DEAR TIM: What is the preferred method to set fence posts? My 5-foot-tall solid-picket privacy fence has had some issues over the past ten years. The latest problem is posts set in concrete that have snapped. I have installed some temporary steel fence posts to stabilize the fence as winter winds have caused it to lean. How would you install wood fence posts? Should I be using treated fence posts? Ron W., Maumee, OH
DEAR RON: Fence posts can be a real hassle. I have fixed my fair share of them over the years. My guess is there are as many opinions as to the right way to set fence posts as there are posts encircling a one-acre field! But about fifteen years ago, I stumbled upon a method of setting wood fence posts that appears to be working really well.
Your fence posts have a tremendous amount of force against them each time the wind blows. You may not think a 5-foot-tall fence is high, but in terms of wind load, it is substantial. To get an idea of what I am talking about, lift a 4x8 sheet of plywood up and take it outdoors on a windy day. If you are not prepared, the wind may either knock you over or rip the plywood from your hands. Imagine the pressure on the entire fence being held back by a single fence post every 8 feet!
The average wood fence post seems to be a square timber that is a 4-inch by 4-inch post or possibly a 6-inch by 6-inch post. The surface area of the post in the soil is what determines how quickly a fence will tip in the wind. Smaller fence posts will yield much faster than larger posts assuming both are buried at the same depth.
The trick I discovered is that you can use crushed, angular gravel to trick the wind into thinking a massive fence post is installed. The crushed gravel is affordable and it is easier to deal with than concrete that becomes as solid as a rock. You are about to discover how hard it is to deal with concrete that is deep in the ground around your fence posts. My guess is that you will be swearing like a sailor by the time you get to removing the concrete around your third fence post.
My method of setting fence posts is quite simple. If I am installing a 4x4 wood fence post, I dig a hole 10 inches in diameter. The depth of the hole is important. My tests over the years have shown that the amount of buried post should be half the length that extends above the ground. In your case my guess is your fence posts were taller than the fence, so your wood fence posts should be in the ground at least 3 feet.
When it is time to install the fence post, be sure it is in line with the other posts and it is plumb. It is also important that the fence post is centered in the hole. Start to add the crushed gravel in 5-inch lifts. This means add 5 inches of gravel around the post and use a long heavy 2x4 to tamp and compact the gravel. Each time you are finished tamping the gravel, check to make sure the fence post is still in the correct position.
Continue to install the crushed gravel until it is just a few inches from the top of the hole. You can add top soil to finish off filling the hole in case you want vegetation around the fence post. Some people like the look of gravel, so it can extend to the surface. You can even use a decorative, colored gravel if you like to finish off the hole.
The mass of interlocking gravel acts like an anchor when wind blows against the fence. It also can facilitate drainage of water away from the fence post as the soil moisture drops in late spring and all summer long.
The gravel is also easier to remove than solid concrete in the event you need to work on the fence in the future. A metal spud bar will quickly loosen crushed gravel that has been in the ground for many years.
Fences that are not solid have less wind load against them. A split-rail fence or a regular picket fence that has spaces between the individual pickets allows much of the wind to pass through the fence thus lowering the tipping force. In these instances the fence posts do not have to be buried as deeply.
If you decide to use treated fence posts, be sure to use the proper timbers. Not all treated lumber is approved for burial in the ground. The treated fence posts should be labeled that they are approved for direct burial.
Always be sure to call the national Call-Before-You-Dig Number a few days before you start your fence project. Simply dial 811 or go to their website www.call811.com. You may prevent death, serious injury or substantial expense by doing this. You would not be the first person to strike an electric line, phone line or even a buried natural gas line with a posthole digger!