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how deep should deck post be in the ground

Deck Building 101: How Deep Should Deck Post Be in the Ground

Decks are a great way to appreciate the outdoors and get away from everyday life, and because of that, a well-built deck can be a lovely addition to any home. But, what is the secret to a solid outdoor deck? Or how deep should deck post be in the ground?

The answer is to use deck posts that are typically 4 feet by 4 inches or 6 feet by 6 inches depending on the estimated load. This is also determined by the sort of wood used.

The deck footing depth is critical for a sturdy, secure, and long-lasting deck. They bear and support the full weight of the completed deck, all furnishings, and all those who occupy the deck at any given moment.

Whether you’re building a new deck or renovating your outdoor space, knowing how to correctly install a platform post on your exterior is an important part of the process. So, read below to learn about how deep your deck posts should be.

How to Choose the Right Post Size for Your Deck

How to Choose the Right Post Size for Your Deck

Before we get into how deep the deck posts should be, you should know what size the posts should be. After all, these are the cornerstones upon which your construction will be built, so selecting the right one with the right size is critical.

The proper size of deck posts is influenced by a variety of factors, the most significant of which are deck height and width. A 4×4 is only useful up to a certain height before becoming unstable.

Furthermore, many jurisdictions demand a 6×6 post as a minimum. As a result, to prevent code violations, many deck makers choose the 6×6 even though the 4×4 is acceptable.

To avoid lateral movement, most codes require posts to be secured at both the bottom and top. The reinforcement posts should also be made of approved wood and pressure-treated or decay-resistant.

Ultimately, a deck support post must be able to endure both concentric compressions as well as eccentric loads. If this criterion is not met, a post may fail, compromising the deck’s structural strength.

How Deep Should Deck Posts Be?

how deep should deck post be in the ground

You must consider how deep the deck posts must be to provide decent support to the structure’s mass, especially since the deck is raised above the ground. It will not yield or buckle under the weight if it has sufficient depth.

Keep in mind that your deck posts should sustain the deck, the persons that will be on it, and the furnishings you will place on top of it. The posts must also be resistant to wind, moisture, and other external conditions.

This part will go over several factors that influence the required depth for your deck posts.

Frost Line

The frost line is the maximum depth at which groundwater is anticipated to freeze.  The primary reason you should think about the frost line when calculating the depth of your posts is that when the earth freezes, the groundwater in the soil freezes as well and frost up to the frost line.

Why is the frost line important to be taken into account? It is a significant concern mostly because water expands whenever it freezes. As a result, the freezing water will generate sufficient pressure to lift whatever building is above it.

It follows that the deck footer and supports must be below the frost line. Because the deck’s footers are so far below the frost line, they will not be impacted when the underground water freezes and expands and there will be zero structural damage to your deck.

Essentially, when you make any structure, your aim is to ensure that the foundation does not change over time. So, much like you don’t want to build on unstable soil, the main reason you want to figure out the frost line is that you don’t want your deck to lift up unequally when the groundwater below it freezes.

Frost Depth

how deep should deck post be in the ground

A frost depth map is a helpful tool for estimating how deep you have to dig into the earth to reach areas where the water table rarely freezes. This map contains contour lines along which any point indicates the frost depth within these towns.

There are hundreds, if not thousands, of frost depth maps available across the nation that show average levels. Unfortunately, if you’re searching for a specific ZIP code for your home, those maps won’t assist you much.

So far, the best source of frost-depth information for your particular location is the local building authority. They are generally aware of the local circumstances and can provide them upon request.

A quick google of your state’s frost depths will give better results, but not as thorough as your local authority. You can also use online tools, but don’t take this information at face value.

Warmer regions, as a norm, do not experience freezing temperatures, suggesting that they are not at risk of frost heaving. As a result, they can meet the minimum depth standard for deck posts.

However, in colder regions, the frost depth can hit 96″. Again, it is best to consult with your local building official about the frost depth for your particular area so that you can figure out the appropriate depth for your deck posts.

Type of Soil

The next factor to take into account when determining the depth of your deck posts is the type of soil you have at your property. It should be clear, compacted, and well-drained ground. Although there are many different varieties of soil types, three main ones are used to determine load-bearing capacity.

  • Gravelly soil.
    This earth is made up of tiny rocks, pebbles, and dirt. It is difficult to form into a shape and drains well, making it the finest soil for load support. In truth, it is rated for 3,000 psf loads.
  • Sandy soil.
    Because it is made up of microscopic particles, this soil has a gritty, coarse, and granular texture. It is frequently light brown in appearance. It requires the addition of water to maintain its shape, but the moisture escapes rapidly. Moreover, it is scored to carry loads as high as 2,000 psf.
  • Soil made of clay.
    This type of soil is often sticky and easily formed into a shape. The soil can be red, brown, or other colors depending on the minerals in it. It retains moisture and can withstand weights of up to 1,500 psf.
  • Bedrock.
    This type of soil is essentially a solid rock layer. The best part about bedrock is that it is not influenced by frost, so it can be used to support a deck footing at any depth.

