Slope and Venting in a Restaurant Remodel
A general contractor is remodeling the kitchen of a 1960s restaurant. The existing 4-inch cast-iron building drain runs 25 feet under the slab at a measured slope of 1/8 inch per foot to a 3-inch line at the property line. The new layout adds a three-compartment sink with a 2-inch trap arm and two 2-inch floor drains. The nearest existing 2-inch vent stack is 14 feet from the proposed sink trap, and the only other vent is a 1-1/2-inch lavatory vent 9 feet from the floor drain. The owner wants to reuse the existing underground piping to save money and schedule. The jurisdiction has adopted the Uniform Plumbing Code with local amendments, and the inspector has already flagged the vent distance. The GC asks you whether the existing slope is acceptable and whether the new fixtures can tie into the existing drain without adding a vent.
1. Evaluate the existing 4-inch building drain slope and the proposed 2-inch trap arm. What is acceptable, and what is the controlling defect?
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Under the adopted UPC slope table, 4-inch horizontal drainage at 1/8 inch per foot is generally acceptable, and the 25-foot run would fall about 3.1 inches. Do not approve it solely on arithmetic: verify with a laser or water test that the line has continuous fall and no backfall or sags from the old cast iron. The 2-inch trap arm must be at 1/4 inch per foot and is limited by the trap-arm rule: it cannot fall more than one pipe diameter before the vent. At 14 feet and 1/4 inch per foot the arm would fall 3.5 inches, exceeding the 2-inch limit. The controlling defect is therefore the vent distance, not the 4-inch slope. Local amendments may require 1/4 inch per foot for 3-inch and smaller; confirm the adopted table. Recommendation: add a vent within the allowed trap-arm distance or relocate the sink trap; do not rely on the existing 2-inch stack at 14 feet.
2. Can the two floor drains tie into the 3-inch line and use the existing 1-1/2-inch lavatory vent as a common vent? Explain.
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Each floor drain needs a trap and a vent. A common vent can serve multiple fixtures only when the fixtures are on the same floor, the vent is sized for the combined fixture units, and each trap arm is within its maximum distance to the vent. The proposed 2-inch floor drain trap is 9 feet from the 1-1/2-inch lavatory vent; for a 2-inch trap arm the usual maximum is about 8 feet, so the distance alone is a problem. The lavatory vent may also be undersized once the floor drains are added. Do not assume the existing vent is adequate. Calculate the fixture units, size the vent from the adopted table, and either install a dedicated vent or a properly sized common vent with each trap arm within the limit. If the floor drains cannot be vented, the connection is not code-compliant even if the drain slope is correct.
3. What should happen before the slab is closed?
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Submit a revised permit drawing showing the new vent and drain routing. Hydrostatically test the new drainage piping before concealment, verify slope, and have the inspector approve the rough-in. Camera the existing 4-inch line to confirm no cracks or backfall. Do not cover until approved, and document the test and inspection results.