Drive shims into the footing on all four sides of the post.
Illustration by Melanie Powell
Good neighbors make good fences and good fences require good planning, materials and construction. The Fence Bible (Storey Publishing, 2005) acts as a reference to construct and repair any fence that might be right for your property, with an explanation of project options and detailed step-by-step instructions from fence-building and home-improvement expert Jeff Beneke. The following is excerpted from chapter eight, “Repairing Fences and Gates.”
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Correcting Footing Failures
If the posts have loosened or fallen out of plumb, it is likely due to a problem with the footing or backfill. Concrete footings can crack or heave, and a tamped earth-and-gravel footing can soften over time. A failed concrete footing is hard to repair, and the best solution is to replace the fence post and footing. You can tighten the hold of an earth-and-gravel footing by digging out as much of the backfill as you can, replumbing the post, then backfilling and tamping. You might also try fastening pressure-treated shims to all sides of the post before backfilling and tamping.
If a concrete footing seems secure but the post has shrunk away from the concrete and loosened, drive pressure-treated shims into the concrete on all four sides of the post, checking for plumb as you do so. When the shims are tight, trim them level with the concrete and run a bead of clear silicone caulk around the top to seal the joint. This may not provide a long-term solution, but it can buy a few more years.
Mending fence posts that have decayed
Rotted posts can be replaced, with new posts being set in new footings in the same location as the old ones. But on some fences, this can be a challenging job that requires a lot of work and threatens the strength and integrity of the rest of the fence. In that case, it often makes better sense to try and reinforce the old post with another, partial post.
Begin by shoring up the fence on both sides of the bad post. Place blocks beneath the rails at the very least, and for a high and heavy fence I suggest you brace the top rail and infill from both sides. Remove the backfill down to the bottom of the post or, with a concrete footing, dig a new posthole right next to the existing one. Cut the old post off an inch or two above ground level. Remove the post; if the post is set in concrete, you should be able to wiggle it loose and lift it out with a helper or two. Clean out the hole, and make sure the fence is plumb.
Now add about 4 inches of gravel to the bottom of the hole. Cut a new post so that it reaches from the bottom of the hole to about 3 feet aboveground. Cut the top at an angle to shed water, and set the uncut end of the post in the ground. Add a couple more inches of gravel to the hole, and tamp it firmly. Attach the reinforcement post to the old post with H-inch carriage bolts spaced 8 inches apart. Fill the hole with concrete as described in Setting Fence Posts: Installing Concrete Footing. When the concrete has set, remove any bracing and blocks. Coat the bottom of the old post and all of the reinforcement post with a water-repellent preservative.
Replacing fence posts that have decayed
In most cases, the best way to deal with rotted posts is to dig them out and replace them with new fence posts set in new footings. On many fences, this can be done fairly quickly, especially if you are able to remove the adjacent fence panels. Often this can be done by removing some screws or brackets, but if the rails are toenailed to the posts, I would use a reciprocating saw to cut through the nails and then carefully set the fence panels aside.
Now dig out the backfill around the post or loosen the earth around a concrete footing. Rocking the post back and forth can sometimes help loosen it up, although if it is badly rotted, this will just as likely cause it to break off. If you have several concrete footings to remove, it might make sense to rent an electric jackhammer for a half day to bust up the concrete into pieces that can be lifted out.
Remove any loose dirt and other debris from the hole, then proceed to set a new post with either a concrete or an earth-and-gravel footing. If you want to use concrete but find that removing the old footings created very large holes, you can set a 12-inch-diameter fiberboard form (commonly referred to as a Sonotube, after one of the brands) in the hole on top of the gravel, then position and brace the new post in the center of the form. Carefully backfill around the form, tamping the earth every 4 inches or so. Finally, pour concrete into the form. Once the concrete sets, you can cut away any of the form that remains aboveground.
With the new post or posts set and the concrete cured, reattach the fence sections. Handle the fence sections carefully. You may find that metal fence brackets make reattachment easier.
Learn more fencing skills from The Fence Bible:
Mesh Fencing: Installation Basics
Setting Fence Posts
Excerpted from The Fence Bible © Jeff Beneke, photographs of hardware, materials, and tools © 2005 Storey Publishing, Illustrations by Melanie Powell used with permission from Storey Publishing. Buy this book from our store: The Fence Bible.