How To Set a Post in Concrete
Setting a post in concrete is a fairly easy project. There are a few key steps to follow and your results should be very satisfactory. We'll cover those key steps, plus we have a few tips that will help to ensure the longevity of your posts.
Post Hole Size
The post hole should be three times the diameter of the post. For instance, if you are installing a 4"x4" post, the hole should be at least 12" across. The depth of the hole should 1/3 the height of the post above ground, plus six inches. For example, if you want 6 feet of post above the ground, then the hole should be 2 feet six inches deep (1/3 of 6 feet + 6 inches).
- Important Tip: Make the bottom of the hole flat and wider than the top to prevent the post from sinking over time. Most holes tend to come to a point at the bottom. The concrete that fills the hole forms an arrowhead shape and the weight of the post and concrete together drive it deeper into the ground over time. Making the bottom wider results in the weight being spread over more area and reducing the chance of your fence or other project becoming uneven over time.
Setting the Post
The extra six inches in the depth of the hole is for gravel. If a wood post, even treated wood, touches the soil or is completely encased in concrete, it will prematurely rot. Metal posts will rust. Setting the post onto 6" of gravel allows the water to drain away instead of ruining the post. Adding gravel will add a lot of time to the life span of a wooden or metal post. Tamp down the sides and bottom of the hole and then add the gravel.
Set the post on top of the gravel. You can check for plumb now, but you'll want to recheck it after pouring the concrete. Checking now allows you to get the braces roughly into position for fine tuning later. Add braces to hold the post in position.
Mix the concrete and pour it in. Some varieties of post hole concrete allow you to pour the concrete mix in dry and then pour water on top afterward. This simplifies the process but may result in weaker concrete. For applications demanding the most strength, mix the concrete first and then pour it in. If you don't want exposed concrete when the project is finished, be sure to leave the concrete half an inch to an inch below the ground level.
- Important Tip: Slope the surface of the concrete away from the post. If it is flat or sloped toward the center, water will pool there and cause the post to rot more quickly.
Now check the post for plumb on two sides. Place a level against the edge of the post and adjust the post until it is plumb. Now move the level to the next side and check it for plumb also. Secure the braces to keep the post in place until the concrete sets. Recheck again to make sure everything is still plumb.
- For Best Results: If you are setting a series of posts and want all the tops to form a level line, it is a good idea put in longer posts than needed and then trim them to level after the concrete has set. If you want all the posts in a straight line, place a stake near the first post and another stake near the last post. Tie a string near the bottom and one near the top. Check the stakes for plumb. As you install each post, align the edge of each post with the string. This will aid in keeping the posts plumb and in a straight row.
The concrete will harden within a few hours, but no heavy load should be placed on it until, at least, over night. If for any reason, one of the posts is not plumb, you can dig out around the concrete base to enlarge the hole. Shift the post into plumb and then add more concrete to fill the hole. Brace the post and allow to concrete to cure.