Deck beam span calculator

Enter the joist length the beam carries and the distance between posts. We pick the smallest built-up beam that DCA 6 allows and show the table cell.

For joists up to 10'-0" in Southern Pine, a 3-ply 2x8 beam spans up to 8'-3" between posts. Your 8'-0" post spacing fits. The beam may also run past the end posts by up to a quarter of that span.

Table 3A for Southern Pine. The column is the joist span your beam carries. Your cell is highlighted.

Allowable beam span between posts, DCA 6 Table 3A
Beam	Joists 6 ft	Joists 8 ft	Joists 10 ft	Joists 12 ft	Joists 14 ft	Joists 16 ft	Joists 18 ft
2-ply 2x6	6'-8"	5'-8"	5'-1"	4'-7"	4'-3"	4'-0"	3'-9"
2-ply 2x8	8'-6"	7'-4"	6'-6"	5'-11"	5'-6"	5'-1"	4'-9"
3-ply 2x6	7'-11"	7'-2"	6'-5"	5'-10"	5'-5"	5'-0"	4'-9"
2-ply 2x10	10'-1"	8'-9"	7'-9"	7'-1"	6'-6"	6'-1"	5'-9"
3-ply 2x8	10'-7"	9'-3"	8'-3"	7'-6"	6'-11"	6'-5"	6'-1"
2-ply 2x12	11'-11"	10'-4"	9'-2"	8'-4"	7'-9"	7'-3"	6'-9"
3-ply 2x10	12'-9"	11'-0"	9'-9"	8'-9"	8'-3"	7'-8"	7'-3"
3-ply 2x12	15'-0"	13'-0"	11'-7"	10'-6"	9'-9"	9'-1"	8'-7"

Design this deck in 3D, free

Carries over: species and a deck depth of 13 ft, which gives joists of about 10 ft on the beam. Not carried: the 8 ft post spacing, which follows from the deck width you pick, and the beam size, which the designer picks for the whole deck. This table sizes one beam only, not the joists, posts or footings.

Planning figures from the AWC DCA 6-2015 guide, not an engineer's design or a permit. Your building department has the final word, and local rules may be stricter.

What the beam span means

The beam carries the joists, and the posts carry the beam. The beam span is the distance between two posts, center to center. A longer joist span puts more load on the beam, so the allowed beam span shrinks as the joists get longer. That is why the table is read by joist length across the top.

Built-up beams

The beams in DCA 6 are built up from two or three plies of dimension lumber, fastened together, and they sit on top of the posts. Deeper boards span further than shallow ones, and three plies span further than two. The calculator lists them from smallest to largest and picks the first that reaches.

Beam depth against joist depth

When the joists hang from the beam in hangers, the beam should be at least as deep as the joists. A 2x10 joist needs a beam that is 2x10 or deeper.

What sits under the beam

Post height and footing size depend on the same two numbers, the beam span and the joist span. Look both up in the post height table, or use the deck footing calculator.

Where the numbers come from

Every limit on this page is read from the American Wood Council, Prescriptive Residential Wood Deck Construction Guide (DCA 6), 2015 edition, a guide written for the 2015 International Residential Code. The deck designer, the builder's plan and these tools all read the same tables. The span tables assume 40 psf live load, 10 psf dead load, No. 2 grade lumber in wet service, and footings on soil that bears at least 1,500 psf.

A number that is our own choice and not in the guide, such as the 24 in joist overhang we draw by default, is never presented as a code requirement. Where a value comes from somewhere other than DCA 6, the page says so.

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Questions

Anything else? Write to us or see every calculator.

Questions people ask

As far apart as the beam on top of them can span. In DCA 6 Table 3A a 2-ply 2x10 beam in Southern Pine carrying 10 ft joists spans 7'-9", and the largest beam in the table, a 3-ply 2x12 under 6 ft joists, spans 15'-0". The table as a whole runs from 2'-6" to 15'-0". Use the calculator above for your joists and species.

Yes. DCA 6 lets a beam run past the end post by up to a quarter of its allowed span. Our designer draws 12 in as a neat overhang and never more than that quarter.

A 3-ply beam of the same size spans further, as the table shows. It also needs a post cap to hold all three plies, and a beam bolted to the side of a post without a notch is not allowed. If two plies reach, they are cheaper and simpler to handle.