I am installing a newer tub over rotten sub-floor that has been masked in the past. I plan to tile on the outside floor around the new tub. Should I remove the old layer underneath where the sub-floor over the joist will be put down or is it alright to just put the new plywood over the old? The reason I ask, the floor under the sub-floor over the joist runs under what I believe to be a support wall around the tub area. This house was built in 1963. I think the joist are 12 inches apart, so I do not know what that means on center, if someone wants to explain that to me.