The span of a beam is dependent on a few variables. The species of lumber, size of lumber and the load it carries. Fewer posts on upper level decks are typically more desirable to the occupants and this drives the use of larger framing materials for longer spans. Beam span maximums are based on a maximum anticipated live load. Building codes for residential decks only require 40 psf.
The longer the joist, the more area of deck the joist supports, and thus the beam supports. For pressurepreservativetreated southern pine no. 2 grade, there is a method that can be used for estimating beam spans for a preliminary design. When supporting joists that span 12 feet with no overhang beyond the beam, a double ply beam can span in feet a value equal to its depth in inches. A double 2x12 beam can span 12 feet; a (2) 2x10 can span 10 feet and so on.
Beam Sizes (Based on No. 2 or better Ponderosa Pine and Southern Pine Treated for weather and/or ground exposure)

X = Joist Length
X 
Species 




Post Spacing 








4' 
5' 
6' 
7' 
8' 
9' 
10' 
11' 
12' 
13' 
14' 
6' 
SYP 
12x6 
12x6 
12x6 
22x6 
22x6 
22x6 
22x8 
22x8 
22x10 
22x10 
22x10 

P P 
12x6 
12x6 
22x8 
22x8 
22x8 
22x8 
22x10 
22x10 
22x12 
22x12 
32x10 
7' 
SYP 
12x6 
12x6 
12x6 
22x6 
22x6 
22x8 
22x8 
22x10 
22x10 
22x10 
22x12 

P P 
12x6 
12x6 
12x8 
22x8 
22x8 
22x10 
22x10 
22x10 
22x12 
32x10 
32x10 
8' 
SYP 
12x6 
12x6 
22x6 
22x6 
22x8 
22x8 
22x8 
22x10 
22x10 
22x12 
22x12 

P P 
12x6 
22x6 
22x8 
22x8 
22x8 
22x10 
22x10 
22x10 
32x10 
32x10 
32x12 
9' 
SYP 
12x6 
12x6 
22x6 
22x6 
22x8 
22x8 
22x10 
22x10 
22x12 
22x12 
32x10 

P P 
12x6 
22x6 
22x8 
22x8 
22x10 
22x10 
22x10 
32x10 
32x10 
32x12 
32x12 
10' 
SYP 
12x6 
12x6 
22x6 
22x6 
22x8 
22x8 
22x10 
22x12 
22x12 
32x10 
32x10 

P P 
12x6 
12x6 
22x8 
22x8 
22x10 
22x10 
22x12 
32x10 
32x12 
32x12 
Eng Bm 
11' 
SYP 
12x6 
22x6 
22x6 
22x8 
22x8 
22x10 
22x10 
22x12 
22x12 
32x10 
32x12 

P P 
22x6 
22x6 
22x8 
22x8 
22x10 
22x12 
22x12 
32x10 
32x12 
32x12 
Eng Bm 
12' 
SYP 
12x6 
22x6 
22x6 
22x8 
22x8 
22x10 
22x10 
22x12 
32x10 
32x10 
32x12 

P P 
22x6 
22x6 
22x8 
22x10 
22x10 
22x12 
22x12 
32x12 
32x12 
Eng Bm 
Eng Bm 
13' 
SYP 
12x6 
22x6 
22x6 
22x8 
22x8 
22x10 
22x10 
22x12 
32x10 
32x12 
32x12 

P P 
22x6 
22x6 
22x8 
22x10 
22x12 
22x12 
22x12 
32x12 
32x12 
Eng Bm 
Eng Bm 
14' 
SYP 
12x6 
22x6 
22x6 
22x8 
22x10 
22x10 
22x12 
32x10 
32x12 
32x12 
32x12 

P P 
22x6 
22x8 
22x8 
22x10 
22x12 
32x10 
32x12 
32x12 
Eng Bm 
Eng Bm 
Eng Bm 
15' 
SYP 
22x6 
22x6 
22x8 
22x8 
22x10 
22x12 
22x12 
32x10 
32x12 
32x12 
Eng Bm 

P P 
22x6 
22x8 
22x8 
22x10 
32x10 
32x10 
32x12 
32x12 
Eng Bm 
Eng Bm 
Eng Bm 
16' 
SYP 
22x6 
22x6 
22x8 
22x8 
22x10 
22x12 
22x12 
32x10 
32x12 
32x12 
Eng Bm 

P P 
22x6 
22x8 
22x10 
22x10 
32x10 
32x10 
32x12 
32x12 
Eng Bm 
Eng Bm 
Eng Bm 
Joist sections cantilevered over beams must be calculated as (2x length) for beam sizing.
SYP= Southern Yellow Pine
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