Fitting a shower tray (concrete ground floor) for the first time. The instructions require: (i) a plinth (for access to the waste) (ii)18mm marine ply as a deck, (iii) then the tray itself is set in mortar on the ply (say 15mm thick mortar). Problem: all the shower traps I have looked at have a dimension between the bottom flange (which seals against the underneath on the tray) and the top of the compression nut (which allows the waste pipe to be connected to it) of between 12mm and 15mm. The total thickness under the tray (ply and mortar) is a minimum of (18mm + 15mm) 33mm. Therefore the waste pipe leaving the trap will foul the underneath of the plywood - and couldn't be fitted anyway as the fitting would be half way up the thickness of the plywood! Am I missing a trick here? What do I do? Grateful for and comments/advice/rolling about on the floor laughing etc.