Even the treated wood often used to construct wood fences experiences decay and rot. Since a rotted fence post or plank might weaken an entire section of fencing, repairing such damage is a necessary task that saves both time and money later on. Thankfully, removing a rotted post or plank and replacing it is usually a simple task, and in some cases that part of the post or plank that remains intact can be saved.
Pull the rails or panels off the rotted fence post. For split rails, you can simply lift and pull to remove the rail. For panel fencing, pry the nails loose with a crowbar or claw hammer. Set the rails/panels down and outside the work area.
Cut the fence post loose from its rotted base with a saw, taking care to make a smooth level cut at a position at least 1 inch above the rot.
Put a wood block into a post spike, and set that post spike above the rotted wood that is still in the ground. Hammer the wood block to drive the post spike into the ground without bending the metal.
Install the good part of the wood post into the post spike's cup, and fasten it by tightening the bolts with a crescent wrench.
Put the rails or panels back on the post.
Remove the old picket, plank or rail. Split rails can be pulled out by hand. Plank rails need to by pried off the post with a crowbar or claw hammer. Pickets, planks and/or rails on a panel-style fence need to be removed from the larger structure by prying as well.
Discard and replace rotted split rails. Because these run from post to post, any trimming leaves them too short and any patchwork ruins their appearance. If you have a panel fence, skip this step.
Pull out any nails that were left behind in the post or panel with a crowbar, claw hammer or pliers. This often happens because the rotted wood breaks around the nail.
Set a replacement plank, picket or rail onto the position occupied by the rotted component. Once it is lined up, fasten it with nails. Replacement parts can be easily pulled off of leftover fence panels.
If you do not have any leftover fence panels in storage and cannot order replacement parts from the hardware store or manufacturer, you will need to make them by hand. Choose a suitable piece of treated lumber that matches to the extent possible the width and thickness of the old picket, rail or plank. Trim the height down to size with a saw, and use a plane to shave the width and thickness down to size.