Oh, bother.
I pulled up the carpet pad and the (at least) fourty-year-old plywood underlayment in a few spots was crumbling apart like stale Wheatabix.
I cut out the rotten sections and I'll be patching with new plywood just to make sure the floor is level before I put down a cork vapor barrier/padded second underlayment and then my fake slate flooring.
There's also hardwood subfloor under the plywood. Good to know, I guess.









