How far can you span a 2x4 for a roof?

A 2×4 can span a maximum of 6' 7” when spaced 16” apart and used as a floor or deck joist. When used as a ceiling joist or a roof rafter, a 2×4 can span up to 7' 3” spaced at 16”, and 6' 4” when spaced 24” apart. Due to the small spans of 2x4s, they often are not used in horizontal load-bearing capacities.

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In some applications, spanning a 2×4 to support a load is necessary. For instance, my neighbor has a very small shed for his snowblower, and the joists are 2x4s. When he told me this, I began to wonder just how far a 2×4 can span without support?

A 2×4 can span a maximum of 6’ 7” when spaced 16” apart and used as a floor or deck joist. When used as a ceiling joist or a roof rafter, a 2×4 can span up to 7’ 3” spaced at 16”, and 6’ 4” when spaced 24” apart. Due to the small spans of 2x4s, they often are not used in horizontal load-bearing capacities. For smaller structures, however, such as a small shed or outhouse, they can provide adequate support for a reasonable cost. In this article, we’ll take a look at your options when it comes to spanning a space with a 2×4, including span lengths for laminated 2x4s, as well as how to use calculators that determine beam and joist spans for 2x4s.

How Far Can a 2×4 Span Without Support?

A 2×4 floor joist can span 6’ 7” when spaced at 16” and using Southern Pine. Other commonly available species of dimensional lumber span slightly less distance, such as SPF, which can only span 6’ 1”. In a span table, you can only find information for a doubled 2×4 beam – also called a girder. A single 2×4 is never used for a girder because it cannot bear the loads required when framing a structure. It is important to know the definition of the term “span” when talking about lumber and framing. “Span” refers to the distance between the inside of the two bearing points of a piece of horizontal lumber. The actual length of the wood will be longer because it has to sit on something to support itself. Many different species of tree are used for dimensional lumber. Most people will only have one choice and it depends on where you live. In the South, you’ll be buying SYP – Southern Yellow Pine. If you are out west, then you’ll be purchasing Redwood or SPF – Spruce Pine or Fir. SYP can span greater lengths than Redwood or SPF, so know what you are buying. Lumber is also graded by quality. While your typical span table defaults to number 2 grade lumber, higher grades such as number 1 or select will hold more weight. The highest grade – select – will have few to no knots or imperfections, hence the greater load-bearing capability.

How Far Can 2×4 Floor Joist Span?

A 2×4 floor joist spans up to 7’ 11” when spaced 12” apart, on center. However, this span is for Southern Yellow Pine, which is one of the strongest softwoods available. Also, this span is for 30 pounds per square foot of live load. Living areas and decks require a minimum of 40 psf for support, so a 2×4 joist, in this instance, could only be for a shed or other non-living space. Realistically, you are not going to use a 2×4 for floor joists in your home. The loads and spans would simply be too great, and the amount of wobble and bounce would be drastic. Instead, using a 2×4 joist would be more practical in a very small shed or platform coming off a deck. If you opt to space your 2x4s at 16” on center, then your maximum span will be 7’ 3” with SYP. This assumes that the joist is not cantilevered and terminates with support on either end. If you opt to space your 2×4 joists at 24” on center, then you can only span a distance of 6’ 4”. These calculations assume a 30 psf live load. If you have Spruce-pine-fir lumber, then your span changes to 6’ 8” for 16” joist spacing. A redwood joist with the same spacing will span 6’ 6” – 9” less than an SYP joist at the same spacing. The differences in wood species are significant, and you need to know the type of wood you are using to properly read and use a span table. For small projects, using stronger lumber like SYP versus Redwood could allow you to have a larger shed or platform.

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How Far Can a 2×4 Rafter Span?

2×4 rafters spaced at 12” on center can span up to 11’ 3”, for select grade SYP. While no minimum slope is given, many span tables differentiate rafter span lengths between roofs sloped 3:12 and greater versus lower slopes. Therefore all spans mentioned in this section should assume a slope of at least 3:12. Span tables that indicate maximum rafter lengths for roofs with slopes lower than 3:12 do not include 2×4 lumber. While no reason is given, one can assume that using a 2×4 rafter in that instance is likely not acceptable and should be avoided. Most roofs have rafters spaced further than 12” apart – usually 16” or 24” on center. If you have that spacing for a roof with 2×4 rafters, then you can have a maximum span of 10’3” and 8’11”, respectively, if using SYP select grade lumber.

How Far Can You Span a 2×4 Ceiling Joist?

