# Deck Joist Span Explained

Building a deck starts with footings, posts, beams, and a ledger, or just beams resting on the footings or deck blocks. The placement of the footings and beams is interconnected and dependent upon the span of the deck joists. If you’re wondering how the deck joist span works, we’re here to help!

Deck joists are usually parallel lengths of dimensional lumber that span the gap between two load-bearing beams or a ledger and beam. They form the framework to which deck boards are fastened and help distribute the loads through the beams or ledger to the ground. Their span depends upon the wood species, grade, dimensions, loads, and other factors.

In this guide, we’ll explain what a deck joist span is, the factors that influence it, and if it includes the cantilever. We provide a deck joist chart and discuss deck joist span calculators. Plus, we’ll identify the maximum span for 2×6 and 2×8 deck joists, and how to determine the number of deck joists required. Our aim is to provide you with helpful information for your deck project.

## What Is a Joist Span on a Deck?

The deck joist span is the distance between two bearing points that support and carry the joists. They may be two beams or a beam and a ledger board. The joists may rest on top of the beams or attach to their face with joist hangers, which is how they are typically attached to the ledger board.

The greater the load-bearing capacity of the joist and support members, the further it can span. Most deck joists are required to support a minimum live load of 40psf (pounds per square foot), although some must be able to support snow loads up to 90psf or more.

## Factors Influencing Joist Span

The distance a joist can span helps determine the location and dimensions of the support beams, posts, and footings. It also can affect the shape and size of a deck, or be affected by the desired shape and size. However, the distance a joist can span is influenced by specific factors that have been scientifically tested in the interest of safety. The following six factors influence the distance a joist can span.

### Joist Size/Dimension

The width and depth or thickness of the lumber used for the joist affect the distance it can span. Most joists are chosen from dimensional lumber stock that is nominally 2 inches wide and 4, 6, 8, 10, or 12 inches thick.

Sistering or doubling joists doubles their width and can increase their span by approximately 25%, and doubling their depth from 4” to 8” or 6” to 12” can increase the span by 75 to 100%.

### Wood Species

The wood species of lumber used also affect the span. Species that grow slower have more growth rings per inch, making them denser and stronger. Softwoods typically used for deck joists are Douglas fir (DF) or Douglas fir-larch (DF-L), hemlock-fir (H-F), Southern pine (SP) or Southern yellow pine (SYP), and Spruce-Pine-Fir (SPF).

Southern pine or Southern yellow pine will typically span further when used as a deck joist than the other species above. Lumber is also graded with select structural (SS) spanning further than #1 or #2 grades, plus the span for deck joists is also reduced for wet service – wet wood isn’t as strong as drier wood.

### Joist Spacing

The spacing between joists affects the distribution of loads, which also affects the joist span. Joists are commonly spaced 16” apart measured from the center of one joist to the center of the adjacent joists, or ‘on center’. Joists are also spaced at 12”oc and 24”oc, and sometimes even 8”oc.

The closer together the joists, the further they can span. Additionally, joist spacing is also affected by the dimension, type, and angle of intersection with deck boards. Many composite and 5/4” deck boards can only span 16” if perpendicular to the joists, while 2×4 and 2×6 decking can bridge joists spaced at 24” centers. However, decking traversing joists at 30°, 45°, or 60° require narrower spacing between the joists, which can also increase the joist span.

The amount of weight or load the joists need to support and transfer to load-bearing members greatly influences the joist span. The dead load is comprised of the weight of the building materials and usually averages 10psf but can be 15 or 20psf depending on structural requirements.

The live load accounts for people, furniture, planters, BBQ, and other movables, and commonly is 40psf. Ground snow load is a seasonal load that is often 40psf but can be 90psf or higher in some regions due to snow accumulations or wind forces.

The joist span commonly needs to support a combined live and dead load of 50psf. Adding a hot tub or spa, though, can greatly increase the load requirements too.

### Joist-to-Beam Connection

Joist-to-beam connections affect the distribution of loads and can affect the joist span. Decks may be attached to a house or other structure or freestanding. Attached decks have a ledger board that acts as a support beam fastened to the building, while freestanding decks have two or more beams.

Joists typically are attached to the face of the ledger board using individual joist hangers of appropriate size. At the beam or freestanding end(s), the joists can butt into the face of the beam(s) and be connected to it/them with appropriate joist hangers.

Alternatively, the joists can rest atop the freestanding beam(s) with spacing blocks inserted to prevent twisting or movement. The blocking can be used to fasten joists to the beams or hurricane straps or brackets can be used to secure individual joists to the beams and counter uplift forces. Joists that rest atop the beams are commonly cantilevered beyond the beam to increase the deck area.

### Local Building Codes

Local Building Codes are often based on State or Provincial Building Codes, which in turn are based on National Codes. National Codes, and thus all others, are developed using the International Residential Building Code (IRC) and information provided by other interested partners who carry out stringent testing.

The IRC is updated every 3 years and the last update was released in 2021. Section R507 of the 2021 IRC addresses exterior deck construction, and the current updates are also reflected in Table R507.6 of UpCodes.

Unfortunately, many small communities or jurisdictions don’t have the resources to keep their local Building Codes up to date. As a result, local Codes may be based on the 2015 or 2012 IRC editions, which in some aspects, makes them obsolete. The current IRC identifies significant changes in their deck joist span Tables (R507.6) compared to previous editions.

