Like it has already been said, you need to use actual roof pieces and not just using floor pieces on top. Also, you don't use any resources on repair, I don't know if you were mistaken about using wood to repair your houses or if you're actually destroying them and rebuilding, but either way the repair option in the build menu requires absolutely no resources whatsoever so there's literally nothing stopping you from spamming it if you have to mass repair a structure, which you shouldn't need to do all that often.