On Thu, 22 Jul 2010 21:57:47 +0000, AL_z wrote:
This side of the pond they do a cement backer-board for tiling onto
(often branded as wonderboard)

Thanks - I was looking at 12mm cement board today in our Travis Perkins
store. It's fairly expensive, Agreed, it's probably the most reliable
option as far as staying inflexible and flat, long term, and least
likely to be degraded by dampness. However, I guess it'll be a bit iffy
trying to screw onto it, so I'm thinking of using WBP 18mm ply (exterior
grade ply). Perhaps not quite as reliable long-term, but more
screwable-into..

You can get special screws for attaching to the sub-surface (it still
needs bonding too, but you use screws as well to keep it all rigid). The
screws are self-starters, but they also have ridges on the underside of
the screw head so that they bed into the cement, rather than sticking out
slightly proud as a normal screw would (which would make tiling over them
harder). A box of 100 over here works out to about 4 quid (and I think I
paid around the equivalent of 12 quid for 4x8' 1/4" cement board sheets).

As for screwing things on later, once the tiling's done, a regular drill
will drill a pilot hole through the cement board easily; far easier than
drilling through tile. It's sturdy stuff, but it drills nicely (and cuts
pretty easily enough with a stanley knife - needs about three heavy
scores with a good blade, then it just snaps right along the scored line)

I'm not sure you'd need to go with 12mm; whatever 1/4" works out to in mm
will probably do, so long as you're retaining the plasterboard wall
beneath (1/2" cement board is significantly more expensive than 1/4" over
here).

cheers

Jules