Determine the depth and height of the water damage. Next, mark at least one or two inches above the rot. Using a circular saw, cut across the post a depth of half the column. Also, determine the height of the joint to be at least two thirds the width of the column. Repeat the same depth on the opposite side at the same height. Finish cutting out the rotted wood with a handsaw, then separate the wood with a chisel. Prepare the new infill material by determining the height of the new material and cutting it to height, then matching the joint together. If necessary, apply wood hardener to the surface of the old wood to add more strength.
Drill large boles (Image 1) in both pieces of the wood so screws can be recessed. Clamp new and replacement wood together; drill pilot holes further into the wood for the screws. Unclamp the wood, then dampen the interior side of the replacement wood with a wet cloth and apply urethane glue in a diagonal pattern (Image 2). Clamp the wood, then connect the two pieces of wood using three inch galvanized screws. Next, cut plugs of wood from the remaining wood. Using wood glue, fill the screw holes with plugs. Allow four to six hours for the glue to dry. Sand the wood smooth.
If necessary, cut off any excess height from the infill material. Anchor the column to the porch using three inch galvanized deck screws. Prime the wood, then caulk the exposed nail holes. Finish with a final coat of latex paint.