Ceramic tile underlay on bathroom floor

I have removed the tiles and screed from my bathroom floor, and want to put down ceramic tile underlay before the floor is re-waterproofed etc. However around the edges of the room the existing waterproof flashing has made a ridge about 40mm wide and 1-2mm high. Therefore the underlay will not be able to sit completely flat if it is layed with the floor as it is. This seems like a problem to me. Is it, and if so, any suggestions for how to make the floor flat before the underlay goes down?

I wouldn't think it would be a problem. You have to glue it anyway and the glue bead will be just under a mm thick. If there is a slight rise to the edge, that can be taken up when you lay the tiles.

JMB,

I did the same thing on the tile underlay I just put down on the floor. I have 75x50mm PVC angled flashing around the perimeter that I glued down. Yes it is raised, but as SilentC has said, it can be taken up when tiling no problem.