If the wood is properly seasoned prior to installation, then it ought
not shrink any further. I would use pressure treated 4x2 for the basic
framework, with a top layer of 19mm WPB ply, and then if its a stone
resin tray, bed that onto a half inch screed of sand and cement -
probably with a bit of SBR in there as an admixture.

When installing the tray, butter the wall facing sides with silicone,
and rule off the bead cleanly at the top edge of the tray. Then tiling
/ boarding - bring those down toward the tray, but leave a 1/4" gap,
to later fill with silicone. Having a wide enough bead ensures it will
get good adhesion, and also will tolerate any small amount of
movement.



Sounds good - thanks


I was thinking of getting a 1700mm tray and that would need chopping into
the wall as is the bath it is replacing, The space is about 1680mm. I am
now thinking this is additional work for no real benefit and am now
thinking of a 1600mm tray. This will leave a small gap to be dealt with.

Do you think it is a better way? Any thoughts on the "gap"