Deck Joist Sizes, Span & Spacing (Complete Guide)

Homeowners looking to build a deck often have questions about the size and spacing of joists. This article will answer all of your questions so that you can get started on building your dream deck!

The International Residential Code (IRC) determines the size of your deck joists. IRC covers the maximum span length of wood joists, from 2×6 to 2×12. IRC also specifies deck joist sizing and spans depending on the joist spacing, commonly 16 inches on center but can vary from 12 to 24 inches apart.

In this post, we’ll go over everything you need to know about deck joist sizing and spans. We’ll go through ways to measure your deck beams and offer some advice on repairing undersized ones.

What is Deck Joist Span and Spacing?

A deck joist span is a distance between two load-bearing points. Deck joist spacing usually is 16 inches on center but can vary between 12 and 24 Inches. The deck joist spacing you choose will depend on several factors, including the size of your deck, beams, the deck foundation, and the type of decking you plan to install.

The IRC also has regulations for minimum joist sizes. In most cases, your deck joists will need to be at least a certain size to support a live load of 40 pounds per square foot. Deck joists are measured in inches by their thickness (e.g., “a deck joist is a 10-inch floor joist”). If you’re unsure what size deck joists to use, don’t worry – we’ll go over this in the next section.

Deck joists are generally 6, 8, 10, or 12 inches in width, with 8 or 10 inches as the most common sizes. Deck joists come in dimensional lumber lengths from 8 to 16 feet or more.

As discussed in this article, deck joist span is important for designing decks to support heavy loads. Deck joist spans depend primarily on how much weight the deck will have to bear. Decks holding hot tubs will require reinforcement and closer joist spacing.

2018 International Residential Code (IRC) Exterior Decks

The IRC is the governing document for all residential construction in the United States. It’s updated every three years, and the 2018 edition was released in November of 2017. The IRC covers everything from minimum floor joist sizes and spacings to allowable roof loads.

One of the most critical sections of the IRC when it comes to deck joists is Section R507. This section covers conventional lumber sizing, spacing, and spans. Table R507.6 gives maximum span lengths for different-sized wood joists based on their location species, size, and spacing.

If you’re a homeowner looking to build a deck, you’ll need to know the IRC’s joist sizing and spans requirements.

What Wood Species Are Available

Many homeowners are looking for an excellent resource to assist with the sizing and spans of deck joists. Here are the deck species available.

Southern Pine

Southern Pine is a good choice for floor joists since it is strong and resistant to shrinking and warping. Southern pine has several benefits compared to other woods, including being inexpensive. The major disadvantage is that it’s vulnerable to decay and termites.

Douglas Fir-Larch

Douglas fir is the most common type of wood used for floor joists and roof rafters. It’s robust, resilient to shrinking and warping, and comes in long lengths. The major disadvantage of utilizing Douglas fir is that it may be costly compared to other woods and isn’t readily available in some locations.


Douglas fir is not readily available in some areas, so hemlock-pine (also known as “Hem-Fir”) is a good alternative. This wood has comparable properties to Douglas fir and can be used in most situations.

Spruce Pine Fir

Spruce is the least expensive species, but it also has the lowest strength rating. It’s a decent choice for regions where moisture is likely an issue since it won’t warp or decay rapidly. Spruce should not be utilized in areas with a lot of humidity or snowfall because it can quickly become damaged.


Redwood is a type of wood that is native to North America. It is one of the most popular types of lumber used for decking because it is strong and weather resistant. Redwood can also be stained or painted to match any color scheme. In general, redwood decks last longer than other decks made from less expensive wood species.

Western Cedars

Western cedars are a type of wood native to the Pacific Northwest. They are often used for decking because they are strong and weather-resistant and have a natural resistance to decay. Western cedar decks can last up to 30 years with proper maintenance.

Ponderosa Pine

Ponderosa pine is a type of wood that is good for building decks. It ranks high in strength, weather resistance, and durability. It has always been popular for building because it was readily available at a low cost. A disadvantage for some homeowners is that Ponderosa Pine often contains knotholes or other defects that require extra work during deck construction. There are also fewer color choices than redwood or cedar decking options, so it may require painting if you cannot find what you need in the natural form of this lumber species.

