Introduction
Building a deck is perhaps one of the most common construction DIY projects. It gives you much more space, improves home value, and doesn’t require specialized tools. However, building a safe deck requires some knowledge of math and physics. Also, knowledge about deck post spacing.
The posts holding up the deck need to be spaced and set so that they can support the deck’s weight (and anything on it).
To decide how far apart your posts should be, determine the size and type of lumber used for your joists. Then, compare this information to the table in your local building code, or utilize an automated calculator for the task.
Spacing Deck Posts
The best advice for spacing deck posts is to build stronger than you think you need to. Nothing bad will happen if the posts are too close together, or if you use lumber with excessive load-bearing capacity.
Not only will “over-engineering” your deck give you peace of mind, but it will also give you flexibility when adding furniture or other heavy objects to your deck later.
The ideal post spacing is about 8 ft in between the midpoint of the posts, although in some cases you can get away with up to 15 ft.
Deck Post Spacing Calculators
Several online calculators exist to help you space your deck posts. I usually do the calculations with multiple methods, as this assures me there hasn’t been a mistake somewhere. Popular calculators for deck building include these by DecksGo and Decks.com.
Using these calculators is quite easy. It can be done in a matter of minutes with a measuring tape and some note-taking.
Code Regulations
Depending on the place you live, you might also need to abide by a building code. These are laws that determine the minimum requirements for how things are built.
Local building codes are generalized guidelines that factor in local ways of doing things such as:
- The types of lumber available in your area
- The way the wood was cured
- Local weather patterns
- Soil type
You can often build stronger than dictated by building codes, but never weaker.
For example, you may decide that you can place a post every sixteen inches, but if your municipality states that posts should be placed every twelve inches, then this overrides your number. However, if your building code states that posts are to be placed every fifteen inches, but you want to place them every eight, then you can do this.
Deck Post Spacing Chart
To determine the correct spacing for your deck posts, it’s important to consult a deck post spacing chart. A deck post spacing chart will provide you with the recommended spacing for your deck posts based on the size and weight of your deck, as well as the local building codes in your area.
The chart provided in your local building code are difficult to read for a novice. You will find that most speak in terms of maximum beam length per joist span and are organized by type of wood and size of the beam. What does it all mean?
Here is an example of a span table in a building code. There are four main pieces of information you need to read it properly.
| Wood Species | Dimensions | Max. Joist Span 12” | Max. Joist Span 16” | Max. Joist Span 24” |
| Southern Pine | 2 x 6 | 9’ 11” | 9’ | 7′ 7” |
| 2 x 8 | 13’ 1” | 11’ 10” | 9′ 8” | |
| 2 x 10 | 16’ 2” | 14’ | 11′ 5” | |
| 2 x 12 | 18′ | 16′ 6” | 13′ 6” | |
| Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir | 2 x 6 | 9’ 8” | 8’ 4” | 6’ 10” |
| 2 x 8 | 12’ 8” | 11’ 1” | 9’ 1” | |
| 2 x 10 | 15’ 8” | 13’ 7” | 11’ 1” | |
| 2 x 12 | 18′ | 15’ 9” | 12’ 10” |
Deck Post Spacing Chart
Beam Length
Beam length refers to the length of a beam that can be unsupported, and is the number listed in the third and fourth columns. This is indirectly telling you how far apart it is permissible to space your posts – you need to put posts a minimum of the listed beam length apart. The beam length is measured from the center of either post.
Maximum Joist Span
This is the distance that your joists are apart. This matters when spacing the posts because it determines how much approximate weight each joist is supporting.
Wood Species
Different woods have different densities (and thus load capacity). You’ll need to know the wood you use when building. Most building codes include information for all the common wood species in the area.
If you are using an unusual building material, then you will need to find the wood on the list with the closest density to the one you are using. The place you bought the lumber should be able to tell you its load-bearing capacity in psi, which you can compare to this list of common lumber types.
Size of the Beam
The size of the beam refers to the dimensions of the horizontal wood you are supporting with the post. Most building codes cover all the common lumber sizes, but if you have a size that isn’t on the list always round down to the small beam size.
Where To Find Building Codes
To utilize the important information contained in building codes, you will need to find the ones relevant in your area. Building codes often exist at multiple levels – national, state/province, and city/municipality are all common umbrellas for building codes to fall under.
National and Regional Building Codes
The national or regional building codes will be the easiest to find – if you live in the United States this website contains lists of building codes at the national and state level.
Outside of the United States, you can search your country’s government website, or rely on the same source you’ll use for city building codes.
City/Municipality Building Codes
Local codes are not always easy to find. Depending on how well-funded and tech-savvy your area is, they may or may not be online. They are, however, by their nature accessible to the public. After all, how else would people follow them? The trick is finding the right person to talk to.
Asking an expert is my preferred way of getting information about local building codes. You can refer to a home/building inspector, as they will surely have access to that information.
In most cases, if you ask nicely where to find the local building code regulations, they will tell you free of charge. This is a mutually beneficial arrangement because they know that someone doing renovations will eventually need the services of a building inspector.
If you are unable to get the information this way, you can also approach a government office that works closely with building codes. Some examples of offices that might know are:
- your local building permit office
- your local government archive
Such offices are usually helpful, although you might have to wait longer before they get back to you.
What Happens If the Posts Are Spaced Incorrectly?
The consequences can be catastrophic if you incorrectly calculate deck post spacing. While nothing bad will happen if they are too close together, too far apart can collapse the structure. This may occur when the wind blows, completely unprompted, or even when someone walks on it.
A structurally unsound deck is a hazard to the health of you and your loved ones. It can also cause property damage to any nearby or attached buildings (such as a pool or your home). Finally, it will almost certainly ruin a large portion of the lumber used to build it, which is money down the drain.
What Affects Deck Post Spacing?
Several factors can affect deck post spacing including:
- Species of wood used.
- Curing method used on the lumber (oven vs. air)
- Thickness of the joists
- Local humidity and weather patterns
Because there are so many factors to consider, I advise relying on the tables found in your local building code rather than trying to calculate the load capacity of a beam yourself.
Setting Deck Posts
Spacing your deck posts the correct distance apart will not do you any good if they come loose from the ground. They should be set deep enough and/or held in place with concrete to prevent this.
Frost
The general rule of thumb is that a post should be at least six inches below the frost line. This is to prevent the freezing and thawing of the ground from shifting the post.
Soil Type
The type of soil you have also plays a role in the needed depth. Sand and rocky soils typically hold better, while clay soil is more prone to shifting.
Before you go …
Your deck post spacing will be a very important step in constructing your deck. Mistakes can result in a dangerous and costly failure of the structure. To prevent overloading any one section of the deck, always build stronger than you think you need to. The ideal spacing for most deck posts is 8 inches, however, local building codes provide more detailed guidance.
