Are you trying to determine what dimensional lumber is best for your project? Is it better to use a 6×6 or three 2x6s? If you’re wondering how much weight can a 6×6 hold, we’re here to help!
An 8-foot (interior north) Douglas fir beam will support almost 1180 pounds evenly spaced or a center load of 4740 pounds. Oriented as a post, the same species and grade will support 34,461 pounds as a 2-foot post and 33,371 pounds if 4 feet high. The grade and species, along with numerous other factors, affect the amount of weight a 6×6 can hold.
In this guide, we’ll identify how much weight a 6×6 can support horizontally and vertically, and what factors affect its load-bearing capacity. We’ll also discuss the use of a 6×6 as a beam and what’s the strongest wood for a 6×6. Plus, we’ll look at which is stronger, a 6×6 or a triple 2×6. Our aim is to provide you with the best information to assist you in your project.
How Much Weight Can a 6×6 Hold?
The amount of weight a 6×6 can support depends on numerous factors. Wood species and grade determine strength, as do the grain or ring pattern and load variables. Using a 6×6 in a vertical or horizontal orientation and its length or span also affects how much weight it can hold.
Moisture, density, wet or dry use, incised or not, and the condition of the timber also need to be considered when determining loads on 6x6s or any wood for that matter. It should be noted too, that many building codes reduce overall load capacity by at least 30% as a safety factor.
Common lumber yard 6x6s are typically Douglas Fir (DF), fir-larch, or hem-fir which are the strongest softwoods commonly used. Spruce and Southern pine (SP) are close seconds and more common in certain regions. Spruce-pine-fir (SPF) and cedar are also used but aren’t as strong as the other softwoods.
Oak and maple are stronger and were commonly used in past centuries, but are much more expensive, so not often seen in new builds. The best practice is to check all numbers with a qualified professional.
A dimensional 6×6 is actually 5-1/2” by 5-1/2” and has a cross-sectional area of 30.25 in², which is almost 2-1/2 times that of a 4×4 (3-1/2”x3-1/2”). So, a 6×6 is significantly stronger than a 4×4 in comparable situations.
Optimally, a 6×6 DF (interior north) spanning 8 feet will support a center load of almost 4740 pounds and about 1180 pounds per foot if the load is uniform. Using DF-SS as a 2-foot post it has an axial compression load capacity of 34,461 pounds, at 4 feet, the load capacity decreases to 33,371 pounds. So, different grades and softwood species support different weights.
How Much Weight Can a 6×6 Support Horizontally?
The amount of weight a 6×6 can support horizontally depends on wood species, grade, moisture content, ring or grain pattern, location of use, and loads, as well as other factors. Using a 6×6 for a beam, header, or lintel is less common today than in past centuries. It is also typically used with 2×6 or larger framing which makes it easier to enclose in a wall.
The timber used for 6×6 beams is commonly #1 or #2 grade, although Structurally Select (SS) is often used in more visible locations. Douglas fir and Southern pine are considered the strongest commonly available domestic softwoods and are used in construction to span greater distances or carry greater loads.
SPF and cedar have similar strength to each other but aren’t nearly as strong as DF or SP. Before building, though, always check designs and values with a qualified pro.
An 8-foot #2 or better interior northern DF or SP can support a uniform load of 1184 pounds per linear foot (PLF) if supported at the ends. Both species and grade will also support a center load of almost 4740 pounds.
A similar length and grade of Black or Red spruce can support about 1065 PLF and a center load of 4262 pounds. Using less expensive SPF the load drops to 814 PLF and a center load of about 3255 pounds.
How Much Weight Can a 6×6 Wood Post Support Vertically?
The amount of weight a 6×6 post can support depends on species, grade, incised or not, where it was cut from on the log, bracing, and base or footing. Incised lumber isn’t as strong as non-incised lumber, nor is edge cut as strong as boxed heartwood.
Bracing will also prevent buckling and a proper base will support more weight without tipping or breaking. 6x6s are commonly used to support deck, floor, or roof beams, or for framing fences or outbuildings.
They help to transfer loads from larger surfaces or tributary areas to the ground. The shorter the post, the less likely it is to bend, so the greater load it can support.
The grade and species, along with the load height, determine the axial compression load capacity of the 6×6. An SS-DF has a load capacity of 34,361 pounds at 2 feet, while a #1 DF is 30,004 pounds, and a #2 is only 21,027 pounds.
