Deck Joist Sizing and Spacing Guide

Adding a deck to a home or yard is a great way to create an outdoor living space. Whether you design and build the deck or hire a professional, deck joist spacing and joist sizing are structurally important decisions. The greater the span and spacing, the more spongy a deck may feel.

The width, thickness, wood type, grade, and spacing influence the span a joist can bridge. Joists can usually span up to 1.5 times their thickness in feet. The spacing between joists is 12”o.c. or 16”o.c. for composite and 5/4 decking, and up to 24”o.c. for 2×6 deck boards.

In this guide, we’ll discuss joist sizing, spans, spacing, and factors that affect material selection when framing a deck. We’ll also explore how they influence the strength and safety of a deck. By the time you’re finished reading this guide, you’ll have a better understanding of how span and spacing work, and what will structurally work best for your deck.

What Is a Deck Floor Joist?

A structurally solid and safe deck is the end product of all the building components, even if all that is seen and admired are the deck boards. The boards and furnishings rest on a deck frame of joists supported by beams, ledger, posts, and footings – all are important. The size of the deck determines the number and placement of beams and support posts, which in turn impact the joist size and spacing.

Deck joists are planks laid on edge in a spaced pattern to which the deck boards are fastened. Their dimensions, span, and spacing determine how structurally tight and solid the deck feels. Joists distribute the weight of the live and dead loads of the deck to the supports.

The spacing, span, grade, and joist dimensions are identified in the International Residential Building Code (IRC) which guides most national and state codes. Whether the deck framing is free-standing or attached to a structure with a ledger board, joists span the distance and help shape a deck.

What is Joist Spacing and Span?

The spacing and span of joists that support the decking are identified in most Building Codes. Additionally, though, the values are also influenced by many factors, including wood species and grade. Southern pine with a select or #1 grade is stronger than #2 grade SPF and can carry more weight and span a greater distance.

Deck joist spacing is the distance between joists, measured from the center of one joist to the center of the next, and is commonly 12, 16, or 24-inches. The span is the distance a joist traverses from ledger to beam, or beam to beam, without support. The narrower the spacing between joists, the greater the span can be.

Tables and span calculators frequently provide joist dimensions, span, and spacing values based on a dead load of 10 pounds per square foot (PSF or psf) and a live-load of 40psf. The height off the ground, wind-load, snow-load, soil-load capacity, and potential seismic forces also factor into the span.

It is advisable to retain the services of a Structural Engineer for correct calculations, especially for decks of large dimensions, loads, and heights.

Why is Joist Spacing Important?

Deck joists, similar to those in any structure, determine the load a floor or deck framing can carry. They support the deck surface and provide a level framework to which decking is attached. The spacing between joists also influences the type of deck material that can be supported, whether 5/4 or 2-inch thick, or cedar, SPF, or composite decking.

Deck joist spacing affects how well supported the deck boards are; if too far apart, the decking will be springy or soft, and can sag. Spacing also impacts the distribution of weight, influencing the structural strength, and deck integrity. Additionally, the inconsistent spacing will create an irregular pattern of fasteners that detracts from the visual look of the deck.

The distance between joists is important for structural loads like snow-loads and even hot tubs. The Building Code stipulates acceptable spacing to prevent injury or death due to improper spacing which can cause structural failure. It’s best to adhere to the Code or involve a Structural Engineer.

Factors that Determine Deck Joist Spacing and Span

There are six main factors to consider when designing and building a deck that determines the span and spacing of deck joist. Each factor affects the span, the spacing, or both, as well as the placement of beams and support posts that make up the deck frame.

Shape of the Deck

The shape and size of the deck influence the layout and number of joists, potential spans, and even the spacing. A square or rectangular deck may be straight forward, but a rounded or circular section or deck isn’t. The shape often determines the direction of the joist grid, which in turn affects the deck board layout and direction.

The shape determines post and beam placement, spacing, and dimensions. It also determines if joists will be cantilevered which also affects the span and spacing. A 2×6 yellow pine at 12”o.c. can span 9’-11” but that drops to 6’-8” with a cantilever.

Raised or Ground Level Deck

Decks close to the ground are permitted greater spans than decks raised further above grade. Elevated decks often require greater post and beam support for shorter deck joist spans.

Type and Grade of the Lumber

Different types of wood, even within species, have different physical characteristics and strengths. Growing conditions also contribute to the variations making timber from one area more desirable for different projects.

