what determines thickness selected 20,28 or 35 mm?
Biggest difference between thicknesses of the board is the slope the shower tray provides. Thicker shower tray provides sharper slope. This might prevent water from pooling if plenty of water will be going through the shower, although even 20 mm thick tray provides sufficient gradient to make water flow towards the drain. As the bottleneck of the flow rate is the waste siphon trap and we include exactly the same to any thickness it does not determine maximum capacity of the shower tray which is always 26 l/min.
For timber flooring 20 mm thick tray is recommended, because floor boards usually are 18 mm thick and by proper installation you can avoid increasing rest of the floor height. If you have much space you can go for thicker tray if you prefer to have steeper shower.
20 mm thick trays are the most popular one, therefore we carry largest amount of them in our warehouse for next day delivery. Some of 35 mm thick trays might be available within 3 business days.
If you have any questions I will be happy to help.
David 020 7193 0953