My Fence Guys and NOVA Build Pros are sister fence and deck companies in Northern Virginia and Hampton Roads. We are your premier contractors for installing your new fence and deck. However, we are always happy to lend a hand if you prefer to do it yourself.
One common question we get again and again is, "how do we set the posts?" We've heard time and time again about deck failures resulting from sinking posts. Before the International Building Code was as well known many deck posts were installed like fence posts. The hole was dug, and post and concrete were installed at the same time. This may prevent a post from toppling. However, as you can see from the diagram, the main forces to account for in deck footers is that of the live and dead load. For the most part, this force is normal (perpendicular) to level ground. The force must support 40 pounds per square inches. In general, that means if you divide the square footage of your deck by the number of supporting posts (not accounting for ledger boards) and multiply that number by 40, the result is the weight that the post must support. Similarly, the footer must also support that weight. The footer must have a larger bearing area than the bearing area between the post and the footer (i.e. 3.5″ x 3.5″) because the soil load bearing capacity is obviously less than the concrete load bearing capacity. Concrete bearing capacity is between 3000 and 5000 pounds per square "inch" in general. The soil bearing capacity is (again, in general) 1500, 2000, or 3000 pounds per square foot. So… that entire force transmitted through the post (1,000's of pounds) must be spread across undisturbed earth. Additionally, the footer bottom should be below the frost level depth. In our area, footers are typically (check w/ your locality) a minimum of 24″ deep, 18″ square and 8″ thick. The post can be set in 8″ of concrete on top of that footer, or can be set on post anchors or by other means. Don't forget to get your permit(s) and schedule inspections "before" you place the concrete.
For fences, the force that must be resisted is usually due to wind loads (or, kids climbing the fence to retrieve their lacrosse balls). There isn't the same risk of the fence settling through the footer. There is a risk, however, of the fence toppling over due to high winds (or kids, mowers, etc). This isn't so much of a problem for decks due to fact that the lateral bracing tends to keep the posts in place vertically. Fences are essentially cantilever boards (i.e. supported only on one end) sticking out of the ground. The earth is the only thing preventing them from toppling over in high winds. So, the larger the cross section of the post (in general) the more the post will be able to resist that moment. Thus, fence posts should be placed approximately 1/2 to 1/3 in the ground and should be surrounded by concrete as they are set. Dry packing concrete is a manufacturer-recommended method that helps to speed along the installation. In a matter of days, the concrete will solidify making removal of the post in tact very difficult. We always recommend setting gate posts in wet concrete (we use fast setting concrete) so the footer will have a high early strength and be able to withstand the changing forces caused by a swinging gate (or a swinging gate banging against it).
If you need any more pointers or would like us to do it for you, give us a call!