Greetings,

1/2" backerboard is normally used on walls. Your subfloor may not be thick
enough.

"
a.. Use minimum 5/8" exterior grade plywood or 23/32" exterior grade OSB,
complying with Local Building Codes and ANSI A108.11, over joists. Joists'
spacing not to exceed a maximum of 16" on center.

a.. Deflection must not exceed L/360 of the span, including live and dead
design loads.*

a.. Do not use glue between grooves of tongue-and-groove subfloor to allow
for expansion and contraction of subfloor.
"

Please read the following
http://www.jameshardie.com/backerboa...stallation.php
http://www.thetiledoctor.com/installations/floors.cfm

Hope this helps,
William

"Adam" wrote in message
oups.com...
Well, the plywood appears to be 1/2". I haven't pulled up the linoleum
yet, so I can't see the joist spans. Can you tell me what
considerations would force me to choose 1/2" backerboard?
Thanks!