Triple 2 x 10 Beam Span

Wood-framed buildings use standard dimensional lumber as the load-carrying members, divided into two categories: posts, which are vertical elements holding up the weight of the structure and transferring that weight to the foundation and beams which support the weight between the posts. Of the two, posts are much more easily understood, as they only have to be able to support a certain amount of weight. Beams on the other hand tend to deflect under weight, the more space between posts that support the beam; the more they will deflect under weight. But only so much deflection is allowed. The maximum limitation allowed is calculated by taking the span and dividing it by 360, with a maximum allowable deflection of 20mm.

In actuality, carpenters don’t do this calculation; they depend on deflection tables, which show how much different types and dimensions of wood will deflect over different spans. Interpreting these tables can be a little tricky for the layman, as we don’t work with them all the time. What we need is basic information on what material we can use and how long a span it can be used for, without any support.

There are several factors that affect what this span is.

  • The type of wood – Not all woods have the same density or grain structure; therefore they don’t all support the same amount of weight. For construction, we are usually looking at some sort of conifer, like Douglas Fir, for which the characteristics are well known. However, different types of conifer will have different strength ratings, so it is important to know the type of wood and read only that type on the table. 
  • Quality of the wood – We normally think of wood grading as affecting the appearance of a board, but those knot also affect the strength of the wood. In most cases, #2 grade lumber is used for framing; but if #1 or select lumber is used, it will span a larger distance. 
  • The dimensions of the beam – All beams are made of 2” thick lumber, although multiple pieces of that lumber may be used side-by-side to gain more strength. For beams or joists, the important dimension is the height of the beam, as that determines the strength. The wider it is, the farther the wood extends from the centerline, increasing its strength. 

When talking about the deflection allowed for those floors, it is important to take into consideration the weight that will be placed on the floor. This adds an additional layer of complication to the equation, giving us one more reason to work from a table, rather than calculating the ideal size to use. That weight or load is broken into two parts:

  • Dead load – The weight of the building or itself. For homes, code requires that we use 10 psf (pounds per square foot) for the dead load if there is no ceiling hung below those beams or joists and 20 psf if there is a ceiling hung below it. 
  • Live load – The weight of furniture, dΓ©cor and people in the home. Building code specifies that a live load of 40 psf be used in living areas of a home and 30 psf be used in sleeping areas. 

So, the most common loads we find in a typical home are:

  • Live load 40 psf, dead load 10 psf – Used for first story floors and decks
  • Live load 30 psf, dead load 20 psf – Used for second story floors
  • Live load 30 psf, dead load 5 psf – Use for roofs


The design of a home, as well as the different uses of different areas can create considerable variance in the size of the joists and beams used in a home. However, most architects design homes to use the same size joists throughout or at least on every floor, simplifying the building process. Having multiple joist sizes for the ground floor (for example) would make the design and building of the basement walls much more complicated, as it would be necessary to adjust the top of the walls to match the difference in joist height. Those changes in height would need to be executed extremely accurately by the foundation contractor or the floor wouldn’t be level.

The following table provides maximum spans for floor joists and deck joists, by wood type, for a live load of 40# and a dead load of 10#. In all cases, it is assumed that #2 grade lumber is being used.

Joist SizeSpacingHem FirSpruce or Pine firDouglas Fir or LarchSouthern Yellow PineSYP Treated DecksCedar Decks
2x612” OC10’10’-3”10’-9”10’-9”9’-11”8’-10”
16” OC9’-1”9’-4”9’-9”9’-9”9’8’-1”
24” OC7’-11”8’-1”8’-1”8’-6”7’-7”7’
2x812” OC13’-2”13’-6”14’-2”14’-2”13’-1”11’-8”
16” OC12’12’-3”12’-7”12’-10”11’-10”10’-7”
24” OC10’-2”10’-3”10’-3”11’9’-8”9’-2”
2x1012” OC16’-10”17’-3”17’-9”18’16’-2”14’-11”
16” OC15’-2”15’-5”15’-5”16’-1”14’13’-6”
24” OC12’-5”12’-7”12’-7”13’-1”11’-5”11’-3”
2x1212” OC20’-4”20’-7”20’-7”21’-9”19’-1”18’-1”
16” OC17’-7”17’-10”17’-10”18’-10”16’-6”16’
24” OC14’-4”14’-7”14’-7”15’-5”13’-6”13’

This second table if for live load of 30# and dead load of 10#, as would be needed for the second floor of a home, where the home design calls for the sleeping areas to be on the second floor.

