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".
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!
When water freezes, it expands about 9 percent—think of how ice cubes are domed above the original water level in the ice cube tray. Ice exerts a pressure of about 50,000 lbs. per square inch—enough force to lift even a large building. A puny little deck on inadequate footings doesn’t stand a chance.
The reason buildings and decks don’t always return to their original height is that surrounding dirt sometimes fills in under the footing while it’s lifted.
Heavy clay soils don’t drain well, so they tend to have more frost heave problems than sandy, well-drained ones. But even if footings are deep enough, ice lenses can latch onto the rough surfaces of wood and concrete and lift footings and posts from the side. That’s why concrete piers poured in waxed cardboard tubes and smooth wooden installing deck posts work well for below-grade support.
- Dig down 35 inches
- The hole width should be at least 3x the width of your post; 4x3 = 12" wide hole
- Place your 5 inches of gravel
- Pour 6 inches of concrete
- Place 6 inches of solid material such as a brick into the center of the concrete to make step #4 possible
- Place your post on top; 2 feet below the ground should leave you with 4 feet above ground
- You may find that it's easier to just set taller posts and cut them down short later since land is almost never perfectly straight. Some posts might be 4 feet above ground and others might be 5 feet.
- Pour 2 feet of concrete around your post
- Use a post level to make sure everything is perfectly vertical
- See https://diy.stackexchange.com/a/226599/42053 for how to use braces if you want perfection
- Pour extra concrete so that it rests an inch or two above the dirt so that you can slope it outwards to prevent water pooling near the wood
To position the posts plumb (upright), you can support them with braces that are attached near the top of the post, and rest on the ground, with a securing peg.
And to offset the bottom of the post from the bottom of the pit, you can lift them using supports placed flat on the soil (illustrated in red, below).
This allows you to pour the gravel and concrete/cement after adjusting the height of the post and aligning its top with other posts. Or if you are pouring only concrete/cement (no gravel) you can now pour it in one go and ensure that the concrete envelopes the bottom of the post.
You can use this same technique to fine-tune the alignment of the tops of all your posts without having to cut the tops after placement.
You can also pre-cut the posts based on the slope of the ground, and use the offset bracing to do the fine alignment.
Crossing the brace is probably not necessary, since its purpose is to lift the post and not vertically stabilize it. So one 2x4 will do. Also, you can run a long 2x4 from one post to another, resting it on the soil and supporting both posts, if it's hard to brace across a hole due to obstructions like rocks etc..
Original image: https://www.construction53.com/2011/09/setting-and-aligning-the-poles/ with edits in red.