I'm planning to add a shower to a downstairs loo with chipboard floor. I've read that I should replace the chipboard with ply and install the tray before the wall tiles, but should I tile under the shower tray (ie. tile the floor first), or leave it mounted onto the plywood and tile up to it (ie. install the tray first)? My gut feel is to go under it, as that will make it easier to get under the tray for maintenance - any advice?