I've just laid 20mm marmox in the bathroom and have left cutouts for the bog and bath feet (wide enough to marry with most standard baths) as suggested by Marmox technical as these are points of higher load concentration (bath moreso but the bloke suggested both).
I need to infill these with screed to level tomorrow - then give it a week curing and drying out time.
I *could* use an SBR mix, but I feel there is merit in allowing the substrate under the marmox to breath slightly. Anyone see any problems with a 1:4 basic mix at 20mm thick? It will be bonded to the substrate with cement slurry.