Slow growth trees have more growth rings which means they are denser and stronger than trees that grow more quickly. The greater number of rings per inch results in more compact end grain and better bending strength.

• High Strength: Douglas fir and southern yellow pine will span greater distances.
• Medium Strength: Redwood, hemlock, and spruce will span almost as far.
• Lesser Strength: Eastern white pine, ponderosa pine, western red cedar, and SPF have shorter span ratings.

Pressure-treated or wood preservative doesn’t affect the strength of wood, incising however does. Incising is the process of infusing preservative deeper into the wood through small 1/4” deep cuts on all surfaces. Incising weakens the wood slightly, so it often is rated for shorter distances.

Lumber grades further identify the strength of different wood. Knots and defects affect the strength of the timber, making it weaker. Lumber with no blemishes is rated select, clear, or #1 and is stronger and more expensive. The most common grade used for joists and deck framing is #2, which has some knots but not enough to weaken it. It is also less expensive than select or #1 grade wood.

Lower grades, economy, or utility lumber have too many knots or large knots that weaken the lumber too much for structural use. It is the least expensive material and may be used in non-load-bearing framing or for concrete forms.

Joist Size

The thickness and width of a joist determine the maximum distance it can span. Most decks use dimensional 2-by lumber which is more readily available. Doubling a joist will increase its width allowing it to span by about 25% while doubling the thickness will add 75% to 100% more to the span.

For example:

A single spruce 2×6 may span 8’-8”o.c, and a double spruce 2×6 joist reaches 25% further, or 10’-10”. A 2×6 yellow pine at 16”o.c. with a live load rating of 40psf will span 9’-9” while a 2×12 yellow pine at 16”o.c. can bridge 18’-6”.

Load on the Deck

The load or weight a deck frame must support influences the span and spacing of the joists. The overall load is made up of the dead load and the live load. The dead load is the weight of all the building materials and is commonly calculated at 10psf. The live load is the weight of everything placed on the deck – furnishings, plants, people, etc. – and is normally rated at 40psf.

Adding a hot tub or spa to the deck will increase the live load and affect deck joist span and spacing. Living in a region with heavy snow loads or winds can also increase the live load values, and so should be included in the design considerations when building a deck.

Decking Boards

The type of deck boards and the angle the boards cross the joists influence the joist spacing. Most 2” thick wood planks are fine traversing joist spacing of 12”o.c., 16”o.c. and 24”o.c. at right angles. Many composite decking brands and all 5/4” wood boards are limited to 12”o.c. and 16”o.c., although some manufactured board brands are designed for the 24”o.c. spacing or greater.

Deck boards that diagonally cross joists at angles such as 30°, 45°, or 60° span a greater distance, and so the deck joist spacing must be adjusted. Composite deck boards and 5/4” boards laid on an angle to the joists require joists at 12”o.c. or less spacing, and 2” planks are limited to 12”o.c. or 16”o.c. joist spacing.

Local Building Codes and Standards

The local Building Code is adapted from the State or National Building Code, which in turn is based on the International Residential Building Code (IRC). Codes address all aspects of residential construction, not just decks.

Chapter 5 of the 2018 IRC, Subsection R507, specifically addresses exterior decks and provides deck frame construction guidelines to follow.

The Codes identify maximum joist and beam span requirements for different wood types to ensure decks don’t sag or collapse under the weight of people or snow load. Local building codes address unique conditions for their specific region, including the load-bearing ability of soil, frost and seismic issues, plus snow and wind load concerns.

The IRC reflects proven construction techniques for wood decks using widely accepted conventional construction materials and practices. Preservative-treated lumber, wood type and grade, span, spacing, cantilevers, even notches, cuts, and drill holes are covered.

R507.2.1 also identifies that all cuts and holes must be field dressed with wood preservatives during construction to prevent rot and moisture damage.

How to Determine the Proper Deck Joist Sizing and Spacing

Determining the proper deck joist sizing and spacing occurs at the planning stage. Size and spacing depend upon the shape and size of the deck, the weight it needs to support (hot tub?), and the height off the ground. Additionally, the beam support structure, type of deck board desired, the pattern for installing the decking, and of course, the Building Code must be factored into the decision too.

