I plan to use Durock backer board as underlayment for tile (in conjunction with plywood). My preference is to use 1/2". However, I downloaded Durock's install guide and got confused. It sounded as if they made two kinds, one for walls and one specifically for underlayment. They call the 1/2" and 5/8" board their "Standard Board" while the "Underlayment" version comes only in 5/16". Is there anything that makes the 1/2" board unsuitable for underlayment or is this just a matter of advertising the 5/16" as underlayment because it's thinner and will not raise the floor as much?
Secondly, convention suggests a gap of 1/8" between boards. However I can't seem to get a straight answer regarding the space that should be left between the backer board and the wall. 1/8"? 1/4"?
Help is a appreciated!