I am doing a shower to bathtub conversion in my basement. I have purchased an above the floor rough bathtub. My question is regarding the support underneath the tub. The installation specs say to use mortar, but the specs are also for the non afr tub. I don't think just mortar will give it enough support. Our plumber said use foam spray, yet he doesn't really want to do the job.... Anyway.... what about using stringers or wood supports under or a combo of both?