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Foundation Post (4)

There are three grades of treated lumber. The best type for posts is rated for ground embedment (ACQ: 0.60pcf, CBA-A: 0.61pcf, CA-B: 0.31pcf). The other two types are intended either for ground contact only or for outside use with no ground contact. Even if you don't intend to bury your posts, use ground embedment rated lumber for your deck or gazebo posts.

On BestDeckSite, we recommend embedding deck posts at least three feet. If you live in an area where the ground freezes, you may need to go even deeper to get below the frost line. To see a frost penetration map for the United States and learn all about the causes and cure for frost heave, review the frost heave section. For this discussion however, we'll focus on why it makes good sense to embed posts a minimum of three feet - even in areas that never get cold.

There are two good reasons to embed deck posts. The first is that embedded posts substantially increase the ability of the deck to resist lateral (side-to-side) loads. The second is that embedded posts help hold the deck to the ground.

If your deck stands over 48" tall, then embedding the deck posts eliminates the need to add bracing - to a point. Since most people don't like the appearance of bracing, the solution is to embed the posts. Posts buried in the ground substantially increase the the lateral rigidity needed to resist high winds that buffet the deck. To read more about this, click on the bracing in the diagram above.

Even if your deck is low enough that it doesn't need bracing, there is another very good reason for embedding the posts. If you read about wind loads in the sizing overview section, you'll see that the uplift on deck posts ranges from between 250 pounds for 80mph winds to over 1,000 pounds for coastal regions. A wind map of the United States shows that sooner or later, everyone will experience at least 70mph winds. Given this, the question isn't whether you need to firmly anchor your posts to the ground. The question is how you choose to do it.

One option for decks not subject to freezing, is to pour shallow concrete footings flush with the ground. The posts are attached to the footings with a metal saddle that is set in the top of the concrete. There are two drawbacks to this type of support. First, it offers no resistance to lateral forces and second, it takes a big footing to keep the deck anchored against even a small uplifting force of 250 pounds.

To be more specific, concrete weighs 150 pounds per cubic foot. This means a footing would have to to be 16" in diameter and 14" deep to weigh 250 pounds. That's a lot of concrete, especially if you're mixing it by hand.

Alternatively, a 6x6 post embedded three feet in "average" soil, has a withdrawal resistance of about 500 pounds. Embed the post another foot, and the force required to pull the post out of the ground increases to over 1,000 pounds. Granted, you'll still need to pour a footing to support the deck, but it will only require a fraction of the concrete needed compared to shallow footings that must be sized so they're heavy enough to resist wind uplift.

BestDeckSite contains information on setting wood posts, attaching posts to beams, and preventing decay. We outline what you need to know to mix and pour concrete piers and even what to expect if you elect to get your concrete delivered. If you want to build a deck that will still be standing when you're old and gray, BestDeckSite will show you how.