How Much Weight Can a 4X4 Hold?

The weight that a 4×4 can hold is not simply answered with a single number. This is because several factors can influence the strength of the lumber, including wood species, wood grade, span, and load. Therefore, when determining how much weight a specific 4×4 can hold, you must consider these factors.

In addition, the direction of the wood also has a major impact on the supportable weight because the depth will change depending on whether the 4×4 is positioned vertically, horizontally, or on its edge. Not only that, but the direction can also affect whether the pressure of the load is being placed with the grain or against it.

Because of the multitude of variables that can affect the weight that a 4×4 can hold, it is good to look at how each factor can change the supportable amount. Then, you can apply those based on the generic number of pounds supported by a 4×4 depending on direction.

What Does Affect 4×4 Load Bearing Capacity?

The various aspects of a certain 4×4 can drastically increase or decrease the amount of weight that it can safely support. So let’s look at each of these variables and how they can make a difference.

Wood Species

The species of the 4x4s that you use can vastly affect the amount of weight that they can support. For example, different tree species used to create 4×4 lumber have different densities and strengths. Not only can the species affect the amount of weight that it can support, but different species are often recommended for different uses of 4x4s.

Denser woods can support more weight than those that are less dense. There are plenty of different wood types to choose from for 4x4s. Weaker options like White Cedar can be used for fence posts or other non-supportive projects. However, if you plan to use the 4×4 to hold substantial weight, it is better to choose denser species like Douglas Fir. If you need something exceptionally strong, then Yellow Pine is another option.

Grade

The grade of a 4×4 makes a difference in the strength and how much weight the lumber can adequately hold. The number of knots, splits and warping in the wood determine the grade.

The grade of wood ranks from No. 1 wood, the strongest with the least number of irregularities, to No. 4 that is economy grade and not usually recommended for anything structural. You should also avoid No. 3 lumber for any significant support.

It is also important when choosing 4x4s to consider the rings. This is because a byproduct of plywood production called a peeler core is commonly used for 4×4 posts, usually 4x4x8 posts. The peeler core contains the pith, which is the weakest portion of the lumber. Having a 4×4 peeler core can significantly limit the amount of weight that the component can hold.

In fact, the farther away from that core, the better. Even if the 4×4 is not a peeler core, if it is close to the center of the tree, then it will not be as strong as lumber made out of the area with larger rings. Look for flat wings without much width and avoid buying 4x4s that show a bullseye shape on the ends.

Load Type

The type of load is also important for 4×4 strength no matter the use. The two types of load are dead load and live load. You will have to keep both in mind when using 4x4s because they account for different variables.

Dead load is the amount of weight that constantly puts pressure on the lumber. For example, if you are using the 4×4 posts for a deck, then the dead load will include the deck itself and all permanent structures and components within that structure. If you have a permanent pergola on top of a portion of the deck, that would also be considered part of the dead load.

The live load is the weight that changes and will not be a constant weight on the 4x4s. Again, with the example of the deck, this would include people, chairs, umbrellas, and grills, as well as weather factors like rain and snow.

After choosing the species and grade of lumber, you can determine the load amount for that 4×4. To put it into perspective, an average deck is designed to hold at least 10 pounds per square foot and 40 pounds per square foot live load. This establishes a design load of 50 psf.

However, if you have heavy furniture or plan on having a lot of guests on the deck simultaneously, you may need to construct the deck to support higher loads. You can multiply the load by the square footage to get the amount of evenly distributed weight that the deck could support. You should also consider load duration, which is the amount of time that the 4×4 can support excessive weight before becoming damaged or weakened.

Span

The span is the distance a 4×4 can travel without needing additional support. When you exceed the allowable span, it will not support as much weight. However, the shorter span will be able to support more weight. This is important if you are using 4x4s horizontally and it will always make a stronger structure if you utilize more vertical support for the horizontal lumber components.

Another aspect of span to keep in mind is the spacing, which is the amount of space between the two 4x4s spanning the vertical support. Closer spacing means more components and a lengthier span. It is important to remember that 4x4s are generally only used vertically and are not designed to offer structural support horizontally. Still, there are existing structures that utilize 4x4s horizontally and there may be some situations where using a 4×4 is acceptable.

Length/Height

Whereas span will affect the amount that a horizontal 4×4 can hold, the length can affect the strength and weight that a 4×4 will hold vertically. As the length (or height considering it is vertical) increases, the number of supportable weight decreases. For example, a Douglas Fir 4×4 that is 8 inches in length can vertically support 7,395 pounds but will only support 4915 pounds when the length is increased to 10 inches.

