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.
The wall needs to be run over a joist. Also, you should check with your inspector on the 2x8's before you get too far. Depending on the span, that might not go over too well. Also, double check with the folks who are supplying any floor tile, and have them check to make sure the floor is stiff enough. You wouldn't want that to crack after you laid it.
The span for that bathroom wall will only be about 9 feet. I am actually going to be renovating the kitchen, which is directly under the wall, though not under the whole bathroom, so I can add an additional joist under the wall from below. There are longer spans (about 18') elsewhere in that floor, but I was planning on taking the ceiling under them down and sistering another 2x8 right beside those long spanning joists, from bearing point to bearing point, and blocking the hell out of the floor structure. Will that be a good solution, you think? As for the tile floor and stiffness issue, I was going to add another layer of 3/4" plywood to the floor. Thanks in advance for any additional comments. I can really use the advice, approval, insight, or criticisms!