Mak Wilson wrote:
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?

The Durock is bonded to the subfloor with thinset. It does not need
the strength of the thicker board used for wall installations as there
is no span, and as you noted the thinner board doesn't raise the floor
as much. You can use the thicker board on the floor if you'd like -
there's no harm in it.

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"?

You don't need a big gap. Whether it's new or old that you're butting
up against, all joints between different wall and floor surfaces should
be caulked. Don't try to grout it.

R