Over the past two weeks we've noticed that whenever an upstairs shower is used there is the distinct smell of sewage (smell is in the vicinity of the shower drain). It lingers for a while but goes away between shower uses. 98 year old house. This particular shower has a toilet right next to it that shares a 4 inch cast iron pipe. The cast is in good shape - I can see it in between the joists from a small crawl space attic in between floors. These are the only two fixtures in the house that have not been replumbed due to accessibility issues. Originally there was likely a tub where the shower is now. It's used once a day by a 7 year old. I swept the drain and didn't find anything and it doesn't run slow. Thoughts?
