I'm about to fit a B&Q quadrant resin/stone tray (once I take the warped
one back and get a flat one - I assume the top should be flat(?)). My
plan currently is to screw some 12mm marine ply to the new (green)
chipboard in the new extension (the waste will drop down between the
joists) and then bed the tray on 5:1 mortar on the ply.

Question: do I really need the 12mm marine ply if it's on green chipboard?

The default answer is "yes" but I'd rather avoid it because of
difficulty making the edge look good - unless someone can suggest a neat
way of trimming it around the quadrant...

--
Dave S
(The return email address is a dummy)