The exterior posts are Architectural Grade Douglas Fir. As long as the lumber is not cast within concrete or buried in tile, it will have the ability to breathe and keep from rotting. We installed these posts above finish floor, and they are mounted on steel post braces to avoid rot. Treating the post bases with a marine grade epoxy can further preserve the material. Alternatively, a more rot resistant material such as Cedar or Redwood could also be used.