How Much Weight Can a 4X4 Hold? [Vertically and Horizontally]

Are you spending time staring at racks of lumber and their prices and trying to figure out which will work best for your project? Different wood species, grades, and dimensions of lumber are rated for varying loads and span, so determining which will work best can be difficult. If you’re wondering how much weight can a 4×4 hold, we’re here to help.

A 4×4 horizontally spanning 8-feet can typically carry 500lbs safely in the center, and 1,000lbs spread evenly across the span before sagging excessively. Properly supported at 16” centers, and it can support up to 4,000lbs. Used vertically as a post, an 8’ 4×4 can support about 6,000 PSI before buckling, and even more if correctly braced.

In this article, we’ll identify how much weight a 4×4 can hold horizontally and vertically, and what can affect its load-bearing capacity. We’ll also look at different uses for 4x4s, how far they can span before sagging, which species is strongest, and whether a 4×4 is stronger than 2-2x4s. Our goal is to provide the information to make the best lumber choice for your project.

How Much Weight Can A 4X4 Hold?

The amount of weight a 4×4 can hold depends on its orientation, wood species, grade, length, height or span, moisture content, grain orientation, condition, and the type and location of the load. When we look at 4x4s, we’re typically looking at softwoods like Douglas Fir-Larch (DF-L), Southern Pine (SP), Hemlock-Fir (H-F), Spruce-Pine-Fir (S-P-F), Cedar, Redwood, Red Pine, or Ponderosa Pine. The species common to the region tend to be more readily available, and sometimes even less expensive.

A 4×4 is actually 3-1/2” by 3-1/2” once it has dried and been planed down. Some have squared edges and others have rounded, which marginally will affect the use and bearing. If one surface will be exposed, a 4×4 has 4 faces the same size to choose from, so hopefully at least one looks good.

Where the 4×4 is cut from on the tree affects its bearing strength too. 4x4s cut from heartwood are typically weaker than those cut from sapwood, so check the end grain of the 4×4 for growth rings before purchasing, or buy a 10-foot 4×4 and cut it to length.

Many 8-foot long 4x4s are the byproduct of the wood veneer or plywood industries. They are the cores of peeler logs, so are essentially heartwood. That means they are more resistant to fungi, shrinkage, and warping than sapwood, but also don’t accept pressure treatment well. They typically work well for fence and railing posts or garden ties but aren’t the best for structural use.

Depending on all factors, a horizontally placed 8-foot 4×4 supported at each end will safely support about 500 pounds at its center before bending excessively, and around 1,000 pounds if the weight is evenly distributed along its length. However, if it is properly supported every 16”, then the maximum weight is around 4,000 pounds, which is also commonly the weight at which it breaks or shears. The same 4×4 oriented vertically for compression will support around 6,000 PSI before failing or buckling, although, if properly braced, it could support significantly more weight.

How Much Weight Can a 4×4 Support Horizontally?

The amount of weight a 4×4 can handle is not only a building concern but also a safety one too. Using a 4×4 horizontally as a header, beam, top-plate, or another purpose isn’t as common today as it once was. Depending on where they are cut from on a saw log often determines how much they will twist, warp, and bend, which can cause structural, safety, and finishing issues.

The species of wood, its grade, number of knots and their location, condition, and span also affect the bearing strength. Southern Pine and Douglas Fir are common in the construction industry and stronger than most other softwoods.

Number 1 and 2 grade 4x4s are used for construction and load-bearing, while #3 isn’t meant to carry as much. Check the end grain to determine if it is cut from the center of the log or edge, as the center cut isn’t as strong. Also, check for twists, crowning, warping, and bowing too, as well as the number of knots and their locations – the fewer the better.

Typically, an 8-foot 4×4 supported at both ends will safely support about 500-pounds in the center. Spreading the weight out along the same 4×4 will increase the load limit to around 1,000-pounds. Properly supporting the 4×4 along its length will increase the load capacity to approximately 4,000-pounds as the span is significantly reduced. However, when in doubt, check with a Structural Engineer or your local building department.

How Much Weight Can a 4×4 Wood Post Support Vertically?

