My daughter lives in a house built before 1940. A plumber told them that their floor drain in the basement doesn't need a plug in the clean out. In fact, they were told that there was a 'built in' p-trap AND the reason floor drain was backing up from the washing machine was because of the mechanical seal that I installed in the clean out when they first moved in. I'm worried about sewer gas. Is a plug in the clean out hole always necessary?
Thanks!