1. Determine the Shape of the Deck

The shape and size of the deck are primary considerations, along with its purpose. Decks commonly follow the perimeter of a wall or structure, or the contours of the landscaping, or both. Attached decks require a ledger for securing it to the building, while free-standing ones need an additional beam and post framework.

Decks may provide access to various rooms of a building and also to the ground which affects the design and height of the deck. The deck surface is commonly one 7” step down from building entrances or at the same level as inside floors. The height available from ground level to entry-level may trigger the need for landings or the inclusion of a lower deck.

Square or rectangular decks are easier to layout a joist and beam plan for. Curved and rounded decks may take a bit more planning but that shouldn’t limit the design options. Windows, trees, neighbors, and utilities are other factors to keep in mind.

When planning a deck, consider the direction the deck boards are to go. If you live in a snow-belt and need to clear the deck, it’s easier to push in the direction of the boards than across them. That affects the joist and beam directions, as well as the live load requirements.

A hot tub may be part of the purpose but it affects the shape and size of the deck too. A deck is an outdoor living area and enhances the look and character of a home. However, be mindful of the size – do you want a house with a deck or a deck with a house?

2. Prepare Your Deck Plan

Transferring the idea to reality begins with a plan. Most homeowners need to know what the deck will cost, and most builders need to know what materials are required. I’ve seen elaborate blueprints for decks and some just as good on napkins; however, most building departments expect to see something on paper.

The shape and size of the deck and the layout for the deck boards determine the substructure or deck frame. How the decking runs determines the joist pattern, which in turn identifies the number, size, and placement of beams and support posts, and the ledger board.

The size of the beams and the span between beam support posts, plus the distances between beams and ledger, affect the distance joists must span. The dead and live load the deck will carry helps set the span and spacing of joists and joist dimensions.

The plan needs to identify the number of beams and support posts, the layout and number of joists, and the plan for the deck boards. Decking may be perpendicular or diagonal to joists, full run or staggered, or something unique. The more elaborate the decking plan, the greater the joist structure requirements.

Once the plan is complete, a material list can be generated for building materials. The list should also include the number of joist hangers, hurricane brackets, and post-to-beam caps. The type of decking selected may require special fasteners and tools too. The more complete the plan, the fewer surprises.

3. Choose Decking Material

The type of decking and its layout affect the joist spacing.

Pressure-treated lumber is protected from moisture and insect damage, as well as mold and mildew. It is available in green or brown tones of preservatives, so the color is reasonably consistent. It doesn’t affect the strength of the wood and is available in 2-by and 5/4 decking formats for decking and frame construction. The wood needs to be cleaned yearly and have a water-proof boost every other year, depending on your climate.

Cedar has a natural wood preservative that protects from rot and insects. However, cedar will gray with age and requires cleaning and sealing or staining regularly. It provides a warm wood glow that many appreciate. It too comes in 2-by and 5/4 dimensions.

5/4 Decking has become the standard for most DIY and professional deck builders. The planks are nominally 6” x 5/4” and usually spaced 1/8” to 3/8” apart. There are invisible fasteners for securing the boards to the joists, or two 2-1/2” screws or nails through the face into every joist. The joists need to be spaced 16”o.c. or less if the decking is laid perpendicular, or 12”o.c. or less if diagonal to prevent sag and bounce.

2×6 Decking is more costly than 5/4 decking of the same wood. It offers greater strength and allows joists to be spaced up to 24”o.c. when laid perpendicular to the joist and 16” on the diagonal. It requires two 3” or 3-1/2” fasteners in every joist and is also spaced 1/8” to 3/8” apart.

Composite decking is commonly 5/4” x 6” and fastens to joists using invisible clips. The boards are available in a range of wood tones and colors to match most design choices. The wood doesn’t rot or discolor but does need to be cleaned regularly. Most manufacturers have deck joist spacing stipulations similar to 5/4 decking, however, some boards are rated for greater distances.

4. Choose Material and the Size for Your Joists

The distances deck joists must span and their spacing influence the size and grade of the lumber, and vice versa. Southern pine is stronger than most other common deck construction wood types and offers the greatest span. It is available pressure-treated at a premium price.

Erecting and spacing beams to allow shorter spans means less expensive and more readily available timbers may be used. It is better to go shorter than to try and maximize span bridging for a more solid and secure platform. Joist size determines span and spacing, so select the grade and type of wood best suited for your needs and budget.

