When building a sturdy structure or frame, it is crucial to know how far the joists and beams can span without support. It is also good to know what factors may affect the span of a 2×8. But first, generally, how far can a 2×8 span without support?
Depending on various factors, the maximum span for a 2×8 floor joist is 16’ 6”. The maximum span for roof rafters is 23’ 9”. For headers, the maximum span is 11’ 2”. Finally, a single deck beam has a maximum span of 5’ 11” and double maxes out at 8’ 9”.
However, the maximum is determined by the species and grade of wood, the spacing, the load, and more. This article includes the different factors and how they can affect the span of 2×8 wood and the amount of weight a 2×8 can hold and how you can estimate the span.
Contents (Jump to Topic)
What Is Span in Building Construction?
Span is a term for the linear distance that a joist, rafter, or board can cover without bending or breaking without support. Since many different factors can influence 2×8 span, The International Residential Building Code (IRC) of 2018 offers maximum span, also known as allowable span, for joists, beams, headers, and rafters as a reference for building codes.
The allowable span is the distance that a structural component can extend without needing support. To measure span, you should start at the center of one support point to the center of the next support point. These points could be a wall, ridge, beam, or any other structural support.
Knowing the allowable span will help you determine where the lumbar requires support for stability. The type of structural component, including joist, beam, and rafter, will determine where support should be placed.
What Factors Determine Span?
While there are maximum allowable spans, they are determined by various factors. Each factor can affect the span and should be taken into consideration for optimal support.
Species of Wood
The tree species will affect the strength, flexing, and other features that play a role in support and span. Popular structural woods include Southern Pine, Douglas Fir, Hem-Fir, Redwood, and Spruce-Pine-Fir (SPF).
Generally, Douglas Fir is very strong and will span farther than SPF and Southern Pine if the other factors are identical. Southern Pine is stronger with a longer span than SPF, but SPF is popular because it is strong for its weight.
The Depth of Structural Element
The depth of the joist, beam, or other structural element is crucial for the allowable span. The rule of thumb is to multiply that depth by 1.5 and change the amount to feet to determine the span. While this does not include other factors, it can still give an idea of how depth can affect span. Using this rule, a 2×6 will span 9 feet, a 2×8 will span 12 feet, and a 2×10 will span 15 feet. As you can see, the larger the depth, the longer the span.
Grade
Lumber is graded for quality. The unique grade will be based on imperfections like knots as well as overall performance. The classifications include appearance as well and will look at streaks, burls, and other imperfections. The performance is based on stress grades for structural use and these grades are standard.
Grade #1 is the strongest and typically utilized for construction projects, while #2 is also commonly used for support. The lowest grade is #4 and is not a good grade for any structural systems.
Spacing
Spacing is the area between joists or rafters and is important to accurately identify the area that the structural element has to support. Spacing between parallel wood components is usually 12”, 16”, or 24”. Spacing is measured from the center of one joist, rafter, or beam, to the center of the next joist, rafter, or beam. The longer the spacing, the shorter the allowable span.
Use
The use refers to the way a 2×8 is positioned and if it is working as a beam, floor joist, ceiling joist, rafter, or header. In addition, it also concerns whether you use a single beam or if you choose to double or triple the beam.
Load
The load refers to the amount that the 2×8 or other structural component can carry before the integrity is compromised. This means that joists and rafters have to support the load of all structural materials and any other weight. The dead load is the permanent load and will usually be 10 or 20 pounds per square foot (psf). Non-living areas often have lower load ratings as well.
The live load is the temporary load and accounts for furniture, people, pets, and anything else that is not permanent. Live load has a higher value than dead load, usually around 30 or 40 psf. A load also has to account for snow, ice, and other factors that can add weight to the rafters or joists. Therefore, in colder areas, the live load must be increased up to 70 psf.
Another consideration is the duration of load, which is the amount of time that the joists or beams must support the load. Full-time loading is the standard that can be multiplied to determine the load. For snow-load, multiply the full-time loading by 1.15 and for 7-day load, multiply it by 1.25. For example, a 2×8 with a full-time load of 1795 Fb would have an approximate snow-load of 2065 Fb and a 7-day load of 2244 Fb.
The combination of joist size and spacing also determines the strength value, Fb, and the stiffness value, E. For example, a 2×8 floor joist made out of southern pine has a 2650 Fb and the same 2×8 will have a higher Fb, 3040 Fb, when utilized for roof rafters. Duration of load will not change the E value.
How Far Can a 2×8 Span Without Support?
So, with all these factors, it is best to start with the use. It can make a huge difference with the same 2×8 component when used for a roof rafter versus using it as a floor joist. If you are unsure about the accurate allowable span, you should check the IRC 2018 or consult with a professional.
Roof Rafter
Rafter span lengths require measurement from the exterior face of the supporting wall sheathing to the center of the ridge beam or board. A 2×8 roof rafter, like any other structural member, will have a span based on various factors.
