Firstly, it's important to understand that the roof once fitted can not simply just crack. The most common cause for this is one of either 2 reasons:

1. The screws have been over-tightened on install and caused a stress fracture which has caused the roof to open up a crack during the heating and contraction of the roof over during use of the shower. Normally this will be evident as it will crack out from where the screw has been installed.
2. The roof has accidentally been hit during washing or cleaning by pushing on the roof too hard.

Roofs are not covered under warranty as both options above suggest install or user error, again to reiterate, the roof can not simply just break as it's a non-moveable part. 

To order a replacement please call our sales on 01908 317512 and they will be able to advise.