In Australia waterproofing is supposed to sit under the tiles and can really only be done as per the building code if all the tiles are removed then the waterproofing is done over the whole floor and part walls and then the bathroom is re-tiled. Anyone with a waterproofing licence...like myself shouldn't issue a waterproofing certificate if it's not done this way...and basically this means a full bathroom renovation. A lot of the bathrooms we have resurfaced have been leaking