The answer - as it often does in fence construction - depends. As a general rule, Yuda Wire Mesh recommends a minimum footing depth of 24 inches for a 4-foot fence, plus 3 inches per foot of height (for example, 27 inches for a 5-foot post). The minimum hole diameter should be four times the maximum cross-section of the post.
These are general recommendations as a basic or minimum guideline. However, a variety of factors can affect the recommended (or specified) foundation depth. Environmental factors such as soil and weather conditions can have a significant impact on the stability of a fence and require installation adjustments. Local ordinances often specify minimum depths for foundations and other elements of fence construction.
This article will take you through the factors that determine the proper foundation depth and then provide basic instructions for setting fence posts. At Yuda Wire Mesh, our sales and service staff have decades of industry experience to support you in installing the highest quality products.
The Chain Link Fence Manufacturers Association has published a guide with a formula for determining the depth of the base. One of the factors in this formula focuses on the stability of the soil based on its composition. There are three types of soil: sand, loam and clay. Although soils are usually a combination of all three, each has its own qualities that affect the stability of the fence.
Sand - The coarsest of soils and therefore porous, sand does not change much with changes in moisture conditions. Contrary to popular belief, sand can prove to be strong and stable once it has been compacted. It provides a suitable medium for fence construction and requires little special consideration.
Loam - Loamy soil, slightly less porous than sand but still quite coarse, does not change much in wet conditions. However, its soft and pliable nature means that it offers less stability. This should be taken into account when planning the depth of footing.
Clay - In contrast to sand and loam, clay is dense and hard. It provides a strong base when dry but is more prone to swelling and shrinking due to water and moisture. This means that when clay absorbs water or dries out, it changes, which can cause problems for fencing.
The USDA maintains a web-based soil survey page that provides information on the soil at every location in the United States. Refer to this map and use your knowledge of soil conditions to adjust the depth of your fencing base.
Wind - Another major factor in the Chain Link Fence Manufacturers Association's foundation depth guidelines is wind. It is probably the most important factor in determining foundation depth, as it is the only reasonably predictable force. The higher the wind force (usually in areas with strong storms), the deeper the post.
Frost line - Freezing and thawing can also exert a strong force on a fence. A force called bulging occurs when ice formation under a fence post causes expansion to "lift" the post. To prevent this phenomenon, fence posts must be set deeper than the frost line, or the maximum depth at which the soil freezes. The National Snow and Ice Data Centre publishes a map of the average winter frost line in the United States.
Often, local governments will consider these conditions and specify a minimum base depth. Cities often post these regulations online (for example, here are Miami-Dade County's fence regulations requiring a minimum base footing depth of 38 inches). These regulations can often be found through a quick internet search, but it is also possible to contact the appropriate government agency to speak with a knowledgeable person.
Regardless of the different factors associated with the depth of the foundation, it is essential that the foundation itself is dug correctly to ensure maximum strength. This means digging a bell-shaped hole, or a hole that is wider at the bottom than at the top. The opposite of a bell-shaped hole is a carrot-shaped hole (thinner at the bottom and wider at the top). Bell-shaped holes provide greater stability by providing more support at the very bottom of the post, keeping it upright and aligned.
At Yuda Wire Mesh, we recommend that the top of the terminal post footing be 10 inches wide and the bottom 12 inches wide. Wire posts should be 8 inches wide at the top and 10 inches wide at the bottom.