One thing to keep in mind is that roof rafters have a slope. This causes the load to apply pressure differently than it does with joists or beams. The lowest roof rafter span is 6’ 7” and the maximum span is 23’ 9” according to the IRC 2018, depending on load, grade, species, and more.
Floor or Deck Joists
The span for a 2×8 floor joist can range from 7’ 1” to 16’ 6”. In addition, floor and deck joists have to support a fairly high live load because they provide support underneath living areas. You may find that a 2×8 does not offer the necessary support on its own. However, you can double the floor or deck joists to increase the span.
Doubling 2×8 floor joists can extend the span around 25 percent with other factors identical. As an example, this means that a single floor joist with a span of 9’ would have a span of 11’ 2” when doubled. You can even triple the 2×8 floor joists for an even greater span.
Ceiling
Ceiling joints have different spans depending on the location. For an uninhibited area with limited storage, the span for ceiling joints will range from 10’ to 21’ 7”. For an inhabited area without any storage needs, the span ranges from 14’ 2” to more than 26’.
Header
The header span will depend greatly on whether it is only supporting the roof and ceiling or if it has to support a center-bearing floor or clear span floor as well. For only roof and ceiling headers, the single header span range is 2’ 10” to 4’ 6” with a double header ranging from 4’ 1” to 6’ 10”
If the header has to support the roof, ceiling, and a center-bearing floor, a single header span falls between 2’ 8” and 3’ 11”. For supporting the roof and ceiling plus a clear span floor, the range starts at 2’7” and goes up to 3’ 6” for a single header and 3’ 6” to 5’ for a double header.
Beam
According to IRC 2018, single deck beams can range from as little as 3’ 5” and as much as 5’ 11”. When doubled, the range increases to 5’ to 8’ 9” and when tripled, it increases from 3’ 8” to 10’ 10”. Southern Pine has a higher span than other popular types of wood. Still, another option is laminated veneer lumber, LVL, which has a span provided by the manufacturer that generally lands between 12’ and 15’.
Pergola
The IRC 2018 does not specify a maximum span for pergola beams. Because of this, it is best to go by the standard rule and multiply the depth by 1.5. This would give you 12 feet. You may be able to get away with another 1 to 2 feet depending on the species and grade of the wood but should avoid going past that.
How Far Can a Double 2×8 Span Without Support?
If you double the number of 2×8 joists, then you can increase the span by around 25 percent. It will also increase by around 25 percent if you double the thickness to 4×8. However, if you double the width, then the span can increase significantly.
For example, a single deck beam can span a maximum of 5’ 11”, but when you double the depth, it will increase the span to 8’ 9”. That is an over 50 percent increase in span. For additional information, refer to the IRC 2018.
How Much Weight Can a 2×8 Hold?
A 2×8 will adequately support a dead load of 20 psf and a live load of 40 psf depending on some factors, including allowable span. This equates to a total load of 60 psf. Therefore, for a 2x8x10, the total load would equal 600 pounds per lineal foot.
It is important to note that the amount of weight a 2×8 will hold is complicated and depends on various unique factors. If you are concerned about the amount of weight it will support, it is best to talk to an expert about your unique situation.
How Far Can You Cantilever a 2×8 Joist?
In general, cantilevers should not extend beyond ¼ of the span of the joists themselves. Take the actual span, not the maximum span, and divide that number by four. That will be the distance that you can cantilever the joist.
For example, for a 2×8 with a span of 12ft, you would divide 12 by 4. This gives you 3. Therefore, the maximum cantilever distance should be 3 feet.
2×8 Span Chart
This chart will give you an idea about how different species, uses, and spacing combine to determine the span. This chart uses a 20 psf dead load and a 30 psf live load for calculations. It also offers only span for grade #2 wood.
| Maximum Span | |||
| Species /
Spacing (inches on center) |
Floor Joist | Deck Joist | Roof Rafters (Snow-Load) |
| Southern Pine | |||
| 12 | 15’ 10” | 13’ 1” | 15’ 10” |
| 16 | 14’ 5” | 11’ 10” | 14’ 5” |
| 24 | 11’ 11” | 9’ 8” | 12’ 7” |
| Douglas fir | |||
| 12 | 14’ 9” | 12’ 6” | 15’ 7” |
| 16 | 12’ 9” | 11’ 1” | 13’ 8” |
| 24 | 10’ 5” | 9’ 1” | 11’ 2” |
| Redwood, Western Cedars | |||
| 12 | 14’ 11” | 11’ 8” | 14’ 2” |
| 16 | 13’ 6” | 10’ 7” | 12’ 10” |
| 24 | 11’ 10” | 8’ 8” | 11’ 3” |
Conclusion
Knowing the allowable span as well as the factors that can affect it is crucial to building a sturdy structure. After selecting the species and the use, you also need to take the load, grade, and spacing into account to determine how far the 2×8 will span without additional support.