The amount of weight a 4×4 post can support vertically depends on a variety of factors such as species and grade. Additionally, the type of base or footing it sits upon, number and placement of supports, plus load parameters all influence weight support limits. Incised lumber won’t support as much as non-incised lumber, nor will center cut-versus edge cut.

A 4×4 is actually 3-1/2” by 3-1/2”, which does impact the load parameters, however, building codes and the industry are rated for the true dimensions. A post typically supports a floor or deck beam grid, roof, railing, or fence. The area it specifically carries is known as the tributary area. The shorter the post length, the more it can support. Longer posts require bracing to prevent bending, otherwise, they can bend or snap like a toothpick compressed from both ends.

Based on species, grade, load height, and whether incised or not, a 4×4 has an axial compression load capacity ranging from 8,071lbs to 23,098lbs at a 2-feet length, and 3,168lbs and 5,598lbs at 10-feet. However, according to the International Residential Building Code (IRC-2018), a 4×4 deck post can be used to a maximum of 6’-9” in height. It also needs to be properly supported and with appropriately sized and spaced beams and joists for a combined live and dead load of 50psf.

What Does Affect the Load Bearing Capacity of a 4×4?

The species, grade, moisture content, condition, number of knots and their location, and span or height without support or bracing all affect the load-bearing capacity of a 4×4. Freeze-thaw actions are also forces that need to be counted too. The depth, width, and spacing of footings affect the load capacity, especially in regards to how much weight can be supported on cohesive and non-cohesive soils.

Where the 4×4 is cut from on the saw log also impacts its strength. Many 8-foot lengths are cores, a byproduct of the veneer and plywood industry, which means they are heartwood or the center pith and not as strong as those cut from the outer part of the log. Incising improves the depth pressure treatment penetrates, but also diminishes the overall load-bearing capacity, so is another factor to keep in mind.

Static and dynamic loads are other considerations that need to be factored into the load capacity. The weight of building materials and stationary furnishings that need to be supported have to be included.

As do seismic motion, wind force, and movements and vibrations from sources such as machines and humans, or live loads. While all these factors affect the load-bearing capacity of 4x4s, they also affect that of any other dimensional lumber for that matter. When in doubt, check with your local building department or a Structural Engineer.

Can I Use a 4×4 as a Beam?

A beam is a horizontal structural element used to support or carry vertical, shear, and sometimes horizontal loads. Beams typically must resist transverse or lateral forces against their axis, in other words, bending forces. While beams today are made of reinforced concrete, metal, wood, and other materials, they, historically, were made of wood.

The species of wood, its grade, and its dimensions were determining factors in how far a beam could span and how much it could support. Beams in most old homes and barns are huge in comparison to most in modern construction. Oak and other hardwoods were prized for use as beams, but lighter and more abundant woods like pines, firs, spruce, and other coniferous trees were also used.

Today, most wooden 4x4s used in construction are softwoods, so their strength, depending on species, grade, and other factors, determines their load capacity and how appropriate they would be for use as a beam. How and where they are to be used, and what loads they must carry determines their span, which in most cases isn’t very great.

A 4×4 today is typically 3-1/2” by 3-1/2”, which gives it a depth of 3-1/2”, and a cross-sectional modulus of almost 5” (4.95”). When used horizontally as a beam, its span can range from 1’-4” to 4’-0” according to the 2018 IRC, although in some circumstances, it can span up to 6’-11”. If planning to use a 4×4 as a structural beam, it’s best to check with a Structural Engineer.

Strongest Wood for 4×4

When selecting a 4×4, check the end grain. If the growth rings form circles, the log is the core of a peeler and won’t be as strong as one that has off-centered rings and isn’t a core. The flatter the rings, the further from the core or pith wood they were sawn from.

The grade of wood is an important factor in wood strength. Select grade and #1 are commonly referenced as the better or stronger wood, with #2 grade being almost as strong. The grade indicates how knots, checks, and other imperfections in the lumber can affect its strength.

Wood species are as important as the grade and end grain. 4x4s used in construction are typically softwoods. Southern pine and Douglas fir-larch are the strongest, with hemlock and SPF (spruce-pine-fir) and cedar having less strength.

The building codes and American Wood Council test and rate or classify lumber based on species and grade for strength, and have different tables for comparison. When selecting the strongest wood, pick #1 grade with flatter end grain, and of the strongest species appropriate for the purpose you need, and your budget.

