Size of Deck Post Needed? (Answer is in the Height and Size of Deck)

Determining what sizes posts you need is crucial in building your deck and can’t be overlooked. However, many individuals don’t even know where to begin because some of the information can be confusing at times. Let’s clear it up.

Depending on how high and wide your deck is, you will use 4X4, 6X6, or in some cases, 8X8 posts. However, careful consideration will have to be given in calculating the tributary area for your deck, the material used, and the post spacing. Then only can you determine what size posts you need.

We will discuss in this article all the necessary factors involved in determining deck posts size, and some calculations will be needed for you to determine the load for the posts correctly.

Before we get into the deep math and figuring out post strength, deck tributary and everything else there may be an easier answer. Your local building code may even require it. So please always check with your permit office before building your deck, including the post and footings.

For example, in Calgary, where I build decks, a deck must be supported by a 6×6 post. End of conversation. It does not matter the height of the deck or how much weight the post is supporting. The post supporting the deck must be a 6×6, anchored to a frost level 10” diameter concrete footing, 4’ below grade. So always check before building but if you have the freedom here is what you need to know to determine the correct post size for your deck.

What Size Posts Should I Use For A Deck?

Deck posts are standardized in size to a degree to make things easier when building. The typical deck posts sizes are;

• 4X4 (4 feet by 4 inches)
• 6X6 (6 feet by 6 inches)
• 8X8 (8 feet by 8 inches)

These posts have been calculated and designed to these standard sizes so that they will appropriately handle the expected load of the deck. Moreover, the most common post types are that of 4X4 and 6X6. This is even true when considering large decks of 250sqft.

To calculate what size posts you need for your deck will require you to consider a few factors. Not only do you need to consider building regulations and then permits (if need be), but you will need to calculate the structural limitations of your deck in order to determine the correct size posts that you will need. This includes calculating the tributary size and taking into account the standardized distance posts have to be placed apart.

*Builder Tip*

Whatever post is required, it must be wide enough to support all plys of the deck beam. If your deck needs 3 or 4 plys, a minimum 6×6 post is required. A 4×4 post is only acceptable for a beam less than 3 ½”.  

Before we jump into the details, let’s consider some logic to prepare ourselves—the taller that a post gets, the less support it can provide. This is because posts are made of materials with strength limitations (Southern Pine is stronger than Cedar). Additionally, the forces bearing down on taller posts are increased.

Calculating Tributary Size For A Deck

The first thing you will need to do before tiring to figure out what size posts you need is to determine the tributary area.

The tributary area is deemed to be the area that will cause load on a particular structural element (in this case, it is the deck).

To calculate the maximum load of your deck, you will take the total area and multiply it by 50psf (Pounds per Square Foot). For example, if you have a deck that is 100 sqft, the deck would only be able to support 5,000 lbs.

The 50psf is a safe value that structural engineers have deemed fit to be used in most applications for deck building.

Going forward with this example, we will consider that your deck is attached to the adjoining house by a ledger board. This means that the ledger board will be supporting approximately 50% of the load. Thus, you will halve the total area of the deck, with the initial half (from the ledger board to the midpoint of the deck) not needing any posts.

Calculating Deck Post Spacing

You will need to place your posts in the second half of the deck (from the midpoint to the cantilevers). Consider that the maximum spacing for 4X4 deck posts should be no further than 6 feet apart. Then 6X6 deck posts should have a spacing of 8 feet. Note that the measurement is obtained from center to center and not the outer edges of the posts.

Take note that, typically, you will start by placing posts in the corners of your deck to provide the most support. There are cases where you will want to distribute the posts, but more than likely, if you are building a deck and have to consider load and area beforehand, you would have approximated to place the posts as close to the corner as possible.

Depending on your deck’s size and where you have “theoretically” placed your posts, you will need to calculate the tributary area for those.

To obtain the area each post will support, you will have them placed with the appropriate distance apart and divide the distance from post to post.   This will be the width of each post. The length will be the distance from the end of the deck to the midpoint.

Calculating Post Height

Now that you have the tributary area for each section of your deck, you should know how big that area is and how much each area will carry.

We discussed earlier that the various types of material would also affect how much load they can handle. As such, you will need to specifically check what material you are going to use in conjunction with the post size to determine if it is able to handle the load you have calculated for your deck. 

However, here is a post height and sizes table based on common materials that you can refer to. The calculations to determine the sizes and heights of the posts in relation to the tributary area are done by using 50psf. If this is not your load capacity, you will not be able to use this chart.

Post Size And Height Based On The Tributary Area (Corner Posts)

Post Size And Height Based On The Tributary Area (Center Posts)

Conclusion

We discovered that it is difficult to decipher what posts you need without first determining the tributary area of your deck and then, subsequently, the spacing of your posts. Additionally, the material used for the posts (type of wood) will affect how high they can go.

Remember that this procedure of determining deck post size should not be overlooked or miscalculated because your deck will probably collapse due to the amount of load exerted on the posts.

Source list

DECK LOAD CAPACITY AND TRIBUTARY AREA: HOW THEY RELATE

SEVERAL FACTORS DETERMINE THE DECK POST SIZE TO SAFELY SUPPORT YOUR DECK

How to Calculate a Post Size for a Deck

How Far Apart Should Deck Posts Be?