Red Pine

Red pine is a type of wood native to the eastern United States. It is often used for decking because it is strong and weather-resistant and has a natural resistance to decay. Red pine decks can last up to 30 years with proper maintenance.

What is Treated Lumber

Lumber that has been treated with chemicals to prevent rotting, insect damage, and fire is known as “treated lumber.” There are three types of common treatments:

  • CCA (chromated copper arsenate)
  • ACQ (alkaline copper quaternary)
  • MCA (mercury copper arsenate)

Although treated lumber doesn’t cost much more than standard untreated wood, it’s much more hazardous for the environment. Arsenic is a poison that pollutes the water supply over time.

Pressure-treated cypress and redwood don’t carry the same hazards as regular treated lumber, making it a much better choice. The primary reason cypress and redwood are not utilized for residential construction is that they’re costly.

How Lumber is Graded

For homeowners seeking information on deck joist sizing and spans, the information below is to be considered. If you’re not sure what wood you’re buying, you should find a white tag stapled to the board with the grade and species marked on it. If the tag falls off, the lumber also has a black stamp branded onto the timber with the grade and species.

The four lumber grades include Select, 1, 2, or 3. Lumber graded as 1 is of better quality than 2 or 3. The highest grade, “Select,” is free of knots and other flaws that weaken it. Select timbers can bear heavy loads.

According to IRC R507.2.1, deck lumber need to be No. 2 grade or higher and preservative treated according to Section R317 or be an approved naturally durable lumber, and termite protected where required according to Section R318.

Floor Joist Load Explained

If you’re looking for information on deck joist sizing and spans, you’ll want to consult your local building codes. In general, deck joists can hold a certain amount of weight based on the grade of lumber, the species of wood, and the dimensions of the joist. Deck joists are typically spaced in 12, 16, or 24-inch increments.

How Far Can Floor Deck Joists Span?

The length of a joist defines the deck joist span. For example, the maximum span length of a Douglas fir-larch, 2×12, supporting a 40 (PSF) live load and 10 (PSF) dead load, with 12″ spacing, is 18 feet 3.

Is There a Standard Floor Joist Span?

Because every house is unique, there is no standard floor joist span. There are just too many variables present to establish a standard for floor joist span.

Any span beyond 20 feet is rare, as seen in the floor joist span tables. It’s difficult to locate framing timber longer than 20 feet, which is usually more costly when available. Contractors will install a beam to break up 20′ joist spans.

Max Span for Southern Pine

Joist SizeAllowable Joist Span Allowable Joist Span Allowable Joist Span

Max Span for Douglas Fir-Larch, Hem-Fir, & Spruce-Pine-Fir

Joist SizeAllowable Joist Span Allowable Joist Span Allowable Joist Span

Max Span for Redwood, Western Cedars, Ponderosa Pine, & Red Pine

Joist SizeAllowable Joist Span Allowable Joist Span Allowable Joist Span

Joist Spacing for Composite Decking

Composite decking is an excellent alternative to traditional lumber, requiring different joist spacing. Composite decking should be spaced no more than 16 inches apart. Before building your deck, consult with your professional contractor to ensure you use the correct joist spacing for your composite decking material.

Deck Cantilevers Explained

A deck cantilever is a deck section that projects out beyond the supporting joists. You can use this to create a covered porch or extended seating area. The maximum cantilever distance is 4 foot 6 inches, but it’s best to keep it within 2 feet for most decks.

Max Cantilever for Southern Pine

Joist SizeMaximum CantileverMaximum CantileverMaximum Cantilever

Max Cantilever for Douglas Fir-Larch, Hem-Fir, & Spruce-Pine-Fir

Joist SizeMaximum Cantilever Maximum CantileverMaximum Cantilever

Max Cantilever for Redwood, Western Cedars, Ponderosa Pine, & Red Pine

Joist SizeMaximum CantileverMaximum CantileverMaximum Cantilever


Deck joist sizes and span can be a complicated subject, but we’ve done our best to simplify it. You can use these tables to determine deck joist sizes, spacing, and span based on wood species.

See our other articles on decks and deck railings:

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started to help people better understand the home inspection process and answer questions about homeownership and home maintenance.
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