Using an 8-foot post those values change to 27,387 for an SS, 24,841 for a #1, and 18,915 pounds for a #2. Switching to Hemlock-Fir and a 2-foot SS post can support 29,205 pounds, a #1 25,494 pounds, and a #2 21,027.
Going to an 8-foot post the SS supports 22,883, the #1 20,849, and the #2 14,906 pounds. We always advise checking all calculations with a certified expert before building.
What Does Affect the Load Bearing Capacity of a 6×6?
Wood species and grade affect load bearing capacity of 6x6s or any dimensional lumber. The straighter the grain the greater its strength. Douglas fir has a very straight grain, which is one reason for its strength and why it is commonly used for 6x6s.
The growth or ring pattern also helps determine load-bearing strength. A circular grain pattern comes from the center of the tree and is stronger than a curved grain pattern from the edge of the tree. Additionally, the closer the growth rings are, the denser and stronger the wood.
Green or wet wood is not as strong as dry wood, and it’s also heavier, the moisture content as a percentage impacts its bearing strength. The orientation of the timber also affects load capacity. Used vertically as a post, the 6×6 is significantly stronger than in a horizontal orientation as a beam.
Lumber typically has a greater axial compression load capacity than its bending or shear limits. The longer the span between supports the less it can support as a beam, while the shorter it is as a post, the more it can support, so length and span impact bearing capacity. However, a longer post with appropriate bracing has greater capacity than a similar length without bracing.
How and where the 6×6 is used affects its load capacity too. Used as a header or beam in interior construction it will usually support more than if used in exterior structural locations. Distributing loads uniformly or as a center load also impacts load capacity.
Other factors that need to be considered are load variables such as live, dead, wind, and snow loads, as well as tributary area, plus the spacing and span of joists. With so many aspects to consider, it’s best to seek assistance from a qualified professional.
Can I Use a 6×6 as a Beam?
A 6×6 can be used as a beam but most Engineers or builders will utilize a beam of double or triple laminated dimensional lumber instead. The wood species and grade, as well as other factors, determine load-bearing capacity and span. So, depending on species, grade, loads, and other factors, an 8-foot #2 dimensional 6×6 can support between 814 and 1184 pounds per linear foot depending on softwood species.
Strongest Wood for 6×6
The wood species, grade, specific gravity, modulus of elasticity, moisture content, number of knots, splits, and other defects all affect strength. It’s also important to check the end rings too as the slope of the grain affects strength. Heartwood is common in veneer peelers and isn’t as strong as boxed heartwood which includes latewood. Latewood is denser and has more rings per inch.
Another practice is to look for straighter growth rings for beams and to orient them perpendicular to the direction of force. Lumber grading uses the slope of the growth rings when identifying different grades, with SS and #1 having the least slope across the longitudinal axis. Too much slope or curvature can cause separation within the lumber when under stress, leading to structural failure.
The strongest softwoods for 6x6s are Select Structural or #1 graded Douglas fir or Southern pine. White oak and hard maples were common species used for beams and posts in the past and are both stronger than comparable dimensional softwoods. However, availability and cost make the use of hardwoods in regular construction very unusual.
Which Is Stronger 6×6 or 3 2×6?
A 6×6 is actually 5-1/2” by 5-1/2” and 3 2x6s are 4-1/2” by 5-1/2”, so the 6×6 is about 22% wider. Presuming the same wood species and grade, the 6×6 beam’s Section modulus (S) and Moment of Inertia (I) will both be 22% greater, making it stronger.
Most strength calculations for built-up beams, however, multiply the fiber strength in bending (Fb) by a 1.15 (115%) Repetitive Member Factor (Cr). The repetitive factor represents the greater bending strength potential of the built-up beam since defects are more visible and don’t transfer across the beam as they could in a solid 6×6. Thus, the 22% greater width is countered by the 15% repetitive factor, which means the 6×6 will have less deflection and be stronger than the triple 2×6 by about 7%.
From a builder’s perspective, though, a 6×6 could have hidden defects. Additionally, the possibility to use a double 2×8 beam which is almost 70% stronger than a triple 2×6 beam of the same grade and wood species, is more than tempting. This makes sense if we’re talking beams or headers, but if we’re using it as a post, the whole picture changes.
Using a 6×6 for a post may be more aesthetically pleasing and mean less risk of moisture or insects getting between the 2x6s and causing rot or other damage. However, a triple 2×6 post of #2 DF-L, whether short or long and fully braced, will support a hefty 36,750 pounds, while a 6×6 of the same grade and species will support 17,995 pounds. So, if wrapping the triple 2×6 post or enclosing it within other construction, the choice is easy.