Sanitary Drainage Slope and Venting in a Renovation

The contractor's plan has two major issues. First, the building drain slope: the existing 4-inch cast iron drain is at 1/8 inch per foot, which is less than the code-required 1/4 inch per foot for 4-inch pipe. The code typically requires 1/4 inch per foot for 4-inch and smaller, but some codes allow 1/8 inch per foot for 4-inch and larger if approved by the authority having jurisdiction (AHJ). However, the contractor cannot assume it is acceptable without approval. The new 3-inch drain must slope at 1/4 inch per foot minimum. Connecting a 3-inch drain at 1/8 inch per foot would violate code and could cause solids to settle, leading to clogs. The 3-inch pipe carries toilet waste, which needs adequate velocity. Second, the venting: the existing 3-inch vent stack serves 10 fixture units. Adding 6 fixture units brings the total to 16, which is still within the 20 fixture unit limit for a 3-inch vent, so the vent size is adequate. However, the new bathroom fixtures must be individually vented or connected to a vent that is properly sized. The contractor plans to connect the new vent to the existing stack. That is acceptable if the stack is sized for the additional load and if the connection is made at least 6 inches above the flood level rim of the highest fixture served. Also, the new bathroom is within 6 feet of the vent stack, so a common vent or wet venting might be possible, but the code requires that each fixture have a vent unless it is a wet-vented combination. The contractor should verify that the existing vent stack is not already overloaded and that the new connection does not create a flat or negative slope vent. The consequences of improper slope include frequent clogging and possible sewer gas backup. Improper venting can cause slow drains and trap seal loss.

To address the slope issue, the contractor has two options: (1) obtain approval from the AHJ to use 1/8 inch per foot for the 4-inch building drain, which may be allowed if the pipe is sized for the load and the slope is approved; or (2) replace or re-slope the existing 4-inch drain to 1/4 inch per foot, which may be costly. For the new 3-inch drain, it must be installed at 1/4 inch per foot minimum. The contractor can install a new 3-inch drain from the bathroom to the main with the correct slope, even if the main is at 1/8 inch per foot. The connection must be made with a wye fitting and the new drain must be properly supported. For venting, the existing 3-inch vent stack is adequate for the additional 6 fixture units (total 16 < 20). The contractor can connect the new bathroom's vent to the existing stack, but must ensure the connection is at least 6 inches above the flood level rim of the highest fixture served by the stack, and that the vent is not obstructed. Alternatively, the contractor could install a new 2-inch vent through the roof if the existing stack cannot be used. The new bathroom fixtures can be wet-vented if they are within the allowed distance and the vent is sized accordingly. The contractor should also verify that the existing vent stack is properly sized for the total load and that it extends at least 6 inches above the roof. The solution should include a plan review by the AHJ and possibly a licensed plumber to perform the work.