Sewer odor from shower drain
My home is 10 years old, single story, slab on grade. For the last month I have been getting a sewer odor whenever I take a shower in my stall shower. I have put some "Liquid Plumber" in the shower drain, checked that the drain runs freely, but the odor persists. The stall shower is equipped with a fiberglass pan, so there doesn't appear to be any leaking around the drain fitting.
As a temporary measure I cover the drain when not in use with a flat rubber stopper. The other drains in the bathroom(tub, double sinks) do not appear to exhibit the problem. Any suggestions?