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I am looking to install a fence as a DIY project. I live in the midwest where the frost line is recorded at 40 inches. I was going to dig my fence post holes 42 inches, and back-fill with tamped crush rock. I am using dog eared picketing and was going to stick it together. So I figured 4x4x8 ft posts should be suitable putting 1/3 of it in the ground sitting on tamped rock the rest of the way down. It got me thinking though, if water can get in and seep through the crush rock, will I have to worry about frost raising the post? Or is it only in the soil where heaving is a problem. Otherwise I will opt for the 10ft posts but was planning on 8ft ones to save a little money.

Thanks!

BTW, what type of rock is normally used for fence posts? All I read is to use crushed angular rock.. Is this the same as crushed limestone?

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  • Crushed quarry rock or shale is best because it will lock together. Don't use crushed river rock because the round sides never pack as well. I don't think I have gone much below 24" even when I lived in Ohio with a 6' solid wood fence it did fine even with the wind during thunder storms that were in the area quite regularly.
    – Ed Beal
    Sep 7, 2016 at 13:51
  • I don't understand what you are intending with depths. 42 inches is 3.5 feet. One-third of 8 feet is 32 inches or 2 and 2/3 feet. Why would you dig a 42 inch deep hole for 32 inch penetration? Does 10 inches of gravel (or other fill) somehow protect against frost heaves?
    – wallyk
    Dec 19, 2016 at 0:39

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A 24-inch depth works well for a 4-foot tall fence, but a fence that is 6 feet high should have posts sunk 3 feet deep or more. So dependent on the fence height above ground you may need to go with the 4x4x10.

Crushed rock works well for supporting fence posts because it contains rock pieces of a few inches in size, fine pebbles as well as fine gravel dust, which can pack together more tightly than using rocks of all the same size. If you use gravel instead, you can mix coarse masonry sand with the gravel for better compaction.

As for frost heaving/line, the best way to address this is how you dig your hole. If it is beveled at top then over time—whether concrete or gravel—it will pop out. So you want to create a lip at the top and that can be done by making the base of the hole wider than the top of the hole.

What causes frost heaving is the inward pressure and is comparable to squeezing a watermelon seed. How can that be prevented? Answer: Dig a bell shaped hole to encase the post as much as possible. Fill hole with gravel leaving a few inches at top and compact that with earth; that helps with enclosing it as well and just looks better (to me).

NOTE: Do not add more than 6" layers of gravel at a time and compact thoroughly between adding layers.

Wrong method in pic 1:

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Correct way in pic 2:

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