Things You'll Need
Attachment hardware to replace original
When a load-bearing post shows signs of stress, such as rot or cracking, it is time to consider a quick replacement before other structural members begin to shift beyond repair. Ensure that the replacement post you choose is equal to the task. Use a post identical to the first, if it has given decades of worry-free support. Otherwise, beef it up with a slightly thicker version. Take your time to properly lift and support the surrounding structure to maintain safety and the structural integrity of the property.
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Raising the Beam
Use a level against the bottom of the beam supported by the old post. Check the indicator; if the bubble is in the center, make your new post the same length as the old; if not, tip the level away at one end until the bubble is centered.
Measure the distance from the end of the level you tipped away back to the beam. Divide the length of the level into the length of the beam to calculate the distance the beam has settled.
Choose a replacement post, equal to or stronger than the original. Measure the height of the original post and measure and mark the new post for cutting, plus any settling you are going to correct. Make the cut on a miter saw for a square end.
Set two post jacks one on either side of the damaged post. Cut 4-by-4 posts to length, 3/4-inch shorter than the space from the top of the lowered jacks to the bottom of the beam above the post you are replacing. Position one post in the bracket on the top of each jack. Slide a 12-inch square cut from 3/4-inch plywood on top of each post and raise the jacks to engage the post against the beam.
Raise each jack a little at a time, going from jack to jack to maintain a level lift. Watch for a separation to occur at the top or bottom of the original post and stop lifting at that point.
Use a sledge hammer to tap the old post out of position. Raise the jacks if needed to account for settling. If it is more than 1/2 inch, raise the jacks 1/8 inch at a time and allow the beam to rest between raises for one hour.
Watch for signs of other damage on each rise. Get professional assistance if the beam pulls away, or significant cracking takes place.
Stand the new post in place and attach any brackets needed at the base and at the top, using hardware and fasteners similar to what was there before. Some posts are not attached at the bottom.
Let the jacks down slowly. If you raised it to correct for settling allow the beam to rest as you did on the way up. Remove the jacks and support posts.