How deep for post?

[Tree-Farmer]

If a person wanted to put up a sign on his own land, advertising his great business or vote for me and beans will go up to $25 a bushel...
If the post is 8 foot above ground, how much post would you need below ground? In North Missouri
Then you could figure out how long the post would be in total length.
Sign size is maybe 8 foot across by 6 foot.

Couple of sign examples. First one looks like it's just a piece of plywood bolted to the post.

 

[used red MN]

Where are you at? Do you get frost in the ground over winter? What type of soil? I suspect since it is a sign that you/he wants noticed I would think it would be in a high spot and not down where it the soil might remain damp for longer periods. In general as deep as the normal farm type post hole digger goes is probably adequate.
I know it’s a joke, but if someone could control the price of beans I wouldn’t necessarily see that as a reason to vote for someone.

 

[wh]

4x6 post Here I would say 3 ft would hold it fine. Think there will be a lot of options to this answer. Frost line here in Alabama is maybe 6 inches.

 

[urbancase]

Here in my part of Indiana it’s 36 inches, I have set many to that depth.

 

[moresmoke]

I would say 4 ft, based on wind load. But I am also where there is a near constant 20 mph breeze.

 

[Texgasser1]

between 3 and 4 is what we have always sunk them

 

[john in la]

A post 4 ft in the ground will hold a pole barn wall up.
A post 3 ft in the ground will hold a gate.
I wouldn’t go less than 3 ft.
Since it is a sign by the road use 4x4 post so they will snap off if anyone hits them.

 

[Bob Bancroft]

If a person wanted to put up a sign on his own land, advertising his great business or vote for me and beans will go up to $25 a bushel...
If the post is 8 foot above ground, how much post would you need below ground? In North Missouri
Then you could figure out how long the post would be in total length.
Sign size is maybe 8 foot across by 6 foot.

Couple of sign examples. First one looks like it's just a piece of plywood bolted to the post.

Any zoning considerations there?

 

[JD Dave]

Part of the equation is the size of the posts your using based on the sign size and height. I've had 4' X 8' signs on 4 X 4 posts snap off like match sticks from wind. I suggest 6 x 6 posts for a sign your size.

 

[Duner Wi]

I would go at least 4 feet. Four feet deep is twice as strong as 3 feet.

 

[da.bees]

2 feet isn't enough, 3 should be fine , I've never seen 11' post for sale, 22' can be ordered but one 22' costs quite a bit more than twice two 11'. Question is do you want to bury 12' posts 4' deep, cut 1' off 12', pay extra for 22' and cut in half?
Diagonal braces add more resistance to wind than deeper holes. Setting three 12' posts in a V holding two 4x8 signs is the ultimate for strength, attracting attention and ease of mowing/weed-eating for clean appearance.

Stop procrastinating and start digging. And to answer the to be asked question of what color to paint posts, green.

 

[Tony in SD]

I would go at least 4 feet. Four feet deep is twice as strong as 3 feet.

Could you please explain the math/physics behind you statement of "Four feet deep is twice as strong as 3 feet."

 

[Tony in SD]

There is more information needed before an accurate solution can be calculated:
What is the soil load bearing capacity at the sign location?
What is the anticipated wind loading? Do you want to design it for worst case or nominal wind loading,
How deep is the frost line? The frost line may be your determine depth rather than the structural forces.
How tall is the sign from the ground level to the top of he sign? This will determine the size of poles and in turn reactive surface area.

 

[john in la]

Part of the equation is the size of the posts your using based on the sign size and height. I've had 4' X 8' signs on 4 X 4 posts snap off like match sticks from wind. I suggest 6 x 6 posts for a sign your size.

Any wood post over 4x4 next to a road needs to be drilled perpendicular to the traffic flow so it will break off if a car runs off the road per federal law.

 

[Duner Wi]

Could you please explain the math/physics behind you statement of "Four feet deep is twice as strong as 3 feet."

Has to do with fence corner post . I don't know where I read it 50 years or so ago .

 

[wh]

Any wood post over 4x4 next to a road needs to be drilled perpendicular to the traffic flow so it will break off if a car runs off the road per federal law.

So any power pole has those holes drilled in them?

 

[wh]

Here in alabama you are not to install anything on right of way. Out side of right of way you can do whatever you want.

 

[john in la]

So any power pole has those holes drilled in them?

Power poles have an exception mostly because they are not in the road right of way.
Same with huge billboards set back off the road.
But a small business sign is usually put out by the street so passing motorists can see it. And anything in the road right of way has to follow federal road sign laws.

Notice I said in my original post “next to the road “.

 

[wh]

True of if on right of way. Off right of way no different than hitting a tree or building here. Had a guy come to school and ask if class could build him a mailbox that kids could not destroy. He said a new mailbox and post lasted less than 2 weeks. Told him sure but needed ok from post office. Post office was aware of his location and trouble. His driveway was in a curve. Mailbox was actually off right of way. Post office told him as long as it had a flag, door and was correct height no problem. He also knew a state trooper and talked to him. Same story. Anything off right of way was fine. We took a schedule 80 8 inch pipe and cut the dome for top of box from it. Used some 3/8 x 4 for sides. Used a 8 inch I beam turned sideways for bottom. 3/8 plate for back and 1/4 inch for door. Used 6 inch square 1/4 inch wall tubing for post. He dug hole with a back hoe and used several sacked of sackcrete to install. End of problem. He did find a chrome bumper by it a few days later along with tire prints leading up to it.

 

[ss55]

True of if on right of way. Off right of way no different than hitting a tree or building here. Had a guy come to school and ask if class could build him a mailbox that kids could not destroy. He said a new mailbox and post lasted less than 2 weeks. Told him sure but needed ok from post office. Post office was aware of his location and trouble. His driveway was in a curve. Mailbox was actually off right of way. Post office told him as long as it had a flag, door and was correct height no problem. He also knew a state trooper and talked to him. Same story. Anything off right of way was fine. We took a schedule 80 8 inch pipe and cut the dome for top of box from it. Used some 3/8 x 4 for sides. Used a 8 inch I beam turned sideways for bottom. 3/8 plate for back and 1/4 inch for door. Used 6 inch square 1/4 inch wall tubing for post. He dug hole with a back hoe and used several sacked of sackcrete to install. End of problem. He did find a chrome bumper by it a few days later along with tire prints leading up to it.

An older neighbor mounted his mailbox on an old mike can with some of dirt in the bottom of the milk can for ballast. Stayed up in high winds, easy to put back in place after Halloween ( had two handles on the top ).

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