Building a deck, shed, or other structure can take a lot of design work, calculation, and research. Determining spans for beams, joists, and headers can be tricky and vary by species, grade, dimension, and loads, plus other factors. So, if you’re wondering how far can a 6×6 beam span, we’re here to help.
A common 6×6 softwood beam span ranges from 3’-5” to 8’-6” according to the International Residential Building Code (IRC) of 2021. Span depends on all the different variables that affect the wood’s strength. Hardwood of the same dimensions will span further, as will steel, but the cost difference may be significant.
In this article, we’ll explain how far a common 6×6 softwood beam or header can span, and its load capacity. We’ll also compare 6×6 beams with triple 2×6 beams, and provide a span chart for 6×6 beams. Our goal is to provide you with the information to make an informed decision about building materials.
How Far Can A 6X6 Beam Span Without Support?
The distance a 6×6 can span as a beam depends on the species of wood, grade, grain orientation, moisture content or seasoning, condition, and load. Southern Pine (SP) 6x6s will span 5” to 8” further than other common softwood species of the same grade. Douglas Fir-Larch (DF-L), Hemlock-Fir (H-F), and Spruce-Pine-Fir (S-P-F) span similar distances to Redwood, Cedar, Ponderosa Pine, or Red Pine.
A 6×6 is 5-1/2”x5-1/2”, so it can be rotated to align so the best grain direction surface is carrying the load. The greatest force can be applied end to end or parallel to the grain. Curved or wandering grain lines aren’t as strong, so span distances are based on the weakest possible orientation, not the strongest.
Knots, splits, cracks, and holes, regardless of the timber’s grade, affect the carrying strength. Wood that is stamped ‘clear’ doesn’t have those defects, and will be stronger than those that do. Orienting defects to the sides, not the top or bottom face improves the strength. If the defects interfere with grain orientation, select a different 6×6 for the task.
A 6×6, like any piece of lumber, can span greater distances than any Code allows. Since the load may be centered on the beam, carried towards the ends, or evenly distributed, wood is graded for the weakest possibility. You may have seen wood bend over time under nothing but its own weight and gravity, so load ratings are an important consideration for span distances.
The IRC-2021 identifies spans for triple 2×6 beams but not for 6x6s, which haven’t commonly been used for beams or headers for the past 50 years or more. Since the depth of a 6×6 and a triple 2×6 are the same, and the edge thickness is the main factor for compression, triple 2×6 span values are regularly used to determine 6×6 spans.
It is possible to calculate spans for 6x6s and get a more accurate and slightly longer span, however, it’s probably better to leave that to a Structural Engineer since most Building Inspectors will use the triple 2×6 span ratings unless stamped by an Engineer.
A #2 S-P 6×6 will span between 7’-0” and 8’-6” for joist spans of 6-feet, and 4’-2” to 4’-11” with joist spans of 18-feet depending on loads. Dead loads of 10 psf commonly reflect the weight of the structure and fixed materials, while the live or ground snow load can fluctuate from 40 psf to 70 psf. So, for design and building practices, use the span distances for triple 2×6 beams when using a 6×6, or leave the math to the pros.
6×6 Beam Load Capacity
A 6×6 beam or header must carry all the structural members above it, plus any loads applied to it. Combined dead and live loads of 40 psf to 100psf may not sound like much, but that takes into account what must be considered for structural integrity. Load capacity depends on species, grade, wet or dry use, tensile strengths, compressive stresses, horizontal shear, and Young’s modulus. If all that means little to you, you’re probably not an Engineer.
The compressive strength for common softwood 6x6s ranges from 500 psi wet to 1850 psi dry, although some species may be higher or lower. The load per linear food depends on the allowable bending stresses (Fb) for different wood species and conditions. Finding the allowable unit stress in extreme fiber bending (Fb) for different wood species isn’t easy, and often depends on the grade and wet or dry use.
