What is the formula for fence height and fence post length?

My neighbor has requested that we remove our current shared old/broken 6' cinder block wall and put up an 8' high horizontal slat wooden fence.

The distance between the posts will be ~5.5'.

My question is how deep do I need to put the fence posts? This will help me determine the length of the fence post.

We live in Los Angeles. I think the frost line is 12".

2 Answers

Are you setting the posts in the ground, or in concrete? How deep is the frost line in your area? Have you contacted your local building department? There may be local codes and ordinances that stipulate the depth.

A common rule of thumb, is to dig the hole 3 times as large as the post, and to a depth of 1/3-1/2 of the post height above ground. so for a 8' high fence, using 4x4 posts. dig the holes 12" in diameter, to a depth of 2 1/2 - 4'. Of course, that's assuming you're setting the posts in concrete.

If you're going to put gravel at the bottom of the hole, you'll have to dig the hole that much deeper. Gravel is optional, and dependent on the drainage characteristics of the local soil.

• I think in Los Angeles it is 12" Aug 24, 2017 at 15:27
• @milesmeow Then you shouldn't have to worry about frost heave. I'd just follow the 1/3-1/2 rule, unless there are some special earthquake (or volcano) rules in your area. Aug 25, 2017 at 17:15
• Does wind force need to be factored in? Or does the depth formula that you've specified already take that into account? Sep 11, 2017 at 2:32
• @milesmeow For a typical installation, yes. However, if you live in a place where it's common to have really strong wind gusts, or overall more wind. Then you might want to factor that in, and adjust accordingly. Sep 12, 2017 at 11:21

The normal in ground length is 2 ft, so you buy 8 ft posts for a 6 ft fence. The normal wood fence has the boards on only one side of the posts so in your case you will have to decide who gets the more finished side and who has to view the posts, or you could put boards on both sides.

• There are styles of fence that look equally the same from both sides. Often referred to as a "good neighbor fence" you can see examples here: fitzpatrickfenceandrail.com/good-neighbor Aug 24, 2017 at 13:38
• If I want an 8' high fence, how many feet ground length? Is it still 2' or do I need more ground length? Aug 24, 2017 at 14:02