In many building supply stores, the choice of grade and type is limited. You can order the grade and type you want, but it often is more pricey. Common in my area is pressure-treated SPF, cedar, and several brands of composite deck boards. The joists are usually not visible, so using expensive premium timbers isn’t necessary as long as the beam structure is properly placed.

Some equations and algorithms will calculate the span and spacing for joists and numerous joist span tables for reference too.

A quick rule of thumb for determining joist thickness for 12”o.c. and 16”o.c. spacing is half the span plus two. Round the span to the nearest foot and add two.

For example: half a 10-foot span is 5, add 2 is 7. So to span 10-feet use a 2×8. Another way for 16”o.c. is that a joist can span 1.5 times its thickness in feet – so, for a 2×8 calculate 1.5 x 8” = 12’ for the span. Regardless of the span, 2×6 joists are usually limited to ground-level decks without railings.

5. Calculate Deck Joist Span and Spacing

There are three ways to determine deck joist span and spacing – formula, calculator, and joist sizing and span tables. Regardless of the method used, span and spacing depend on the type and grade of the wood, cantilever, beam size, height off the ground, footing size, and live load values.

Formula

Some formulas take the bending, compression, and breaking values for wet or dry wood that are based on predetermined values for different species to determine safe loads over specified distances. The formulas have been used to calculate the spans and spacing on different tables in the Building Codes, and are the values used by different calculator algorithms.

There are so many variables and potential for error when calculating from scratch, that unless you’re a Structural Engineer tasked with the job, it’s better to use another method. My preference is to use span and spacing values readily available in Code tables or calculators.

Calculator

Span and spacing calculators, whether on-line or hand-held, use algorithms to identify the span based on wood type selection, joist size, and spacing. Most calculators allow for the selection of three or four common wood types or species, standard dimensional lumber from 2×6 to 2×12, and on-center spacing of 12”, 16”, or 24”. With the input of the three factors, the algorithm provides the maximum span. Check out the calculator below.

Joist Sizing and Span Tables

There are hundreds of Joist Sizing and Span Tables in print and on-line. Many reflect the information found in the IRC or National Building Codes. It is important to note that not all values in some tables are the same as what is in the IRC, so it is important to check the local Code for accurate current values for your area. Check out our table below which is based on the information on Table R507.5 of the 2018 IRC.

Chapter 5 of the 2018 ICR has a variety of tables that are full of useful information about minimum footing sizes (R507.3.1), deck post heights (R507.4), beam spans (R507.5), floor joist spans based on different dead loads (R502.3.1(1&2)), cantilevers (R502.3.3(1&2) & R507.6), and joist spacing for different decking (R507.7).

Most span and spacing tables present values for common dimensional lumber sizes and species. They often have 2×6, 2×8, 2×10, and 2×12. Although 2x4s may be used for low-profile joists near ground-level they aren’t normally included in structural joist tables. They bow too easily to span much distance and won’t support heavy loads.

Joists commonly are spaced at 12”, 16”, or 24” centers depending on the load the deck frame will carry and the size, type, and orientation of the deck boards. Spacing may be less, but shouldn’t be more. The maximum spacing for 5/4 wood and most composite decking is 12”o.c. when laid diagonally to joists, and 16”o.c. when perpendicular to joists. 2×6 deck boards can span up to 24”o.c. when perpendicular, and 16”o.c. on an angle.

The spacing helps to determine the span. Using SPF 2×8 joists at 24”o.c. the span maximum is 9’-1”. Downsizing to 16”o.c. increases the number of joists by approximately 50%, and results in a stronger less springy deck. There is an increase in cost due to more joists but it’s usually marginal in the overall cost.

Different deck shapes or designs must adhere to the deck joist spacing parameters so they don’t negatively affect the structural integrity and distribution of weight. Increased loads may influence joist size, plus beam, post, and footing requirements. The more elaborate the deck design, the greater the need for the input of a Structural Engineer.

Example: How Far Apart to Place Deck Joists for 12 x 12 Deck

To show how the joist spacing, span, and sizing interact, here are several examples based on a 12×12 deck. The design utilizes a double 2×12 beam with 6×6 posts spaced 6-feet apart, and ledger with joist hangers. Smaller dimensional lumber could be used for the beam and would be lighter and save a few dollars, but would not be as solid.

The distance of the beam from the ledger determines the deck joist span. Placing the beam so the outside face is 12’ from the structure or 10’-6” doesn’t affect the cost, but does impact the structural strength.

