I am helping my brother remodel his bathroom which is we will say 7'x5.5'. I have the old vinyl floor and sub floor plywood up and we are down to the 1/2"x6" cross plank sub floor which rests on the 2x10 floor joists spanning 11'. The cross plank sub floor is not t&g. I have checked the deflection with an online calculator and it is ok for tile. My main question. I plan to lay 1/2" ext. grade plywood down over the cross plank and then 5/16" durock down on that. How should I screw these two layers down with respect to floor joists, orientation to the subfloor (perpendicular or parallel)? And should either layer be screwed to the floor joists? Is it best to keep the full sections of plywood or durock around the toilet where there is a lot of weight or in the higher traffic areas? I forgot to mention that it is just a sink and toilet bathroom, no shower or tub. Please explain your answer because I like to understand why I should do something rather than just knowing what to do? Thank you so much for your help!