We have a sewer smell coming up from our basement shower. The drain doesn't hold a constant level of water -- that is, two days after filling it, the trap is empty (and the sewer smell comes back). Any idea what this is and how I can fix it short of digging up the basement?
either there is no vent for the drain, or it is clogged allosing water from upstair to syphon the water out of the drain. to verify syphoning action, fill the drain and then watch as someone upstairs flushes the toilet or drains a sink. it there is syphoning action, the water levle in the trap will drop. it may take several flushes to emply the trap.