Hi all, 13 years ago I placed cement boards in the shower area on an old style shower tray. I have just removed all tiles and the cement boards with the 'rubberised' tanking has held up well - (more than I can say for the plasterboarded walls elsewhere in the bathroom. I intend to put a shower tray former in place, would it be ok to fill the bottom void (about 100mm) where the shower tray sat with marmox board of the same depth and then tape and give the whole area another lick of the waterproofing? I am hoping I don't have to remove what are good solid cement boards. Thanks for any help