I am having a new bathroom fitted in a couple of weeks and am having 12mm travertine wall tiles fitted. They are big 400mmx 400mmx 12.5mm and likely to be pretty heavy. The supplier recommends hardy backer board dabbed and then screwed to the existing walls once old tiles are removed. Plumber who is doing the work reckons this is not necessary and plans to strip old tiles, seal the walls with pva and tile as normal. This bathroom job is costing a bit and want the tiles to stay up there!! Any advise would be very welcome :!