Additionally, many building codes use a maximum deck joist span based on #2 or better (#2BTR). This means Table or Chart values will only show the maximum span for #2 lumber. A certified Structural Engineer could provide the specs for SS or #1 which could increase the joist span, but that increase might not be worth it when cost is factored in. Building codes set the minimum requirements to ensure personal and structural safety.

## Deck Joist Span Chart

Deck joist span charts or tables identify the maximum distance a joist of dimensional lumber can span to ensure structural integrity and safety. The charts typically identify the wood species, dimensions, spacing between joists, span, and loads. Other information such as wood grade, deflection (L/Δ), wet or dry service, incised or not, and snow load may be identified in the chart or in its footnotes.

Deck joists are exposed to moisture, so are considered wet service. That means their maximum span is less than that of an interior floor joist which is considered dry service. The Chart below identifies the maximum span for #2 graded Southern Pine with a live load of 40psf and a dead load of 10psf.

 Maximum Joist Span #2 Southern Pine (10psf dead load, 40psf live load, L/360) Lumber Size Joist Spacing 12” OC Joist Spacing 16” OC Joist Spacing 24” OC 2×6 9’-11” 9’-0” 7’-7” 2×8 13’-1” 11’-10” 9’-8” 2×10 16’-2” 14’-0” 11’-5” 2×12 18’-0” 16’-6” 13’-6”

## Deck Joist Span Calculator

There are numerous online deck joist calculators that can assist in determining the maximum joist span. Most require the selection of wood species, grade, dimensions, use, deflection, joist spacing, loads, and service conditions. You input or select the values, and click on ‘calculate’.

The program does the rest and provides you with the maximum span based on the information provided. The American Wood Council (AWC) has a calculator that is very helpful. It even allows you to compare species and grades for the best option.

## Does Joist Span Include Cantilever?

The joist span is the distance a joist can span between load-bearing structural supports based on the type of wood, its width and depth, grade, loads, and other factors. Cantilever is the distance that joists can extend beyond those structural supports. It is usually calculated as 1/4 of the joist’s back span.

For example, a 2×12 #2 Southern pine spaced at 16”oc can span 16’-6”, so it can have a maximum cantilever of 4’-0”. Table R507.6 of the 2021 IRC and UpCodes identify both the maximum deck joist span and the maximum cantilever based on a variety of factors.

## What Is the Maximum Span for 2×8 Deck Joists?

The maximum span of a 2×8 deck joist depends on wood species, grade, spacing, and loads, as well as other factors. As the Table above identifies, a #2 Southern pine with an overall load of 50psf will span 13’-1” if spaced 12”oc, 11’-10” at 16”oc, and 9’-8” for 24”oc.

Using the AWC joist calculator and selecting Dense Select Structural Southern pine or Douglas fir-larch (north) under the same conditions, the span increases to 14’-5”, 13’-2”, and 11’-6” respectively. Increasing the overall load due to heavier snow load expectations will shorten the maximum spans for all wood species and grades, as will an increase in the deflection limit. Always check spans and other structural values with a qualified professional.

## How Many Joists Do I Need for My Deck?

The number of joists typically depends on the length and width of the deck structure, and the spacing between the joists. A rectangular or square deck with only two load-bearing points is usually spanned by single joists without sistering or doubling. This makes it easier to identify the required number of joists. The length of the two beams or ledger and beam typically identify the ‘length’ of the deck.

Determining the number of joists for that ‘length’ depends upon joist spacing – 12”oc spacing will require more joists than 16” or 24”oc spacing. An easy way is to use a joist count formula such as

# of joists = { [ (beam or deck length x 12”) ÷ spacing] +1 more for the outer edge}

For example, a 16’ long beam or deck with joists spaced at 12” will require

{ [ (16’ x 12”) ÷ 12”] +1} = 17 of joists

The same deck with joists spaced at 16” will require

{ [ (16’ x 12”) ÷ 16”] +1} = 13 of joists

The same deck with joists spaced at 24” will require

{ [ (16’ x 12”) ÷ 24”] +1} = 9 of joists

## How Far Can a 2×6 Deck Joist Span?

The distance a 2×6 deck joist can span depends on wood species, grade, spacing, loads, plus other factors. The Table below identifies the maximum span for four common softwood species and grades of deck joists experiencing a combined load of 50psf, a deflection of L/360, and a wet service factor as well.

 Maximum 2×6 Joist Span (10psf dead load, 40psf live load, L/360) Wood Species Grade Joist Spacing 12”oc 16”oc 24”oc Southern Pine SS 10’-9” 9’-9” 8’-7” #1 10’-4” 9’-5” 8’-2” #2 9’-11” 9’-0” 7’-7” Douglas Fir (North) SS 11’-0” 10’-0” 8’-8” #1 10’-4” 9’-5” 8’-0” #2 10’-4” 9’-5” 8’-0” Hemlock-Fir (North) SS 10’-7” 9’-7” 8’-5” #1 10’-4” 9’-5” 8’-0” #2 10’-4” 9’-5” 8’-0” Spruce-Pine-Fir SS 10’-2” 9’-2” 8’-0” #1 9’-11” 9’-0” 7’-10” #2 9’-11” 9’-0” 7’-10”