—Anita & Elliot Seitzman, Newburgh, N.Y.
A: Norm Abram replies: Judging by the condition of the paint, rot has pretty well ruined the exterior of your columns. But it's possible that rot hasn't reached the underlying porch posts, the structural elements that hold up the porch roof.
The first step is to remove the boards and inspect the post. If it's also soft and rot-infested, you'll have to shore up the roof temporarily and install a new post that's treated to resist rot. While you're at it, check the condition of the porch foundation, deck, and roof, and fix any deterioration before you install the posts. No sense putting new posts on a weak foundation.
If the post is sound, you only need to replace the exterior boards and trim. I'd recommend using cellular PVC for both because it never rots, and it looks just like wood when painted. Caulk and screw the edges of the boards together, face-nail them to the post, then caulk their lower edges to keep water from reaching the post. By the way, if that rotted band below the boards is just decorative, I'd run the trim boards all the way to the deck and then apply the band. That would eliminate an unnecessary joint.