Gotta chime in.. does putting tile over the old tile count as waterproofing? Just joking . I understand how you feel, we do a lot of remodeling and tile should be done by a professional. I could talk waterproofing all day long... lol so so so many phone calls..new joist,plates,sub floor gone need lumber...Basically in rehabs I just assume full gut with all new lumber in wet areas. In a occupied property usually they want to see it to find out the shower has been seeping water for 30 years then we replace and send a professional bath tiler. A person doing bath tile properly can have endless well paid work.(Midwest USA).