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My research has shown that fence posts should be dug to a depth of at least 1/3 of the height of the post—for a six foot fence, the post depth would be 3 feet.

I purchased a 6 foot vinyl privacy fence from Lowe's, and the manufacturer's recommended post was a 5"x5"x8' post, an entire foot short of what I would have expected. Indeed, Lowe's doesn't even appear to sell a 9 foot fence post (without routed holes). Should I return the 8 foot posts and find something longer? Or is there some way to approximate a longer post (e.g. concrete+rebar going deeper or something?)?

  • My soil is extremely rocky (on a mountain in Utah County), but otherwise loamy.
  • It's not usually windy, 40 mph sustained winds with 60 mph gusts would be the strongest wind you might see in a year. Most of the length of the fence will separate my house and the neighbors, about 20 feet away from either house, which I'm hoping should should act as a windbreak.
  • The frost line here is 36 inches
  • I'm planning on using concrete to set the posts.
  • 1/3 of 6 is 2 1/3 of 8 is 2.666 actual panel height is 5.6. 8 footers will be fine. Or use 10 foot posts and cut off excess after panel is placed – Kris Jun 12 '18 at 20:08
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    For a 6' tall fence, 2' buried is one 3rd of the height above ground, resulting in an 8' post. But this is the minimum recommended post length. – brhans Jun 12 '18 at 20:08
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    Wind is one concern but frozen ground is a more serious concern. You want the post below the freeze depth. – RoboKaren Jun 12 '18 at 20:13
  • With a frost line that deep if use the 10 foot post and cut to fit panel height – Kris Jun 12 '18 at 22:40
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I'm about to put some of these in myself. I believe the manufacturer's instructions answer this question in Step 2c, shown below. Note that the post does not extend the entire depth of the hole, but the hole is back-filled with concrete to a 30-inch depth before placing the post. That way, the concrete holding the post extends past the frost line, even though the post itself does not.

Post hole design

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