In message , Russ
writes
We have just made a frame and plywood base for a stone resin shower tray. A
small note says lay it on a weak bed of motar but thats the only info. Any
idea of motar mix

I don't think it really matters that much, I would use say 6:1 or 8:1
(Sand:cement) mix.

and how thick it should be.

I probably used bed of about 1cm - 1.5cm

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Chris French, Leeds