Deck Joist Spacing: Your Complete Guide to a Proper Build

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deck joist spacing

Building a deck as a DIY project is an exciting way to get comfortable with your tools and equipment while having something useful to show for it.

Aside from the build material, though, the most important part of any deck build is the framing.

You need to have proper spacing for your joists, beams, supports and railings.

There are several factors that go into the proper spacing, and reasons to switch from one to another.

This article will cover the best deck joist spacing options and help you understand why they need to be set where they are. Read on to learn everything you need to know about joist spacing.

The Important Deck Build Spacing Measurements

When building a deck there are a lot of measurements that need to be made and double checked. The most important ones are listed here:

  • Total size. You need to know how big your finished deck will be. This affects permits and materials.
  • Post spacing. The main posts have detailed and specialized spacing requirements for the base structure of your new deck.
  • Beam spacing. Some decks only have edge beams and others need support beams in the middle. These measurements will prevent sag in your deck.
  • Joist spacing. The joists hold the deck boards and have the most variable spacing requirements.

Local Building Codes

local building codes

When doing any form of renovations or construction on your home, it is important to know the building codes for your city. Below we outline some of the major factors of building codes and what they mean for you.

Span Requirements

The composition of each city, state and region is different. Temperatures, base soil and other factors go into figuring local span requirements. City codes will help your deck maximize support to prevent sag and collapse no matter the conditions.

Railing Needs

In most areas any deck or porch with more than two or three steps will require a railing. When planning the layout of your deck and framing, you need to account for both the steps and the railings. The styles and materials used are usually left up to you, but their position and attachment to the frame and joists is important.

Posts and Anchors

When framing out your deck, the joists and beams will need to be anchored. The posts and anchors may be required to be a certain depth, a specific distance apart or using a certain fastener system. Make sure everything is in accordance with the building codes and you will be fine.

Frost Lines

In colder climates with snow, runoff and freezing ground temperatures, there will be a frost line. Some regions require that deck footings extend beyond the frost line so the deck doesn’t rise during a freeze. Other regions may allow a floating deck system that rises and falls with the temperature.

Attached Decks Requirements

If your deck is attached to the house, you will use a ledger. Building codes in these instances will be explicit on how many fasteners, what type and their spacing to use. While you may or may not be forced to use metal flashing, whatever the material, be sure you comply. 

Deck Footing, Posts & Railings

When dealing with footings, you will need to comply with building codes. Colder climates will have a frost line that needs to be accounted for. Warmer climates can have more shallow footings.

The size of the footing and posts will also be determined by the type of base soil (sand, clay, etc.) and the height of the deck. In most cases, for example, 4×4 lumber is the standard. However, for decks more than 6 feet off the ground a 6×6 post and frame may be required.

Calculating Deck Beam Spans

The beams are often confused with the joists, so let’s clear this up a little. The short, easy explanation is that a beam is a larger support board that carries the load, distributes the weight and moves the load from the floor, walls or roofs to the columns.

On a deck, these are the main sections of the frame that connect the joists and decking boards to the footings and posts. The beam boards can be a single board or two put together.

The general recommendation is to use two boards as they help distribute the weight and are generally stronger than a single board. Using two 2×8 boards instead of a single 4×8, for example.

The board type will also be a factor in the beam spacing. Southern Pine and Fir have a different beam spacing than Cedar and Red Pine. Smaller decks can also get away with beams around the edge distributing the weight to the footing and posts. However, larger decks may require more beams in the middle section of the deck.

As a general rule, the beams must span the entire width of the deck. Their spacing, though, is dependent on the depth of the boards. The board depth in inches is the space in feet it can span.

For example, if you have a 2×8 inch beam, it can span 8 feet before another beam is required. A 2×12 inch beam can span 12 feet.

However, it is also important to know that the span of the joists that the beam is supporting also determines the amount (in feet) that a beam can span between support. The larger the joist spacing, the shorter the beam spacing.

So as an example, a 2×10 beam supporting joists that span 8 feet will need support beams every 9 feet, but a 10-foot joist spacing can only span 6 feet without a support.

