Roll top bath fitting and bathroom move around.

The thing with a roll top is that all of the weight of the bath, water & person will be placed upon the four exposed feet. Standard acrylic baths have 5 feet and most plumbers will also secure the bath to two or more walls to take the strain along as rigidity.If new pipe work is going to laid out for a new bathroom arrangement I would recommend remove all of the existing flooring, lay required pipework, ensure all joists are in a good condition and repair as required, and refit the floor with 18mm WBP (best) plywood. screw the floor into the joists at a minimum of 300mm intervals. (I screw every 150mm) and map out the pipes on the floor before screwing. avoid pipe channels by 50mm to be safe. It will cost about £120 in materials but will save problems later on!

Yes agree with everything Mercia bathrooms has to offer. only thing i would add is that if you are looking to tile the floor, do make sure that you are using a good quailty flexible adhesive and that it is trailed on evenly as opposed to dotting it onto tiles which may leave cavities beneath and cause tiles to crack at the four contact points. a cup full of latex added to the grout mixture will also give flexability and prevent cracking. good luck!