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