Why is there a choice of tray thickness. Are the different thicknesses for different types of flooring. Is the thinnest tray not as strong as the others?
The biggest difference between each thickness is the slope, thicker shower tray provides larger slope, however even 20 mm thick tray provides sufficient slope for free water fall.
As they all have to be underboarded, even 20 mm thick shower tray is strong enough to bear a lot of weight.
For timber flooring 20 mm thick shower tray is the best option, if you have more space then you could go for thicker one, it might prevent water from getting outside the shower if you're going to have plenty of water going through it.
I am happy to help if you have any questions.
Matt B 020 7193 0953