Basement floor drain backs up when the washer runs: what it says about your sewer line

He described the problem on r/Plumbing on Oct. 31. Standpipes are one of the common backup defenses in Chicago-area basements: a large PVC pipe sealed into the floor drain that lifts the lowest opening in the house above the sewer, WBEZ reported. The note on the washer limits how much water hits the drain at once. It does not tell anyone where the water is getting stuck.

A washer pump sends water out faster than a 2-inch drain can take it, so the standpipe holds the extra water briefly, according to a code summary by Seamless Home. The International Residential Code requires that standpipe to rise 18 to 42 inches above the trap weir and to sit on a 2-inch trap. Roto-Rooter's list of causes covers lint and soap in the standpipe, a kinked hose, too much detergent, a partly blocked main line and, in older houses, pipes too small for a modern machine. If water backs up after a load or two, the clog is probably past the washer and has to be cleared from the main cleanout, the company says.

In an older suburb the prime suspect is the private lateral running from the house to the street main. Roots, grease or debris can choke it, and cracks or extra connections can let in more water than it can carry, WBEZ reported. A plumber's camera inspection of that line is how owners find out which it is.

A backwater valve solves a different problem. A check valve installed in the private line, often through a pit dug in the front yard, lets wastewater out but stops it from flowing back from the public main, WBEZ reported. That matters in Chicago's combined sewers, which the Metropolitan Water Reclamation District's modeling shows are overwhelmed by two-thirds of an inch of rain in an hour. Wheaton, Oak Park and Morton Grove are among suburbs that have offered rebates or cost-sharing for check valves and overhead sewers. A valve does not clear a clog in the house's own line.