I'm fitting a 900x900 stone resin shower tray in an upstairs bathroom with conventional T&G floorboards. I'm fitting the tray at floor level with the waste going under the boards. The fitting instructions aren't clear, but say to remove "suspended" flooring & replace with 18mm marine ply, before bedding the tray on sand & cement. Is it really necessary to pull up the T&G boards and replace with ply, or can the tray be bedded directly onto the T&G boards? The instructions also refer to a "tiling upstand". What's that? I was planning to tile down to the tray & finish with silicon where the tiles meet the tray. Many thanks in advance for any advice.