How Far Can A 8×8 Beam Span Without Support?

A beam is a load-bearing structure. It supports and carries live and dead loads and transfers their weight to vertical supports which then pass them to the ground. They can be made of solid timber, layers or plys of dimensional lumber, engineered materials like glu-lam, steel, or reinforced concrete. If you’re wondering how far can an 8×8 beam span without support, we’re here to help!

The location of the beam and the loads it must support, along with wood species, grade, and other factors, affect its span. Depending on all factors, an 8×8 central beam supporting a floor can span up to 12 feet without support. However, the same beam used for more decorative purposes, can span up to 18’-5”.

In this guide, we’ll explore how far an 8×8 wooden beam can span, its load capacity, and its actual dimensions. We also include an 8×8 span chart to further assist you in your beam selection. Our goal is to provide you with the information to make the best choice for your project.

How Far Can A 8×8 Beam Span Without Support?

A beam is a horizontal structural member that carries loads perpendicular to its horizontal length or axis. They play a critical role in most residential structural builds to provide open spaces for numerous purposes.

Their width and depth, along with wood species and grade, determine and affect load-bearing capacity, deflection, and span potential. Other factors, including span and spacing of studs, joists, and rafters, plus location, type of loads, and other forces also affect its span too.

When we think of beams, solid wood or laminated 2-by dimensional lumber visible in many residential basements comes to mind. The laminated beams are typically 2, 3, 4, or more plies of common lumber fastened together to increase the load-bearing capacity and span. Headers, joists, rafters, and even stair stringers are also considered beams as they transfer loads in a horizontal manner to vertical supports.

Solid wood beams are the forerunner of all other choices but haven’t been commonly used for the past 50 years. Dimensional lumber has been the usual choice for the past half century or so.

Engineered wood, steel, and reinforced concrete have joined the list of options for beams, but currently aren’t as widely used in residential construction. However, in most situations, a Structural Engineer calculates or at least signs off on the dimensions and types of beams required to meet the structural needs of the building.

Most framing lumber is dimensional softwood with known strengths and deflection. The distance different dimensional lumber species and grades can span often determine where support columns or bearing walls are placed, which in turn, affects the size of open spaces within a structure.

We know that changes to width or depth will affect the span length. Increasing the width typically increases the span while increasing the depth often decreases the width required for similar spans.

For example, the maximum span based on all factors of a 2×8 beam is 5’-8”, while a 2×10 can span 7’-0’ and a 2×12 up to 8’-3”. Doubling the width takes it to 8’-9”, 10’-4”, and 12’-2” respectively while tripling increases the spans to 10’-10”, 13’-0”, and 15’-3”. So, increasing the width increases the span, while increasing both depth and width will also increase the span and improve resistance to deflection.

How much a beam can carry and its deflection along with wood species and grade are key to beam selection and span. A wood species and lumber grade that can carry more weight and has greater bending tolerance, modulus of elasticity, or resistance to deflection, will span further than one with less.

A #1 Douglas fir 8×8 can span 18’-5” while a #2 D-F can span 16’-9” with a total load of 125 pounds of tributary weight per linear foot and a deflection of 360/live load.

For example, consider an eastern white pine 8×8 with actual dimensions of 7-1/4”x7-1/4 and supported at both ends. If it is carrying no load but itself and that of gravity, it will develop a deflection of 2-3/8” over a 20’ span. Adding any load will cause the beam to fail.

8×8 Beam Bearing Load Capacity

The load-bearing capacity usually depends on the wood species and grade along with other factors like span and tributary loads. The width and depth of the beam also impact its load capacity too.

Doubling the width decreases the deflection by up to 50%, whereas doubling the depth will reduce deflection to about 1/8th its original value. Additionally, doubling the width usually doubles how much it can carry, but doubling the depth usually quadruples how much it can carry.

Depending on all factors, an 8×8 has a bearing capacity between 8,000 and 12,000 pounds. The longer its span, the lower its bearing capacity per linear foot.