Calculating Deck Joist Spans

calculating deck joist spans

Joist spacing is not weight distribution important as much as beam spacing is, but it is important for placement and deck finishing. The joists are the boards you will place your decking on as well as your grill, chairs and lighting.

The biggest factor here is the board material. Treated lumber is still the most common, but composite and PVC are making more appearances in new construction than in years past.

When dealing with composite joists (see below) you will get explicit instructions when you place your order. Each brand will have their own requirements. Your only job, aside from following those instructions, is to make sure you also stay within code compliance.

For lumber, though, it will depend on the type of wood you are using, as well as the size of the boards. To help make it more clear we have a chart with the most common wood types and measurements based on 16 inch joist spacing.

 2×62×82×102×12
Southern Pine9 ft11 ft 10 in14 ft16 ft 6 in
Fir8 ft 4 in11 ft 13 ft 7 in15 ft 10 in
Redwood, Cedar, Red  Pine8 ft10 ft 6 in13 ft15 ft

Trex Joist Spacing

Spacing for the joists is generally 16 inches from center to center. This means you measure the board from it’s center 16 inches. That mark is where the center of the next joist will be. You do not measure end to end or edge to edge.

Trex as one of the leaders in composite decking also makes great composite boards for joists and beams. Almost each of the Trex options will require a 16 in OC (on center) joist spacing.

The other option you will find from Trex is 12 or 14 inches OC when you are using angled or picture framing applications.

Composite Decking Joist Spacing

Other composite brands also tend to follow the standard 16 inch OC joist spacing. It will also depend on the brand, the type of composite and the conditions where you place them. Some composite, like Deckorators, is rated for on water or ground installation. This means you can use wider spacing, as long as it is within code.

Deck Joist Spacing for 5/4 Decking

When you are using 5/4 decking (1-inch thick decking) the joist spacing must be a maximum of 16 inches OC. It is not uncommon to find the 5/4 joists positioned at 12 inches.

A 12-inch OC joist spacing is also used in commercial decking where 5/4 joists are more common. You also find it in residential areas where you have a larger deck and want a more rigid feel underfoot.

Frequently Asked Questions about Deck Joist Spacing

faq deck joist spacing

In this section we will answer the most common question about deck joist spacing. If you have more questions please use the comment section below.

Q. How many joists do I need for a 12 x 12 deck?

  1. The actual number will depend on the beam width and joist material. However, you calculate how many joists you need. A 12-foot deck is 144 inches long and with a 1-inch board spaced 16 inches apart (On Center) you will need 10 joists.

Q. Can I use 2×4 for my decking frame?

  1. Unfortunately, 2×4’s are not strong enough to support weight for a decking frame, and will require more bracing and extra layers to make it work. It would be much more feasible and less work to use standard frame sizes (2×6, 2×8, etc.), for your beams and joists.

Q. Do I need a building permit for my deck?

  1. In most areas of the US cities follow what is called the 30-inch rule. If your deck is less than 30-inches off the ground (to the top) it is considered a porch or patio and generally does not require a permit. However, you always need to check with city hall and the building inspection department to make sure before you begin construction.

Q. What is the maximum distance between deck posts?

  1. Regardless of footing, beam and joist spacing and material, the maximum post spacing should be no more than 8 feet. 

Conclusion

Deck joist spacing is one of the most crucial and important measurements for your build. If you are building your deck as a DIY project, the joist spacing will determine how strong your deck surface is, whether there will be sag in the future and how rigid the deck becomes.

While the general guide is 16-inches on center for the joist spacing, the actual measurement will depend on the deck size, joist material and location.

Now that you know how to space your beams and joists, you are one step closer to finishing your new deck project.

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AUTHOR

Nora has more than 5 years experience in the floor covering industry, acquiring vast knowledge about installation and material selection. She now enjoys working as a writer and an interior decorator. Her work has been featured in The Spruce, Homes & Gardens, Southern Living and Real Homes. See full biography here.

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