What Size Fence Posts are Best?
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 4x4 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 fifteen of the original thirty-seven posts because they've cracked. What's going on and how do I stop the posts from cracking? Marcus P., Golden, CO
DEAR MARCUS: Three things come to mind, although it could be something few want to discuss. You haven't heard about any strange crop circles in your neighborhood or the open range near you, have you? On a more serious note, I think your cracked fence posts can be traced to one of the following: wind, animal scratching, mischievous teenagers bored with cow tipping.
If I were a betting man, my money would be on strong winds. The photo of your fence tells the tale in my opinion. 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. To experience this, all you have to do is try to hold up a single piece of plywood on a breezy day. I've been blown over before trying to carry sheets of plywood on windy days.
I'm also willing to wager the fifteen posts you had to replace might have been 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 the 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.
I'm sure you've had your fill of breaking out the concrete so you can reset your new posts. That's very hard work. I did it one time and swore I'd discover a better way. 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 fenceposts.
You can also switch to larger fenceposts to see if that helps. The only issue I can see is a 6x6 post might not look right and could appear to be too massive. But on the other hand, they could look very handsome.
You can soften the appearance of the 6x6 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 6x6 post to see if you like the look. You only need to cut in just 3/4 of an 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 #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 too 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 that is in charge of all 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?