The reason for this is similar to the reason a longer span affects the amount that a horizontal 4×4 can hold. When there is more pressure and more area to bend, the weight will compress the wood until it begins to break. Shorter wood is more likely to remain steady and supportive.

Moisture

Moisture in the 4x4s that you purchase can affect the amount of weight they can hold. This factor is often disregarded, but the fact is that dry wood is stronger. Typically, 4x4s that you get at your hardware or home improvement store have moisture content around 25%, but there is also kiln-dried lumber available with a much lower moisture content, as low as about 6%. Dryer 4x4s are more resistant to warping and, in some cases, can hold a much higher amount of weight.

You can also choose to air dry your 4x4s, but it can take a while to drop below 20 percent moisture. However, given enough time, it can drop as low as 12%, depending on the temperature, humidity, and other factors. You can expect it to take somewhere between 2 and 12 months to air-dry 4×4 lumber.

How Much Weight Can a 4×4 Hold Horizontally?

4x4s are not commonly used horizontally, but there may be some situations where you come across a structure with horizontal 4x4s or that requires them for whatever reason. It is important to know how much weight each 4×4 can hold in those situations.

A 4×4 made out of high-quality, dense wood, like No. 1 Yellow Pine, can support up to 2 tons, or 4,000 pounds, as long as it has adequate vertical support. With corner boards at each end of a spanned 4×4 and vertical support every 1 ¼ foot, it can support the complete 4,000 pounds.

However, if there is no vertical support between the two ends of a 4×4, then the middle of the span could only support a couple of hundred pounds or less depending on the length of the lumber. With a greater span, this number will rapidly decrease and the same goes for No. 3 or No. 4 grade or less dense species.

How Much Weight Will a 4×4 Hold Vertically?

4x4s are often used vertically to support significant amounts of weight, like in decks, stairs, or balconies. The height of the 4×4 can alter the amount of weight that the 4×4 can safely hold.

Generally, you can expect a vertical 4×4 to support at least 4,300 pounds, but the amount of weight could be significantly higher than that in some cases. Even at 12 ft high, a vertical 4×4 made out of No. 2 Southern Pine will hold over 3,000 pounds.

At 10 ft, a No. 2 Southern Pine 4×4 will hold even more at around 4,300 pounds, and at 8 ft., it will hold greater than 6,500 pounds. This increases even more as the post gets shorter and it could hold over 17,000 pounds at 3 feet tall.

What Is the Compressive Strength of a 4×4?

The compressive strength of lumber is the amount of load that a piece, in this case, a 4×4, can hold parallel to the grain without breaking. This means that it equals the amount that a vertical 4×4 can hold.

This can differ depending on the factors that influence the weight that a 4×4 can hold. However, it does decrease as the height of the 4×4 posts increases. For No. 2 SPF 4x4s that are 4 feet long, the compression strength is 14,477 pounds, but that amount decreases to 3,858 pounds for 4x4x10s.

How Much Weight Can a 4×4 Pressure Treated Hold?

Pressure-treated 4x4s will hold the same amount of weight as any other 4×4 of the same species, quality, length, grade, moisture content, and load. Pressure treatment only allows the wood to resist the elements better.

The only time when a pressure-treated 4×4 will be stronger than one that is not pressure treated is when the non-treated lumber has rot or other problems that weaken the wood. Therefore, pressure treatment can extend the life of the 4×4 and keep it strong for longer if it is in a location that offers unfavorable conditions.

Can a 4×4 Be Used as a Beam or Floor Joist?

You usually do not want to use 4×4 as a floor joist or beam. Instead, it is best to use 2x6s or doubled 2x4s, which are strongest on their edge. However, if you have to use 4x4s as a beam or floor joist, just ensure that they do not have to support a significant dead load, especially near the middle of the span because they are likely to bend and sag. You will also want to have vertical support every 16 inches at the shortest and we recommend having vertical support every 12 inches to ensure proper support for the horizontal 4×4.

How Far Can a 4×4 Span Without Support?

While 4x4s are not usually used to span horizontally, as long as they are not structural or supporting substantial weight, you can use a 2×4 spanning chart as a guide. With 16 inch spacing, you can span a 4×4 up to 6’ 7”. You should avoid spanning any 4×4 over 8 feet, regardless of spacing and other factors because they tend to sag, which can be a safety hazard.

Conclusion

If you plan to build a structure or repair an existing one that uses 4x4s, you want to make sure that you make it structurally sound. Going beyond the amount of weight that a 4×4 can safely hold could compromise integrity and stability. Not only should you know the average or estimated amounts that a 4×4 can hold, but it is also a good idea to have some knowledge of the factors that can influence the strength like the species and grade, the length or span, the moisture content, and the live and dead load. Then you can make a structure that is sure to be supportive and lasting.