Backer board vs. plywood for tile underlayment
A friend of mine with virtually no experience decided to remodel the only bathroom in his house about a month ago (still an ongoing project, proving the advice that if you don't know what you are doing you are much better off hiring someone to do it!). It's a ~70 year old wood frame house so there was lots of termite damage in the joists and at least 4 layers of flooring that needed to be removed (3/4" pine t&g boards, mosaic tile, linoleum, and finally more tile) that all told was around 1.5" thick. I recommended he remove everything so weak joists could be sistered and new plywood decking be installed (3/4" t&g). I also recommended that he use 1/2" CDX plywood on top of the decking as underlayment for the new tile floor, as I felt that cement backer board might be too difficult for him to install properly. With the joists sistered and 1 1/4" of plywood for a subfloor, I can attest that everything feels very solid. Why, then, would the tile supplier make such a big deal about how he should have used backer board instead of the 1/2" ply as underlayment? Backer board doesn't improve the structural strength of the floor, and as far as I know, the only real improvement it offers over plywood is improved adhesion for the thinset. I do feel it is has merit in tub surrounds, but for floors I just don't see the benefit. Am I missing something??? I'd like to offer my friend a "third-party" opinion on this, even if it means I'm wrong and the tile lady is right.