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Fierce winds can crack fence posts

Sun., June 22, 2014

DEAR TIM: I’ve got a 5-foot-6-inch tall wood privacy fence with very little gap between the vertical wood pieces. When I built the fence I used 4-by-4 posts in between the wood panels. The posts were buried 2 feet down and surrounded by concrete. I come home from work or wake up and my fence is leaning. At this point I’ve replaced 15 of the original 37 posts because they’ve cracked. What’s going on and how do I stop the posts from cracking? – Marcus P., Golden, Colorado

DEAR MARCUS: I think your cracked fence posts can be traced to one of the following: wind, animal scratching or mischievous teenagers bored with cow tipping.

My money would be on strong winds. You’d be stunned at the amount of force the wind can place on the side of a building or a solid fence like yours.

I would also wager that the 15 posts you had to replace were doomed from the start because they were installed such that the wood grain’s weakest face was aimed at the wind. You can crack a wood baseball bat with ease if you hold it wrong and hit a fastball. The same thing is true with wood fence posts. The grain lines you see at the ends of the posts should be oriented at 90 degrees to the fence line direction.

The concrete surrounding the posts might also be contributing to the post failure. The stiff concrete at the ground level doesn’t offer any pressure release and allows the posts to snap just as you might crack a dry twig in your hands.

I maintain that fence posts should be set in crushed stone, not concrete. The stone will provide good drainage and allow the posts to give ever so slightly in high wind situations. If the fence tilts after a windstorm, you can replumb the fence posts with minimal effort by digging out some of the stone on the one side of the post.

For decades I’ve set fence posts in crushed stone and they perform quite well. Farmers and ranchers typically set their fence posts in compacted soil, and it’s not uncommon for livestock to rub against their fence posts.

You can also switch to larger fence posts to see if that helps. The only issue I can see is a 6-by-6 post might not look right.

You can soften the appearance of the 6-by-6 posts by clipping each of the four corners at a 45-degree angle. This requires you to put them through a table saw or use a circular saw to make these modest cuts. Try doing this on a short piece of 6-by-6 post to see if you like the look. You only need to cut in 3/4 inch from each corner to make a distinctive chamfered edge.

Realize that not all wooden fence posts are equal when it comes to strength. There’s a vast difference in bending and cracking resistance across wood species. Douglas fir, oak and No. 1 southern yellow pine are some very strong wood species that you should be able to locate with minimal effort.

Cedar posts, while very attractive, are not as strong as you might think. The lighter-colored spring wood in cedar typically has a somewhat open cellular structure, and as a result it’s just not very strong when you stack it up against the beefy species I mentioned above.

To verify you’re making the right choices, it might be a good idea for you to contact at least two fence companies in your area that advertise they install solid wood fences. Ask to speak with the owner or the manager in charge of installations. A great company that’s been in business for decades will usually offer five minutes of their time to you.

All you want to ask them are four questions after showing them photos of your fence: What is the wood species they would use? What size would the fence post be? How deep should the post be buried? Is crushed stone a good fill material?

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