Decomposition happens to all wood eventually, but pressure treating, sealing, painting, and capping all help extend the life of your wooden fence.
When the wood has begun to rot, it loses strength to hold screws and other fasteners, as well as rigidity to stand upright. This doesn’t mean you immediately need a new fence though – it can likely be repaired or selectively replaced to save on costs.
Let’s look specifically at 4×4 pressure treated posts. Generally these are buried one-third in the ground (e.g. a fence 6 feet tall should have 9-foot posts buried 3 feet deep), hopefully based and surrounded by gravel and not concrete to help with drainage. The top of the posts can begin to rot over time, but that doesn’t mean you need to rip out the entire post and rebuild from scratch.
Tools Needed:
- Circular saw with adjustable miter angle shoe
- Reciprocating saw
- Miter saw (optional)
- Wood sealer, copper-based
- 3″ Decking-grade screws
- 4×4 Pressure treated wood (or matching post material)
- 2×4 Pressure treated wood (or half-width matching post material)
- Level
- Pencil
- Builder’s square (optional)
How Deep is the Rot?
First test the post to see how deep the rot has penetrated. Get a screwdriver and press it into the top of the post. It may easily sink in part or all of the way. We need to find where the solid wood begins. Sometimes the “knock test” is helpful to gauge approximately where this transitions. Knock on the post starting at the top, notice the sound (hopefully) changing from hollow to solid part way down the post.
If the post is solid at least halfway up, we can repair it instead of replacing it.
Cut Below the Rot
Next we will cut the post just below where it sounds solid. Remove any hardware, beams, or slats attached above this point, and give enough clearance for a circular saw and a reciprocating saw. For best results, attach a jig to the post to guide the path of your circular saw cut. Use the level and 2 screws to make sure it holds in place at the correct angle, parallel to the ground and perpendicular to the post.
Set your circular saw miter angle to 45 degrees and use the deepest depth setting so you cut diagonally through the post. The face of the post should have the straight edge and the sides of the post should get the angle. The circular saw should cut about halfway through the post and you can finish the cut with either the reciprocating saw or the circular saw (with miter set back to 0 degrees) cutting up both sides.
If the wood is still rotten, there’s no shame in making a lower cut. You just need to cut the post down to in-tact wood in the top half of the post.
Seal the Freshly Cut Wood
Once you’ve got a nice angled cut through in-tact wood, seal it with a generous coat of the copper-based wood sealer.
Next, measure the amount of post you’ve removed – add up the multiple cuts if you need to – and cut a fresh post segment the same shape as the removed wood. A table-top miter saw is the best tool here, but you can also use your circular saw. Cut 2 2×4 segments twice as long as this 4×4 segment. I prefer to use a 45 degree cut on one side that will be the bottom side of this support.
Apply a generous coat of sealer to all the cut ends on all the segments.
Attach the New Tops Using Braces
Attach the new 4×4 post segment using the 2x4s. It’s helpful if you line up and screw in one 2×4 support segment to the new 4×4 segment while it’s on the ground, and start the other screws. Use 4-8 screws per side depending on the length of post segment you’re replacing. Attach the other support board to the other side of the new post segment with the same amount of screws, spaced evenly.
Now it’s time to reattach your hardware and the rest of the fence structure. If any of those boards are rotten, it’s best to replace them now too.
Congratulations on repairing your fence, saving time and money while adding a design flare to your fence (if your posts are visible)! Use a self-priming or specialized fence paint to finish it up.