At a 4-feet span, a #1-S-P-F 6×6 can support 1,040 pounds per linear foot (PLF) or 4,160lbs per span, and a DF-L D-SS can carry 2080 PLF or 8320lbs per span based on allowable bending stresses (Fb). Run that span up to 20-feet, and the values drop to between 41 PLF or 820lbs per span and 83 PLF 1660lbs per span respectively.
Use the ‘Allowable Bending Stress (Fb)’ table to find the Fb for the species and grade being used. Then reference the ‘6×6 Beam Load Per Linear Foot by Span’ table to identify pounds per linear foot for the desired span.
Multiplying that value will provide the allowable load distributed evenly over that span. However, the Building Code may differ due to structural issues and requirements and should take priority, or consult a Structural Engineer.
|6×6 Beam Load Per Linear Foot by Span
Base On Allowable Bending Stress (Fb) in psi
|Load Per Linear Foot
Information from “Wood Structural Design Data” 1986 Edition, with 1992 Revisions – 2004
Information from: Maximum Allowable Fiberstress in Bending Values (forestryforum.com)
How Far Can a 6×6 Header Span?
A headers’ location affects the loads it may carry. Exterior or interior, the width of the building, stud spacing, bearing or non-bearing, load expectations (snow and wind), plus what floor level it’s on also impact the span. Although a 6×6 is adequate to span 6-feet in some locations, it’s more common to use standard dimensional lumber. Additionally, a 6×6 would be too wide for 2×4 frame construction.
Although a 6×6 is wider than a double or triple 2×6 header, it is still the same depth. Thus, the compression values are similar. A 6×6 header can span from 2’-1” to 7’-6” depending on location, loads, and other factors. For more accurate span values, we recommend consulting a structural engineer.
6×6 Beam Span Chart
A 6×6 is only 5-1/2”x5-1/2”, so its depth is the same as a 2×6, although its width is about an inch or 22% wider than a triple 2×6 beam. However, it’s the depth that is important to prevent bending. The reality though, is that a 6×6 is only about 7% stronger than a triple 2×6, so the values in the IRC-2021 span tables are a reasonably accurate representation of how far a 6×6 can span under different conditions.
The ‘6×6 Beam Spans’ table reflects spans associated with #2-grade lumber for wet service (outdoor) use. The values are based upon 10psf dead loads and 40psf live loads, and also ground snow loads of 50 to 70psf.
|6×6 Deck Beam Spans|
|Species||Loads||Joist Span Length|
|Maximum Deck Beam
(feet – inches)
|Southern Pine||40 psf||8-6||7-5||6-8||6-1||5-8||5-3||4-11|
Data from: IRC-2021 Tables R507.5 (1-4)
Which Is Stronger 6×6 or 3 2×6
A 6×6 is 5-1/2”x5-1/2” while a tripled 2×6 is 4-1/2”x5-1/2” in actual dimensions. The 6×6 is an inch wider, or approximately 22% larger than the 3-2×6 beam. However, it is still the same depth or edge thickness, which is very important when calculating compression, bending, or deflection.
The greater width does affect the resistance to bending since the Section modulus (S) and Moment of Inertia (I) are also both 22% larger. However, since the tripled 2×6 is less likely to twist or warp than a 6×6 due to the grain layering, its fiber strength in bending (Fb) is multiplied by a factor of 1.15.
The 1.15 or 15% ‘Repetitive Member Factor’ (Cr) recognizes that the built-up beam, predictably, is stronger than timber of comparable dimensions, and is usually factored into span tables.
The bottom line then is that although a 6×6 beam is 22% larger than a triple 2×6 beam of the same length, the ‘Repetitive Member Factor’ of 15% decreases the difference to 7%. So, a 6×6 beam is about 7% stronger, or less likely to bend or deflect under the same loads as a 3-2×6 beam over the same spans. If you require exact specifications for a build, use span values for a tripled 2×6 beam or consult a Structural Engineer.
A 6×6 softwood beam may be able to span 20’ with limited loads. However, for structural building and compliance with Building Codes, the span ranges from 3’-5” to 8’-6” depending on a variety of factors. When in doubt, check with the experts. Hopefully, we’ve provided you with the information to make an informed decision for your building project.