I'm planning to replace two posts that rest on a concrete pad and support a porch roof. The home was built in 1998. Each post is formed from four 1x8's that are nailed into a box cross-section. They are structural; there is no column inside them. They are rotting at the base because the end-grain wood sets directly on the concrete and wicks up moisture. In addition there is decorative plinth which has a hard time keeping up with the seasonal swelling of the posts. Each plinth is formed from 1x10's mitred cross-grain and joined as a box that surrounds the post. So, the cross-grain of the plinth is nailed to the long-grain of the post. I've oversized replacement plinths by 1/4" on each side and they are still forced apart by the swelling of the columns.
I'm thinking there are two solutions. One is to use different materials, MDO perhaps. The second is to create a flat base from pressure-treated 2x10 on which the post will rest. What's the right track to solve both the rotting and the swelling problems?