Building a hearth based on the Recomendations in the wood stock keystone stove manual. 3/4 plywood base, 1/2in durock and then slate. Have two questions I have a seam wear two sheets of durock meet. Do I need to seal them with anything to for safety or can I just mortar and tile over that second, Can I run a screw done through the durock to secure to sub flooring, I thought I read somewhere that was not a good idea Possible heat transfer through screw. if i can do that how do i secure the durock to the plywood base or is it not nesscary to do so