While softwoods are typical in construction, it should be noted that different species of hardwood are also used too. Oaks and maples historically were used for beams and as decorative structural elements, but their price keeps them from common use.

How Far Can a 4×4 Span Without Sagging?

The distance a 4×4 (3-1/2”x3-1/2”), or any wood for that matter, can span is based on its species, grade, weight supported, time or duration, and other factors such as gravity, moisture content, and how the load is distributed – uniform or center load. Spreading the weight along the span reduces the incidence of sag, as does the reduction of the unsupported distance or span. Inertia is also a factor that needs to be considered too, but that’s another story.

With most construction softwood species of #2 grade or better spanning 8-feet, the acceptable initial incident of sag is 0.02”/foot with no load or weight other than itself and gravity. An 8-foot 4×4 with a moisture content (MC) of 12% weighs about 27.3-pounds and has an incident of sag of 0.001/ft over an 8’ span. A 16-foot 4×4 weighs around 54.6-pounds at 12% MC and has a total incident of sag of 0.07” over a 16’ span. Both are well within acceptable sag parameters.

Hanging 250 pounds in the middle of a 4×4 with an 8-foot span produces a total sag of 0.07”, or 0.019” per foot which is acceptable. Upping that weight to 1000 pounds produces a sag of 0.3” total, or 0.075” per foot, which is considered excessive or unacceptable. Spread that 1000 pounds along the length of the 4×4 beam, and the sag drops to 0.15” or 0.037” per foot, which is considered borderline.

Support the 4×4 at 16” centers, so the span is around 12-1/2”, and the amount of weight that can be carried without the incident of sag becoming unacceptable significantly increases. Theoretically, a 4×4 can support 30,000 pounds per foot, provided the supports don’t buckle and the 4×4 compress. Spread the support to 24” centers so the span is approximately 20.5”, and the load that causes too much sag drops to about 10,000 pounds per foot.

Time or duration are also factors to be considered. The initial sag or deflection will typically increase by as much as 50% over time, even with nothing other than gravity affecting it. By itself, with no load, a 4×4 will span 8 feet without noticeable sag.

Which Is Stronger 4×4 or 2 2×4?

A 2×4 is actually 1-1/2” wide by 3-1/2” deep, so a doubled or sistered 2×4 is 3” wide by 3-1/2” deep, while a 4×4 is usually 3-1/2” wide and 3-1/2” thick. Depending on orientation, the two have either the same measurement, or the 4×4 is greater by 1/2″ or approximately 15% than the doubled 2×4. The greater dimension makes little difference when supporting horizontally on the 3-1/2” thickness, but does make the 4×4 stronger in a vertical position.

The cross-section of a 4×4 is about 4.95” and the twinned 2×4 is 4.6”, making the 4×4 stronger and less likely to bend under similar loads. However, a 4×4 is more susceptible to twisting and through cracking, while two 2x4s properly fastened together are less likely to twist, bow, or develop through cracks, thus are less likely to fail.

Additionally, a #2 or better 8-foot pine 2×4 currently costs $4.15 US in my area, and a #2/BTR Douglas Fir 4×4 runs $13.37 US for an 8-foot length. So, a doubled 2×4 will cost about $8.30 versus the 4x4s price. Two 2x4s with a 12% MC will have a combined weight of about 18lbs, and the 8-foot 4×4 is around 27.3-pounds, making the double 2×4 both cheaper and lighter to move around on a job site.

While the 4×4 is technically stronger than sistered 2x4s, they are basically treated the same by building codes when used horizontally. 4x4s used vertically are recognized as stronger, so are more commonly used as posts and supports than two 2x4s. Since 2x4s are significantly more common in construction, they are more likely to be doubled and used in place of a horizontal 4×4.

Conclusion

An 8-foot 4×4 can typically carry 1,000lbs horizontally along its length without excessive sag, or 500lbs in a center load. Supported at 16” centers and the load jumps to 4,000lbs. When used as a post, the same 4×4 can support up to 6,000 PSI and even more if properly braced. Hopefully, you now have a better understanding of how much weight a 4×4 can support or carry, and the different factors that can affect its strength. All the best on your project.