Yes they are, currently half tiled with 6x6 bog standard ceramic tiles with white emulsion walls. The existing tiles are dropping off and the plaster underneath looks like new (24 year old house). I thought take the existing off, make good if required, Unibond all walls, use Mapei waterproof paint for where the shower over bath will be and then add the new 600x300 ceramic tiles. My main concern is the weight on the paramount walls (new to me) though I see what you may be getting at, as I suppose the skim could break away under the weight? Thank you for your time.