Ceiling joist span tables have two different categories: habitable and uninhabitable. A habitable space would be a loft or other area where people might sleep or live – or both. Some homes may have both types of ceiling joists – one to support a living area above while the other set of joists supports a storage area. The two sets of joists use different span tables. For an uninhabitable attic space, a select grade SYP 2×4 can span 10’ 3” if spaced at 12”. The term “uninhabitable” means a live load of 20 pounds per square foot – enough to hold a person from time to time but not much more. If the space above the ceiling joists is habitable, then a 2×4 of the same type can span 7’ 11” at 12” spacing to account for live loads of 30 pounds per square foot. If the attic area is a bedroom, then it must be designed to account for a 40 psf live load. In that case, a 2×4 of the same type as above can span 7’ 3” at 12” spacing.

How Far Can a 2×4 Beam or Header Span?

An interior 2×4 beam – made up of two 2x4s nailed together – can span up to 3’ 1” without support beneath a set of joists for a home no greater than 20’ wide and supporting no more than 1 floor. For homes that are 28’ wide, a double 2×4 can support a span of 2’ 8”. For a home 36’ in width, a span of 2’ 5” is possible. For interior beams that support more than one floor, a doubled 2×4 can support a 2’ 2” span for a home 20’ wide. There are no span tables that include 2×4 beams – also known as “girders” – for decks. Deck beam span tables begin with 2×6 lumber. Due to the exterior nature of the structure, 2×4 construction is not adequate for use as a deck beam. For the beam mentioned above, any type of commercially available construction lumber will adequately suffice to cover the spans as described, whether that is SPF, SYP, Redwood or another type commonly found at home reno stores. Exterior wall beams and headers use a different span table. A doubled 2×4 can support a span of 3’ 6” if the only structure above the beam or header is the ceiling and roof for a 20’ wide home. If there is a floor on top, then the same beam or header can span 3’ 1” if there is a load-bearing wall in the center of the structure, and a 2’ 8” if there is no center bearing wall. Using a 2×4 for a header is common above a door or narrow window, but be sure you are using the right span table when calculating whether or not you can use a 2×4 header. It depends on what is above the header. If there is just a roof, then you are probably fine. If you have the floor above your header, then check to see if you need a 2×6 header instead.

How Far Can 2×4 Decking Span?

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An SYP 2×4 deck joist can span 5’ 5”. There are no span tables for using 2x4s for the actual deck boards, which is a very uncommon and cost-ineffective investment. A 2×4 deck joist can be useful when building smaller portions of a deck that juts out from the main portion, either as a platform for stairs or a barbecue area.

How Much Weight Can a 2×4 Hold?

A 2×4 can “hold” different weights depending on its placement in a home, deck, or shed. For instance, a 2×4 beam can hold vastly more than a 2×4 joist because the beam must be doubled. As well, a vertical 2×4 will have a different capacity to bear weight than a horizontal 2×4.

Horizontally

A two-ply 2×4 beam is meant to hold up to 40 psf of live load and 10 psf of dead load. An SYP 2×4 double-ply beam can hold 500 pounds per lineal foot (plf) for an interior space 20’ in width. The span must adhere to the span tables as mentioned above. Thus a 3’ 1” SYP two-ply 2×4 span can hold just over 1,500 lbs. A 2×4 joist beneath a living space has a max span of 7’ 3”, if it is Select SYP spaced at 12”. The joist can hold 50 pounds per linear foot, or around 380 pounds total. Understanding the plf of your beam is important. Still, span tables already take the plf of a beam into account and simply tell you the allowable span of your 2×4 beam depending on the width of the structure above it and other factors. Knowing how much your beam or joist can hold is useful, but the beam span table is all you need to determine what type and how far a girder you need. If you are installing something extremely heavy above your 2×4 beam, then it might be a good idea to calculate the bearing capacity of your beam because it might be beyond the 40 psf live load the beam is capable of holding.

Vertically

A 2×4 placed vertically – a stud – can hold around 1670 pounds when spaced at 16” on center for SPF lumber. Hemlock-Fir lumber can hold around 1590 pounds at the same spacing. For two 2x4s placed together, the bearing amount can be multiplied by the number of pieces of lumber fastened together. So two SPF studs affixed to one another can hold over 3000 pounds at 16” spacing. The weight these pieces of lumber can hold is their compressive strength, perpendicular to the bottom plate of the framing.

Maximum Span Calculator

Looking up span tables and reading them accurately is crucial when constructing any type of framed structure, from a deck to a shed to a house. However, a max span calculator makes looking up maximum spans much faster. When using the calculator, you must know the type of lumber you are using, the type of framing member such as a joist or rafter, the loads it must be rated for (i.e. 30 psf live load), and several other factors. When complete, the calculator will give you a maximum span for that framing member. Remember to double-check with an actual span table to ensure you have the right span.

Conclusion

While 2x4s are not commonly used as joists or girders, they can be effective for smaller projects. As you would with larger framing members, be sure to check your span tables, use a span calculator, and use the proper fasteners and brackets to ensure your project gets done right.

weekendbuilds.com - How Far Can a 2X4 Span Without Support? - Weekend Builds