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In this video, Ask This Old House general contractor Tom Silva shows how to permanently patch a decaying porch post.

Steps for repairing a porch post:

  1. Unscrew and remove the handrail attached to the rotted porch post.
  2. Temporarily support the porch roof on either side of the rotted post with a hydraulic jack and a long 2x4 post.
  3. Use a reciprocating saw to cut the top of the post free from the overhead beam.
  4. Remove the rotted post and lay it across two sawhorses.
  5. Cut away the rotted section from the bottom end of the post using a circular saw.
  6. Cut a new post section from a rough-sawn cedar 6x6.
  7. Mark and cut a half-lap scarf joint into the new post section using a circular saw and reciprocating saw.
  8. Set the new post section on top of the old post and trace the half-lap scarf joint onto the old post.
  9. Cut a mating scarf joint into the old post.
  10. Sand the joint smooth with a random-orbit sander, then fine-tune the joint with a hammer and chisel.
  11. Apply several beads of construction adhesive to the scarf joint cut into the post.
  12. Evenly spread the adhesive over the entire joint with a flexible-blade putty knife.
  13. Fasten the new post to the old post with four 5-inch-long structural screws. Use an impact driver to drive the screwheads about ¼ inch below the surface.
  14. Use a power plane to trim the new post section flush with the old post.
  15. Fill all screw holes and cracks with exterior-grade wood putty.
  16. Once the putty dries, sand the post smooth.
  17. Coat all bare wood surfaces with exterior-grade spray primer.
  18. Screw the base plate to the bottom of the post.
  19. Stand the repaired post back in place on the porch, then release the hydraulic jacks and remove the temporary 2x4 posts.
  20. Secure the top and bottom of the post with screws, then apply a paint top coat to the post.
  21. Reattach the handrail to the post.