Deck structures consist of several sets of framing members, each dependent on the size and spacing of the others for support. Beams support deck joists. The distance between a beam and an adjacent building depends on the allowable length of the joists that run from the beam to the building. Building professionals use the term "span" to describe the maximum allowable length of a joist or beam. Building departments and industry standards organizations publish span tables that outline joist and beam spans according to lumber size and species. Become familiar with lumber spans and design a safe, code-compliant deck structure.
About Deck Framing
The framing members of a conventional deck structure include decking, joists, beams and posts. Decking fastens to horizontal support boards called joists. Your decking material choice influences the spacing between joists. For example, strong lumber, such as 2-inch redwood, often has a maximum span of 24 inches and weaker lumber, such as pine, has a maximum span of 16 inches between joists. Joist size affects the allowable span between adjacent beams or between a beam and a building. Larger, stronger lumber safely spans longer distances than smaller lumber or weaker species.
Decking and Joist Spacing
As you enlarge the spacing between joists, you must reduce the joists' span between beams or beams and buildings. For example, if two sets of joists of similar material and size support the same type of decking, the more closely spaced set of joists may span farther than the more widely spaced joists. Span directly correlates to the distance between beams. If a span table states that your joist can span 5 feet, that means that a beam or building-mounted ledger must support the joist at 5 foot intervals.
Joist Size Affects Beam Spacing
Most deck framing lumber is nominal 2-inch lumber, meaning that its thickness actually measures around 1 1/2 to 1 5/8 inches. The width of the joist lumber typically ranges from approximately 5 1/2 inches, called 2-by-6, to 11 1/4 inches, called 2-by-12. Joist span is proportionate to lumber size; large joists have longer spans than narrow joists. Therefore, as the designer of the deck structure, you can affect the maximum distance between beams and deck buildings by increasing the size of your deck joists.
Maximum Distance Between Beams and Buildings
Maximum joist spans and beam spacing vary according to local code and your lumber provider's specifications, particularly actual lumber dimensions, lumber species and grade. However, an overview of various span tables provides a general idea of the maximum allowable distance between beams and buildings. For example, 2-by-12, "#2 or better" grade hem-fir lumber has a maximum span of roughly 17 feet. Stronger species' spans often near 20 feet.
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