Duplex Sewer Lateral and ADU Connection on a Surcharging Main
A 1948 duplex has a 4-inch cast-iron sewer lateral 70 ft long to the city main. A laser level shows 7 inches of fall over the 70 ft, and the city main has surcharged into the duplex floor drains during two recent storms. The owner is adding a 600-square-foot ADU and proposes to connect its 2-inch building drain to the existing lateral 10 ft downstream of the duplex, using 1/16 inch per foot slope and a 2-inch vent through the roof. There is one existing 4-inch cleanout at the building and no backwater valve. The contractor also wants to route the ADU roof downspouts and a new area drain into the same lateral to avoid a separate storm line. The inspector has observed standing water in the lateral and roots at the clay joint near the city main. The owner wants to know whether the existing lateral can be reused and what must change before the ADU is approved.
1. Evaluate the existing lateral slope and the proposed ADU drain slope. What corrections are required?
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The existing lateral has 7 inches of fall over 70 ft, or 0.10 inch per foot. That is less than 1/8 inch per foot, which is the minimum commonly allowed for a 4-inch building sewer; some local amendments are stricter. The line also has standing water, roots, and a clay joint problem, so slope alone does not establish adequacy. The proposed ADU 2-inch building drain at 1/16 inch per foot is not permitted; a 2-inch drain requires 1/4 inch per foot. At 1/16 inch per foot it will not achieve self-cleansing velocity. Require CCTV inspection of the lateral. If the lateral cannot be corrected to the code slope and condition, replace it. The ADU connection must be made with an approved wye, not a tee, and the 2-inch line must be re-piped at 1/4 inch per foot.
2. What backwater protection is needed given the city main surcharge?
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Because the city main has surcharged into the duplex floor drains, backwater protection is required. Install a listed backwater valve on the affected branch and likely on the entire building sewer where fixtures are below the surcharge level. The valve must be accessible for maintenance and not buried without an access cover. The existing floor drains may need individual backwater valves or raising. The ADU connection should be protected by the same building backwater valve or by its own listed valve if the ADU is below the surcharge level. Do not rely on a cleanout cap or a manual gate valve as the only protection. Check the Los Angeles amendments for the required type and location.
3. Can the ADU roof downspouts and area drain connect to the sanitary lateral? What testing and cleanout requirements apply?
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No. Stormwater from roof downspouts and area drains cannot be connected to the sanitary sewer. It must go to a separate storm drain, drywell, infiltration planter, or approved LID system. In Los Angeles, the project may trigger stormwater and LID requirements, and adding storm flow to the sanitary lateral would overload it and violate the code. The new sanitary work must be tested by hydrostatic or air test as required by the Los Angeles Plumbing Code, with joints inspected before cover. Provide cleanouts at the ADU and at the connection, and repair the root-damaged clay joint by excavation or an approved trenchless method. Re-test the lateral after repair.