Hi - I have just had an acrylic shower tray installed. It seems to have a GRP reinforcment underneath. There was some debat how to support it though, and the plumber eventually put some large blobs of mortar on the floor before squashing the tray on top. Unhelpfully, the underside was sloped in two dimensions, making support difficult.
Most of the shower is supported fine, but there is one area which, if stood on with a foot, flexes by maybe a cm or so up and down. The sides of the tray by the tiles do not flex.
How much of a problem is this likely to be wrt premature cracking? Since the trasy is in place putting more support in is very difficult...
Ben