Bruce, perhaps I didn't state my point very clearly. If the floor meets L/360 it could flex up to 1" and the tile and grout should be ok. If it does not meet L/360 and flexes more that 1" there could be a problem. Obviously, we can't tell from here whether the defect is in design or execution. I guess I'd be supprised of individual joists were flexing, after all, they are 16" oc and have 1-1/8 of plywood between them; but, I've been supprised before :-). Any way, it's an interesting problem. I hope we will here more about it.