Would 21 inches of post below ground be enough to support 4 feet picket fence (wooden)? I have dug holes 25-26 inches and planning to fill 4-5 inches of gravel. I am planning 8 feet panels (only 3 or 4 panels are 10 feet). 1st time DIYer and recently learned about frost heaving. I am in Missouri (zone 6) and the frost line here is 30". Half of my property has very clay soil and half is very rocky (took appox and hour for 1 hole). I have already bought 4x4x6 post and now wondering if I should replace those by 4x4x8 and dig deeper below 30".
A suitable post hole depth for a 4′ fence
fencefrost-heave
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Best Answer
Assuming that you plan to set the posts in concrete then you should follow these rules:
For wind strength:
The post should be buried 33%-50% of the exposed height
You'll be exposing 4 feet above ground so the buried portion should be 1.3-2 feet; your 6 foot post will suffice for wind strength especially since a picket fence has large gaps between the vertical boards.
For frost protection:
You're in zone 6 so the bottom of your post should be at least 30 inches below the ground.
You plan to use 5 inches of gravel? You need to dig down 35 inches.
Do not skimp on protecting against frost!
https://www.familyhandyman.com/project/how-to-build-a-solid-frostproof-deck-footing/
In conclusion: