Proper Fence Post Depth to Build Strong Fences
Frost Heave Prevention For Fence Posts
Reading Time: 5 minutes
Fence post depth, size, and anchoring systems can make or break your fence line. Despite what some people believe, making a long-lasting fence is not always as simple as sinking a post in the ground and moving on to the next post. There are a few great tricks you should know before you start making holes with your post hole digger.
Choosing The Right Post
Picking the right post for the job is just as critical as setting your fence post depth correctly; possibly even more so. Cedar poles are a great way to hang fence wire. Depending on your needs, you could investigate the use of fiberglass fence post rods and steel t-posts if your application permits. Economics will likely play a big role in your decision making.
Classic cedar poles make for a good low-cost construction technique, but they’re not up to the task of supporting and bracing your corners or gates. The corner posts, posts stationed at rises and valleys as well as posts that your gates hang on are under much more stress. Compared to your interstitial posts that simply support your fencing from flopping or drooping, these posts need to be more substantial.
For fence posts charged with important tasks like these, bigger is better. Overkill as it may be, I’ve found that local farmers in New England who use retired telephone poles for their high-stress points have great long-term success; especially at corners, bar-ways and gate openings. If you’re in the market for telephone poles, look on your regional Craigslist.org website, freecycle.org, or talk to linemen you may know.
If you don’t have any luck procuring retired phone poles, then my alternative favorite is 6×6 pressure treated landscape timbers. These can be found at your local big box store for a reasonable cost, and are widely available. In a pinch, you could select, cut and shape a tree from your property to use as a post, but that could result in premature replacement because of relatively quick rotting. Additionally, this method takes time, tools and effort you may not have.
Frost Heave Prevention
Ever notice that telephone poles are thicker at the bottom? This is the natural shape of the trees they make them from, but the tapered shape also helps keep them seated in the face of frost heaving. Posts that are improperly buried can work their way out of the ground, but if we sink our posts with the fat end down, the tapered shape will actually help keep the post from rising over the years of frost-thaw cycling. Cedar poles have this shape too, so be sure to bury them correctly with the thicker end down.
Frost heaving occurs when the water in the soil freezes and expands. The pressure caused by this expansion forces soil upward and anything within it, including your posts. When posts are set correctly, the tapered shape makes it harder for them to be pushed out. Think of it like a watermelon seed between your thumb and pointer finger. If you squeeze your fingers the seed goes flying away from you or toward your palm, depending on which side of the middle you squeeze. The same principle is in play here.
When we bury the fat end of a post down, the frost heave pressure will push the post further into the ground. This downward pressure locks it against the frozen ground below, and your post stays put. Conversely, putting the skinny end of your tapered pole into the ground lets the frost heave push it right out of the ground. You don’t want all your estate fencing popping out of the ground after all the time and effort you spent putting it there, so make sure you bury your posts fat end down.
In northern climates with significant heaving or with posts that are supporting a significant weight, consider cementing them into place. Wood that contacts cement is notorious for rotting quickly, so when you’re anchoring your posts in cement, be sure to follow some basic rules.
- Be sure to add gravel to the bottom of your hole for water drainage. Fence post depth, size and anchoring systems can make or break your fence line.
- Make sure your cement anchor is tall enough to end above ground, preferably in a tapered shape to shed off groundwater.
- Use rot-resistant posts like cedar, pressure treated timber, or good quality steel if your application warrants it.
- When pouring a proper post anchor, do your best to make it bell-shaped to take advantage of the frost heaving pressures to keep your post seated. Water will rot your post eventually, the grade of metal or wood you use simply dictates how long it takes to rot.
- You can prolong that rotting time by having your cement anchor peak above the ground to reduce the amount of groundwater that seeps between your post and concrete, and having a gravel base for water to escape into will also extend the life of your post.
Fence Post Depth
Unlike a footing or poles for a pole barn, fence posts don’t typically extend past the frost line. The rule of thumb for setting your fence post depth is this; not less than one-third the overall length of the pole and not more than half the overall length. More is actually okay, it’s just overkill. Setting your fence post depth to one-third the overall length is the bare minimum since you run the risk of it giving way to lateral pressure, such as livestock rubbing against it, heavy winds or snow drifts.
This is not an either/or rule. Depending on how much post you need above grade will largely dictate your fence post depth, and as long as its depth is somewhere between one-third to one-half the overall length, you should be just fine.
Take into account how much post you need to have above grade when purchasing fence posts. As an example, if you want four-foot tall posts you have a choice of buying a nominal overall length post of six, seven, or eight feet long. Most people like to have fence post tops level with each other, but the grade of the land may not cooperate. If you run the minimum length of six feet, you won’t have wiggle room to make that happen, but if you use a seven foot or eight-foot-long post, you’ll have plenty of length to compensate. To achieve that professional look of level post tops, either painstakingly adjust your fence post depth to fit your level line, or set all your posts to the same fence post depth, snap a level line and cut the excess post to length once they’re set in place.
What About You?
Have any quick tips to add? There is so much more to proper fence building than I can cover in one article, so I’m sure you have ideas. Share them with us below in the comments!