Ran some laundry Sunday afternoon and the last washer-ful wasn't done until after sunset -- which was when I found the basement sink half-full of graywater, a puddle at the floor drain and evidence of a small overflow at the washer drain, too.
Floor puddle is still there this morning; the float is up in the floor drain. The main drain is probably partially blocked.
I'm off to try some drain cleaner ("Thrift," usually good stuff). Wish me luck!
Update: Ran the drain cleaner. (99% sodium hydroxide -- Wikipedia shows a container marked "Certified Lye." It's true!) The big basement sink appears to be draining okay. Guess I'll be able to risk a quick shower.
Update 2: Took shower. Checked basement sink, a bit over half-full. I see plumbers in my future. I hope a serious augering will do the trick.
Update 3: Whatever it is, it's still there tonight after a very large dose of Thrift this evening. The drain seemed to be draining. Waited 20 minutes, then did up a few dishes by hand, checked the basement sink and -- a third full. I need a plumber -- and time to be here with him. Hoping hoping hoping to be able to put it off until at least Wednesday.
2 months ago