How Deep Do Deck Posts Need to Be In the Ground?

A good deck – like every other construction project – starts with a rock solid foundation. You need to be sure that your deck posts are firmly planted in the ground.

How deep is deep enough, though? Well, you want to be sure that your footers are set at least 12 inches below the frost line. Your deck posts are going to connect to your footers underground. That means they are also going to have to set about 10 inches to 12 inches below the frost line as well.

Getting your deck posts dialed in is a huge piece of the deck building puzzle. Let’s go over how deep you want to set them underground right now.

How Deep Do Deck Posts Need to Be In the Ground?

Your deck posts – even if they are sitting on concrete piers that run down to the footing instead of running through themselves – are a huge part of your deck foundation.

These are big, chunky pieces of pressure-treated material. They are often 4 x 4, 6 x 6, or 8 x 8 pieces of lumber that can be best described as “beefy”.

They have to be if they are going to carry the kind of load that a deck is going to require.

To keep your deck strong and stable, though, those posts are going to either have to run deep underground (at least 12 inches below the frost line to your footers) or can be set atop concrete piers that handle the underground part of the construction for them.

The choice is yours.

At the end of the day, though, whether it’s a post or a pier you absolutely need to get down below that frost line.

The frost line in your area is the deepest level that moisture will penetrate when your soil freezes up in the wintertime (or when temperatures drop). Code requires you to set your footings 12 inches below that line.

You can find this information online or if you’re in a hurry you can reach out to any local contractor as well as any code enforcement officer. They’ll be able to let you know exactly how deep you want to set these core components of your deck project.

Get a foot (or so) below that line and you are rocking and rolling.

Fight Frost with Deeper Posts

The best foundations in the world of decking are foundations that you build once and then never – ever – move.

The last thing you want to worry about with a brand-new deck (or an older deck, for that matter) is a deck built on a foundation that shifts, wiggles, or moves around whenever the temperature changes.

No, you want to guarantee that your foundation is as locked in as possible. You want a foundation that isn’t going to jiggle short of a major earthquake shaking things up.

This is why it’s so important to get deep beneath that frost line.

Building on the frost line (or even a couple of inches one way or the other from that frost line) is a recipe for disaster.

As soon as below freezing weather kicks up any of the moisture in the earth is going to freeze up to a certain depth, and that’s going to cause a lot of expansion underground.

That expansion might not look like much when there isn’t anything built on top of the ground. But if you have a deck there (or anything else) and your foundation was above the frost line you’re going to have to contend with some heaving.

The thing that makes heaving so bad – when frost changes the position of your foundation – is the fact that it is so unpredictable, so uneven, and changes from year-to-year.

You might go a couple of years without your deck changing at all only to have one really nasty winter, or a really long winter, that turns your previously level deck into a roller coaster!

Though deeper beneath that frost level with your posts and piers, though, and that becomes a problem you never have to worry about.

If you go with a footing and pier system you want to be sure that you get below that frost line. If you go with a post and footing foundation, though, you’ll need longer piers to get at least 4 feet – and maybe even deeper – into the soil to get down underneath that frost line.

How Deep is Too Deep?

Truth be told, you really don’t want to be going too much further than 12 inches below the average frost line in your area if only because it’s not necessary.

Sure, you could go even deeper than 12 inches below the frost line to guarantee that heaving won’t be a problem for you. But at that point you are dealing with serious diminishing returns.

Every part of your deck has to be bought and paid for, and your deck posts are going to be large, heavy, oversized materials that set you back a pretty penny. They have to be if they are going to carry this much weight.

Every inch longer you go beneath the frost line is another inch of post you have to buy. This can add up in a hurry, especially when you are talking about 4 x 4, 6 x 6, or 8 x 8 pressure-treated materials.

Stick to a foot below the average frost line in your area and you’ll have nothing to worry about.

Closing Thoughts

Setting posts underground becomes a whole lot easier when you know how low you have to go to hit your frost line (plus another foot).

Some people living really mild areas where they might get a handful of below freezing days every year. Those folks aren’t going to have to bury their deck posts all that deep.

Other deck builders, though, aren’t that lucky and have to make sure that they are getting down below the frost line to prevent their deck from popping up, buckling, and causing a nightmare scenario that requires a total teardown and rebuild from the foundation up.

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How Deep Do Deck Posts Need to Be In the Ground?