How to Properly Set Up Deck Posts

How deep should deck post be in the ground?

Aside from the factors that influence the depth of your posts, you should be aware that there are two methods to set the deck post: below and above grade. The method you choose will also decide whether your deck posts require additional depth.

  • Below Ground Method
how deep should deck post be in the ground

Deck posts are merged together with footers beneath the ground in this method. Concrete is put into the support hole first, then the deck post. Eventually, gravel is applied to fill the remaining hole around the deck post to guarantee that it is tightly packed and that the post does not shake or move.

When using this deck footing installation method, keep in mind that approximately a foot of concrete is injected from the bottom before the deck post is set. As a result, the post’s depth should be approximately equal to the depth of the footer minus 12 inches.

Despite the fact that some deck builders object to the practice, decks have been constructed with support posts below ground for so long. The critical reason against submerging wood posts is that they will not last as long.

Another criticism directed at submerged posts is that they are difficult to repair. Whatever method you favor, the above-ground method is here to stay and will continue to be used by deck builders.

The most essential thing to remember when using this method is to use the proper wood. The post material should be as decay resistant as possible and appropriate for contact with the ground.

More Properly Set Up Deck Posts
  • Above Ground Method
Above Ground Method

The deck post is set on concrete footers above the ground in this approach. Furthermore, unlike the below-ground method, the posts are not submerged at all, so only gravel and concrete are used to cover the post hole.

Nonetheless, the footings should indeed be placed below the frost line, and a portion of the concrete must protrude above the ground. This is where the deck post will be attached and secured.

This method is favored by many builders because it helps to avoid wood rot. Because the post is not submerged below ground level, the wood material does not come into continuous contact with moist soil, reducing the risk of decay and rot.

So, how should you go about installing deck posts? Most people advise against placing deck posts in concrete because it may damage the entire deck as concrete does not have the ability to prevent moisture in the soil from contacting the wooden deck supports.

Furthermore, the municipality department does not allow the below-ground method in some areas because these posts could collapse and create significant mishaps. Deck posts installed above the ground, on the other hand, are simpler to replace and less susceptible to breaking.

If you choose the above-ground approach, keep in mind that the size of the deck footings must also be considered. They should be at least as broad as your deck posts. Experts suggest that they be at least 8 inches broader than the posts.

Furthermore, it is essential to secure the deck post in a position to prevent it from collapsing. Metal supports can be used to increase stability even further. And, if the deck posts are properly installed, they will last for a long period. So, how deep should deck post be in the ground?

Is It Possible to Build A Deck Without Posts?

how deep should deck post be in the ground

It is common to think that making deep holes for the posts is too demanding, particularly if you live in an area where the frost line is well below ground. So, what if you can create a deck without using footers?

Some decks can, in fact, be constructed directly on the ground. They are often referred to as floating decks because they are ground level and do not require the construction of deck stairs, which include guardrails and supports.

However, the codes for these decks vary by location. So, you should verify with your city’s code requirements to see if such a deck is permitted in your backyard.

These decks are subjected to a variety of environmental stresses and are particularly susceptible to shifting. Furthermore, the deck will be in continuous contact with the soil, which can quicken the decomposition and deterioration of the hardwood.

Alternatively, deck blocks can be used to raise the platform a few inches off the ground. These are inexpensive, but you must acquire the number of blocks that are required to equally sustain the weight of the deck.

But even so, you must make sure that the land on which your floating deck will be built is solid and not vulnerable to erosion. Furthermore, make certain that water does not pool in that area. Your floating deck is useless if the ground doesn’t meet these requirements.

That’s a Wrap, but Before you go …

Deck posts provide a stable basis for a deck and keep it from tipping or sinking when burdened. Furthermore, the depth of the posts guarantees the deck’s safety and integrity.

Because the stable soil formations, bedrock, and frost level differ from location to location, so does the depth of the footing. If the deck posts are not sufficiently deep, the deck will adapt to underlying shifts or movements. 

The points above explain why understanding the required depth of the platform footing is critical. If the posts and soil are not powerful enough to hold the loads, some decks will lift, dip, or collapse.

Do you have no idea where to start with your deck-building project? Or how deep should deck post be in the ground? Use the above guide to help you decide the best post depth for your needs. If you are unsure, seek guidance from a local construction expert.

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