The table shows the maximum span permitted for 2×8 and 2×10 joists for 12”o.c. and 16”o.c. spacing. The last columns identify the spans for the beam at 12’ and 10’-6” with the cantilever.

*Values from Table R507.6 of the 2018 IRC

Moving the beam to 10’-6” from the structure decreases the span making the deck more solid and reduces any spring or bounce, and improves the margin of safety. It also permits the use of 2x8s at 16”o.c. The cost difference between 2x8s and 2x10s is approximately $7.00 each. Joists at 12”o.c. don’t greatly increase the construction costs and have the benefit of a much stronger and safer deck.

The span between beam supports (6’) multiplied by the joist span plus any cantilever (12’) identifies the “Tributary Area” (72sqft) of the deck which is used to determine the allowable beam post height and footing size. The live load also affects the footing size.

The greater the load, the larger the footing dimensions. Most residential decks are designed for loads of up to 10 people and a live load of 40psf, whereas commercial decks must meet live loads of 100psf or more.

Joist Span Calculator

A Joist Span calculator commonly identifies the maximum span and cantilever for dimensional lumber joists based on wood type, joist size, and spacing between joists. All you have to do is input the information, press calculate, and the maximum span is identified.

We recommend this joist span calculator, which has the following functions:

• Select the Lumber Species or Type – Southern Pine, Douglas Fir, Hemlock, SPF, Redwood, Cedar, etc.
• Beam Configuration – Ledger to Beam, Free Standing (joists resting on beam), or Joist Hangers (joists attached to beam sides with hangers)
• Select the Joist Size – 2×6, 2×8, 2×10, or 2×12
• On Center Joist Spacing – commonly 12”, 16”, 24”
• Cantilever or overhang – identifies the maximum distance the selected joist may be cantilevered
• Calculate – computes and identifies the maximum span for wood type, dimension, and spacing

Deck Joist Span Tables

Deck joist span tables commonly identify the maximum span for different wood types and joist spacing. There are Tables that also provide sizing for beams based on span, joist wood type, and grade, as well as beam size, support post spacing, and footing requirements.

The 2018 IRC is a good place to start as is your local Code which may adjust values or loads for regional conditions.

Values from Table R507.6 of 2018 IRC

Joist Spacing for Composite Decking

Deck joist spacing for composite decking is the same as that for 5/4” boards. Generally, the maximum joist spacing for boards running perpendicular to the joists is 16”o.c. The spacing reduces to 12”o.c. for boards laid at an angle to the joists.

Manufacturers of different composite decking products commonly include instructions with their products. Stringer spacing for stair treads is commonly 12”o.c., although some products are 16”, 11”, or 8”, so it is best to check the instructions. For commercial applications, joist spacing is 12”o.c. for perpendicular and 8”o.c. for diagonal placement.

Deck Joist Installation Options

There are several construction methods for installing deck joists. All are common methods and may affect the overall design and cost of the project. Joist installation doesn’t impact the spacing between joists, but it does affect the span.

Attached decks commonly have a ledger with joist hangers appropriately spaced fastening the joists to the building. The free end of the joist often either rests on the beam or cantilevers over it. The cantilever reduces the span but usually increases the overall length of the joist that may be used.

Free-standing decks don’t have a ledger. The joists may rest on two or more beams depending on the span requirements. The joists are fastened to the beams to maintain spacing and prevent movement.

The joists may be cantilevered at each end beam or terminate flush with the end beams. Joists that cantilever allow for the use of longer joists than those that terminate at the outside face of the beam.

Low profile free-standing decks or ledger supported decks fasten to the beams using joist hangers. The top of the joists are level with the top of the beams or ledger and beam, making for a thinner profile deck structure. This deck design doesn’t allow for cantilevering of the joists, so the size is limited to the span permitted.

Conclusion

The span of deck joists depends on the selection of dimensional lumber available in your area. The grade, wood type, spacing, thickness, and width determine the distance joists can safely bridge. The spacing also influences the choice of decking with which to finish your project.

I hope you have a better understanding of material selection for framing a deck and how Codes can be used to ensure the deck is structurally safe and sound. If you found this guide helpful, please share it with others. Your comments and suggestions are always appreciated.

Eugene Sokol

Eugene has been a DIY enthusiast for most of his life and loves being creative while inspiring creativity in others. He is passionately interested in home improvement, renovation and woodworking.