Mhtiler, I read this thread an hour or so ago and have been trying to think of a constructive response to what I've read. You've been a member on here for nearly 4 years and yet you have just posted that you are fitting slate on a wooden substrate with a ply overlay, this really isn't the best way to be approaching this job IMO. As a bare minimum you need a deflection free floor with a cement sheet overboard, many of the established pros on here would also be including a decoupling membrane.