The depth of the hole necessary for a fence post varies depending on many factors, but generally fence post holes are between 20 and 48 inches deep. The major factors to consider in deciding on the depth of the hole are the material used for the fence post, the type and condition of the soil, and the height and weight of the fence.
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Corner or Gate Fence Post Holes
Corner or gate fence posts, which bear significantly more weight, need to be planted deeper to provide a more stable foundation for the entire fence. The general rule of thumb is to add 20 per cent to the depth you determine as appropriate for the post holes for the normal fence posts.
Material Used for the Fence Posts
Relatively slender metal fence posts need to be planted a little deeper than broader wooden fence posts as they are a bit less stable due to the reduced surface area. If it is a relatively lightweight fence of less than 6 feet in height, then 24-inch-deep post holes are appropriate for average soil compaction conditions. If you are installing a heavier or taller fence or the soil is loose, consider making the post holes as deep as 36 inches. Standard squared wooden posts can be well planted in post holes as shallow as 20 inches deep in compact soils.
Type and Condition of Soil
The type and condition of the soil is another important factor in determining the appropriate depth for fence post holes. Looser, less compact soils require that the hole be deeper for good stability, while in very dense soils post holes can be dug more shallow for sufficient stability and weight-bearing capacity.
Height/Weight of the Fence
The load on the fence posts, that is the height/weight of the fencing between each set of fence posts, is another important factor in determining the best depth for the post holes. Taller and heavier fences require that the fence posts be planted deeper to support the additional weight. The heaviest fences, 8-feet or taller, sometimes require post holes as deep as 48 inches and/or setting at least the corner posts in concrete.
Err on the Side of Caution
It is always better to err on the side of caution and dig your fence post holes a little deeper than necessary. That way you might put in a little extra effort, but you can sleep easier at night knowing your fence posts are going to stand the test of time.
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