Our downstairs has been gutted due to 3' of flooding from hurricane Sandy. The sheetrock was removed 4' from floor...plywood floors will be removed after drying out period. I want to put hardi board down (tile man told me..I know NOTHING) I know 3/4 exterior plywood, then all that is necessary to get level floor so hardi can be installed. I like the thought of this being able to withstand any water contact. But what about the 4' of sheetrock that must be replaced. Can hardi board go there? I like my builder and I think the tile people do the right thing. I just want it done correctly and am trying to educate myself so I don't have to completely trust others to do the right/best thing for me.