How deep should your chain link fence post holes be?
The depth of your post holes can vary depending on factors such as possible frost depth, fence height, wind resistance and if it is a line post, terminal post or gate post.
Frost Depth
Let’s start with frost depth. When digging post holes it is common practice (and industry standard) to dig to a depth past the anticipated frost line. This will prevent the frost’s heaving effect from getting under your footing and lifting your post out of the ground. When determining potential frost depth you should consider the type of soil that you are digging in as this can change the impact of frost. It will have more of a significant effect on top soil and clay over gravely/sandy soil. If you are trying to figure out what the depth your post hole should be to avoid the impact of frost, you can try contacting your local building inspection department and find out what the minimum foundation bury depth is. This depth should be below the freezing boundary. In the Okanagan our typical frost depth does not exceed 24”.
Fence Height
Next let’s look at the fence height. The higher the post is out of the ground the more leverage there will be to potentially move the post laterally. So, we need to think about what kinds of lateral forces will affect your chain link fence. The most common for a fence is a human/animal pushing against it or a gate hanging from a gate post. Also, if there are privacy slats in the fence this will create wind resistance and force on the fence. If the fence is unlikely to have much pressure against it a 24” deep footing for the line posts and a 30” deep footing for the terminal posts should be sufficient. Examples of fences that wouldn’t have much pressure on them are yard fences, garden fences, fences without privacy slats or fences for small dogs. For most man gates in a residential fence application a 30” footing would be sufficient. Once you start increasing the potential for more leverage on the post you will want to increase your footing depth and likely the width as well. For a residential 6’ high fence we would use 30” deep footings for the line posts and 36” deep footings for the terminal and gate posts. Increased footing size to compensate for the installation of privacy slats will depend on how much wind the fence will be exposed to. Generally, the above noted depths will work for a fence with privacy slats but bigger and deeper is always better.
Wind Resistance
As touched on in the above section, adding privacy slats to your chain link fence will increase the effect that wind will have on your fence. The privacy slats will increase the wind load on the fence proportional to the amount of privacy they provide i.e. A 90% visual barrier slats like Winged Slat will create a 90% wind barrier and 70% visual barrier slats will create a 70% wind barrier. If the fence with privacy slats is going to be located in an area sheltered from wind the additional wind resistance might not be a factor. That being said you may still want to increase your post footing depth and diameter in case factors change and the fence is exposed to wind. One thing that is very important to consider is the fence’s proximity to a retaining wall. If your fence is going to have privacy slats in it, possibly exposed to heavy winds and it is located next to a poorly constructed retaining wall, there is a good chance the fence will start compromising the retaining wall. *Do not install a fence with privacy slats, or any privacy fence for that matter, on a glued down cap of a block wall!* The force that could possibly be applied to a fence installed in such a manner can rip the glued down blocks right off the lower blocks!
Gate Posts
The last thing to consider is swing gate posts. The free-floating leaf of a gate can add significant pull to your gate post depending on the size of the gate. Standard man gates up to 5’ wide tend not to have much of an effect on a gate post footing assuming that it is installed correctly. The minimum depth that should be considered for any gate post is 36”. Once you start increasing the width of that gate leaf over 5’ you should consider increasing the diameter of the hole and possibly the depth. For instance, if we were going to install a gate post for a 6’ high 10’ wide gate, we would install a post footing around 42” deep and 12” wide. Of course, you would need to consider all of the other factors that we previously discussed as well. One thing that can be utilized to decrease the required footing size for a gate post would be to add a wheel on the gate. If the wheel can be touching the ground or resting on something in the closed and open positions, you would not need to enlarge its size over a standard terminal post as the ground and/or resting point would be taking the weight of the gate.
Conclusion
Post footings are the foundation of your fence, so it is worth doing correctly. The bigger the better is always the case but it may come at the cost of more concrete and a sore back from digging the holes. Ensure that you consider all of the factors we have discussed when planning your project. If you are not installing the fence yourself, make sure the contractor or person you hire has the equipment and/or ability to achieve a proper depth. Finally, always “Call Before You Dig” and have underground services located prior to starting any excavation.
This is not okay! (under any circumstances)