A #1 Douglas fir-larch beam can support 1,545 pounds per linear foot over 8 feet at 80% of its total load capacity, but only 458 pounds at 12 feet and 136 pounds at 18 feet under similar conditions. So, the longer the span, the smaller the tributary load area.

8×8 Beam Span Chart

The beam span depends on wood species and grade, loads, the width of the structure, and whether it is supporting one, two, or 3 floors. It also depends on if it is supporting exterior walls or providing interior support. Interior beams may support a floor or a floor + ceiling + roof, or multiple floors, while exterior girders support those plus exterior finishes and other environmental loads.

A central beam also typically supports joists of equal length spanning to the left and to the right. Floor joist lengths of 8’ mean an overall width of 16’, and 20’ joists result in a 40’ width. The longer the joist span, the shorter distance the beam can span without support.

In the same sense, the longer the beam span, the shorter the joist span. Additionally, the spacing between joists also impacts the distance they can span, with those spaced at 12” centers reaching further than those at 16” or 24” centers.

A typical beam today is made by fastening lengths of dimensional lumber together. An 8×8 beam could be four 2x8s fastened face to face to form a nominal 8×8, but since a 2×8 is actually 1-1/2”x7-1/4”, it’s actually 6”x7-1/4”.

Whereas a 5-ply 2×8 beam is 7-1/2”x 7-1/4”, so closer to an actual solid wood 8×8 beam. The Table below identifies the beam span for different wood species based on joist span in 2-foot increments for combined loads of 50psf.

Maximum 8×8 Beam Span (Supporting one-floor using common #2 or better dimensional lumber)
Wood Species	4 Ply							5 Ply
	Floor Joist length							Floor Joist length
	8’	10’	12’	14’	16’	18’	20’	8’	10’	12’	14’	16’	18’	20’
	Beam Span							Beam Span
Douglas Fir-Larch	11-2	10-0	9-1	8-5	7-11	7-5	7-1	12-6	11-2	10-2	9-5	8-10	8-4	7-11
Hemlock-Fir	11-7	10-5	9-7	8-10	8-3	7-10	7-5	12-6	11-7	10-8	9-11	9-3	8-9	8-3
Spruce- Pine-Fir	11-0	10-3	9-8	9-2	8-7	8-1	7-8	11-11	11-0	10-5	9-10	9-5	9-0	8-7
Northern Species	9-9	8-8	7-11	7-4	6-10	6-6	6-2	10-9	9-9	8-10	8-3	7-8	7-3	6-10

What Are the Actual Dimensions of an 8×8 Beam?

The actual dimensions of an 8×8 are considered to be 7-1/2”x7-1/2” or 7-1/4”x7-1/4”. However, much depends upon if it’s milled from a single timber or made up of dimensional lumber fastened together. A solid timber may be milled and planed to actually be 8” by 8”. It could also be 7-3/4” by 7-3/4” rough cut and smoothed to be 7-1/2” by 7-1/2” or 7-1/4”x7-1/4”, much depends on who is doing the milling and where.

If the 8×8 is made up of 2×8 lumber then its dimensions will differ too. A 2×8 is actually 1-1/2”x7-1/4”, so an 8×8 beam of four 2x8s is nominally an 8×8, but is actually 6”x7-1/4”. Using five 2x8s will result in one that is 7-1/2” wide by 7-1/4” deep, so closer to the ‘actual’ dimensions of an 8×8. It should be noted, though, that the actual dimensions do impact span and load-carrying capacity, so be sure of the true measurements before calculating anything.

Conclusion

The distance any beam will span depends on its width, depth, the wood species, grade, location it will be used, load variables, and other structural factors. The width and depth of the beam greatly impact its strength, too.

A 4-ply 2×8 beam can span between 9’ and 12’ depending on various factors, while a 5-ply 2×8 beam can span 12” to 16” further. The lighter the loads, the further the span. So, if you need to use an 8×8 beam for structural purposes, check with a Structural Engineer.

Hopefully, you have a better understanding of how far an 8×8 beam can span without support and are ready for your next project.