On Tue, 04 Oct 2005 21:29:02 +0100, Kalico wrote:

Please can anyone tell me what they think the best board to use behind
the tiles of a shower?

One of the deliberately made waterproof cement-and fibre-based boards
for use in showers. Wickes sell "Aquapanel", I use "Versapanel" from
Avon Plywood and there are others. Plywood is very expensive for a
grade that's remotely likely to survive and plasterboard is terrible
with this degree of water exposure.

PVA glue will also soften and creep with long moisture exposure.