How to Compute the Spans for a Deck

Building a deck? Then you'll need to know how to compute the spans. Find tips and information on lumber types, decking, standard lengths, and more.

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Five basic components make up any deck: decking, joists (and when attached to a house, a ledger), beams, posts, and footings. To ensure that a deck is safe and strong, local building departments have strict code requirements for these components. The following guide will walk you through basic requirements and best practices for these parts. 

More Tips for Building a Deck

Types of Lumber

The strength of a board depends on two factors: its species and its quality. Among lumber commonly available in pressure-treated form for deck framing, Douglas fir and Southern yellow pine are strongest, but Southern pine has more of a tendency to warp. Hem-fir is a designation that includes hemlock, fir, and other species that grow in the same stands. Some hem-fir is strong and stable, some is weak and likely to warp and crack. Consult with a lumber dealer and the building department before using hem-fir.

Lumber quality also has a bearing on a board's strength. Boards graded No. 2 and better are the best choice for deck building. Construction grade, common, and No. 3 stock have defects that make them unsuitable for a deck. "Select grade" or No. 1 boards are top-quality, but are usually not worth the extra cost for use in deck building.

Editor's Tip: Lumber comes in even lengths. Your computations, plans, and construction will be easier if you plan your deck with this in mind. You'll do a lot less cutting if you build a 12x16-foot deck rather than one that's 11 feet 7 inches x 15 feet 10 inches. There will be less waste too.

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Footings and Posts

In areas with freezing winters, most codes require concrete footings that extend below the frost line. In warmer areas a shallow footing is allowed, but codes will specify a minimum amount of mass in the footings. If an area has marshy or sandy soil, massive footings may be required.

For most decks, 4x4 lumber is strong enough for structural posts. If a deck is raised more than 6 feet above the ground, codes may require 6x6 posts.


The farther a beam must span—that is, the farther apart the posts are—the more massive a beam must be. The chart below lists approximate recommended spans. Beams made of two or more pieces are usually at least as strong as solid beams of the same size. For instance, a beam made of two 2x8s is probably stronger than a solid piece of 4x8. They are also less likely to crack.

Beam Spans: Distance between ports, using No. 2 and better Southern pine or Douglas fir

Beam: 4x6 Joists span up to: 6' Beam span: 6'

Beam: 4x8 Joists span up to: 6' Beam span: 8'

Beam: 4x8 Joists span up to: 8' Beam span: 7'

Beam: 4x8 Joists span up to: 11' Beam span: 6'

Beam: 4x10 Joists span up to: 6' Beam span: 10'

Beam: 4x10 Joists span up to: 8' Beam span: 9'

Beam: 4x10 Joists span up to: 10' Beam span: 8'

Beam: 4x10 Joists span up to: 12' Beam span: 7'


The required width of a joist depends on its span—how far it must travel between beams or between a beam and a ledger. It also depends on the joist spacing; for instance, joists that are placed 24 inches apart must be wider than joists placed 16 inches apart. See the chart below and study your local code.

Beam Spans: Distance a joist spans between a beam and a ledger or between beams, using No. 2 and better Southern pine or Douglas fir

Joist: 2x6 If joists are spaced: 16" Span: 9 1/2'

Joist: 2x6 If joists are spaced: 24" Span: 8'

Joist: 2x8 If joists are spaced: 16" Span: 13'

Joist: 2x8 If joists are spaced: 24" Span: 10 1/2'

Joist: 2x10 If joists are spaced: 16" Span: 16 1/2'

Joist: 2x10 If joists are spaced: 24" Span: 13 1/2'


Decking boards span from joist to joist. If you use 5/4 decking, joists must be no farther apart than 16 inches. Decking made of 2x4s or 2x6s can span up to 24 inches. If you will run decking at an angle, you may need to put the joists closer together; know your local codes.

Local Codes

Your local building department has regulations designed to ensure a strong and durable deck. While a few requirements may seem dated or unusual, most are based on the following common concerns:

  • Span requirements listed on the accompanying table ensure your deck won't sag or collapse during a party or under snow load. Codes specify maximum spans according to the type of wood used.
  • A deck more than 2 feet or two steps above ground must have a railing. Codes dictate how high the railing must be, as well as how far apart the balusters can be. Small children must not be able to climb it. Some codes require handrails, not just cap rails, for stairs.
  • If you attach the deck to the house with a ledger, code states how many fasteners of what type must be used. Metal flashing of a specific type may be required.
  • Many codes demand that posts be held in place by a specific type of post anchor.
  • In cold areas many codes require footings that extend below the frost line so that the deck does not get raised up in the winter by frost heave. Other departments may allow a floating deck with shallower footings that rise and fall during freeze-and-thaw cycles.


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