Joist SizeSpacingHem FirSpruce or Pine firDouglas Fir or LarchSouthern Yellow Pine
2x612” OC11’11’-3”11’-8”10’-9”
16” OC9’10”9”-11”10’-1”9’-4”
24” OC8’8’-1”8’-3”7’-7”
2x812” OC14’-4”14’-7”14’-9”13’-8”
16” OC12’-5”12’-7”12’-9”11’-10”
24” OC10’-2”10’-3”10’-5”9’-8”
2x1012” OC17’-6”17’-9”18’16’-2”
16” OC15’-2”15’-5”15’-7”14’
24” OC12’-5”12’-7”12’-9”11’-5”
2x1212” OC20’-4”20’-7”20’-11”19’-1”
16” OC17’-7”17’-10”18’-1”16’-6”
24” OC14’-4”14’-7”14’-9”13’-6”

Please note that in cases where the wood type is not known, it is best to assume the weakest possible lumber for that application and use the span shown for that type.


Homes actually don’t have a lot of beams in them, with more joists than beams. What’s the difference? Beams are the major load-bearing horizontal elements, while joists are secondary ones. In a typical floor, built over a crawlspace, there would be beams around the perimeter, as well as down the center of the floor, allowing the joists to attach to the beams at the edges and the middle.

Typically, beams are made of multiple 2”x 10”s or 2”x 12”s, while the floor joists are single layers of the same size boards. Stacking multiple boards together increases their strength. This is the critical weight-bearing part of the home’s structure, as the beams must carry the weight of the rafters and all the weight they are carrying. The weight of those floor joists and the weight they carry is figured into the maximum span for the beams.

In the table below, the same spans are allowed for both #1 and #2 grade lumber. Note the column for β€œSupported Joist Length.” This refers to the floor joists that are to be attached to the beam. The longer the floor joists that the beam is supporting, the shorter the distance between supports that the beam needs. That distance is also affected by the size and number of pieces of wood that are being used to make up the beam, as shown in the six columns to the right. Of course, in the case of a full basement, where the perimeter beams are sitting directly on the concrete basement walls, support and span are not an issue, as the beams have continuous support. Hence, it is possible to have perimeter beams that are only two boards thick, while a center beam might need to be three or even four boards thick. This table is for a live load of 40 psf and a dead load of 10 psf.

SpeciesJoist Length3-2x8 Built Up Beam Size 4-2x8

3-2x104-2x103-2x124-2x12 Built Up Beam Size
Douglas Fir8’9’-8”11’-2”11’-10”13’-8”13’-8”15’-10”
Spruce, Pine, Fir8’10’11’12’-10”14’-1”14’-11”17’-2”

This second table is for a live load of 30 psf and a dead load of 20 psf.

SpeciesJoist Length3-2x8 Built Up Beam Size 4-2x83-2x104-2x103-2x124-2x12 Built Up Beam Size
Douglas Fir8’7’-3”8’-4”8’-10β€œ10’-2”10’-3”11’-10”
Spruce, Pine, Fir8’7’-10”9’-1”9’-7”11’-1”11’-2”12’-10”

The beam span lengths listed in the tables above probably aren’t going to be sufficiently long for whatever structure is being built. The idea is that supporting columns or posts would need to be installed with that frequency, so as to ensure that the beams have sufficient support.

Triple 2x10 beam span
Triple 2×10 beam span, Butch Austin

Building 3-Ply Beams

The next question is how to build those beams. Obviously they will be built out of standard #2 dimensional lumber, 2” thick using a width that will be adequate to support the weight. As mentioned earlier, it is normal to use the same sized lumber for the beams as for the joists. To gain extra strength, additional pieces of wood are laminated together, making a thicker beam.

It is often impossible to buy this lumber long enough to make a three-ply beam out of only three pieces of lumber. That lumber is going to have to be spliced together someplace. Care and planning must be used in determining where to make those splices, so that the beam will have the necessary strength.

If we assume a three-ply beam, the ideal position for the splices on the middle ply is directly over the support posts. That will make the middle ply function essentially as if it is one continuous piece. But the other two plies cannot be joined at the same place. The joints will need to be staggered for those pieces, so as to maintain the overall structural integrity of the beam. They should be a minimum of four feet away from the joints in the center ply and four feet away from each other. If possible, it is even better if the joints in those plies are between different pairs of columns, so that there is no place along the length of the entire beam where there are two seams between any pair of supporting posts.

To connect the boards together, use a combination of construction adhesive and screws or nails, spiral nails are preferred. All screws should be #10 at least 3” long and nails should be 10d or longer.

Before nailing, liberally apply construction adhesive between the boards. Then nail them in a zigzag pattern, spacing the nails no more than 16” apart. Be sure to double nail at both ends of every board, so that there are nails in the top and bottom edges of the board. The finished beam will probably be quite heavy and some help will be required to set it in place.

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