity and venting. Tell the contractor to run the 3-inch branch at 1/8 inch per foot with continuous support and no sag, and to verify the local inspector's position before framing is closed. If the jurisdiction or the engineer requires 1/4 inch per foot, the ceiling must be lowered or the route changed; do not hide a 1/4-inch-per-foot line in an 8-inch space.
2. Is the 7-foot lavatory trap arm with a 1-1/2-inch trap legal? What about the 5-foot tub trap arm?
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No for the lavatory. The maximum developed length from the trap weir to the vent for a 1-1/2-inch trap arm is 6 feet; 7 feet exceeds that limit, so the lavatory would be a crown-vented or unvented trap arm. The 5-foot tub trap arm with a 2-inch trap is within the 8-foot limit for 2-inch pipe and is acceptable if the vent is sized correctly and the arm slopes uniformly.
Correct the lavatory by one of these methods: move the vent connection to within 6 feet of the trap weir; enlarge the trap and trap arm to 2 inches if the fixture outlet and code allow it, which raises the limit to 8 feet; or install a separate revent within the limit. Do not simply connect the 1-1/2-inch arm to the stack at 7 feet and call it vented. Because the lavatory is also proposed as a wet vent, the trap-arm correction and the wet-vent correction must be designed together.
3. Can the 1-1/2-inch lavatory drain wet-vent the water closet as proposed? Explain the requirements and the needed correction.
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No. A wet vent serving a water closet must be at least 2 inches in size and must meet the length and arrangement limits for wet venting. The proposed 1-1/2-inch lavatory drain is too small to serve as the wet vent, and its 7-foot trap arm is over the individual trap-arm limit as well. A 1-1/2-inch lavatory drain is acceptable as a fixture drain, but not as the wet vent for a water closet.
The correction is to make the lavatory drain and wet-vent section 2 inches, keep the lavatory trap arm within the applicable limit, and connect the water closet downstream of the lavatory so the toilet is the last fixture on the wet-vented branch. The vent must be sized for the fixtures served and must rise vertically or at the required slope. If the layout cannot meet the wet-vent length and size rules, individually vent the water closet and connect the lavatory to its own vent. The branch also serves a tub/shower, so check the total fixture units and the vent size before calling the design complete.