Framing a wall, which will be a shower stall in the future and clad with stone tile. This wall is upstairs, in an upper level (attic) I hope to make into living space. It runs parallel to the joists, but I can't tell if it is directly over a joist. The joists are 2x8's at 12" O.C. and there are two layers of 1/2" plywood (or maybe a 1/2" and 3/8") put down as an existing subfloor. Because of the weight of the future tile, do I need to rip up the subfloor to see where the joists are, and possibly block between the ones the wall will go over, or will the plywood be enough to span the joists and support the finished wall? The wall will be about 9' long and about 8' high. Thanks in advance for your replies.