Recommended Beam & Joist Spans for Deck Framing

International Residential Code

• The International Code Council, the umbrella organization that is responsible for the administration of all commercial and residential building codes, provides recommendations and requirements related to deck construction. These elements are found in the International Residential Code (IRC), a set of recommendations and requirements that are revisited and updated about every three years. The deck requirements from the IRC of 2009 are the most current version that must be referred to in all residential deck construction.

Deck Joist Span

• The factors involved in all deck and joist span requirements are the type and size of the wood members as well as the size of the deck itself. The larger the deck and, consequently, the longer the span, the more you have to add beams and joists. For instance, 2x6 Southern Pine joists that are spaced at 16-inch intervals should not exceed 9 feet 9 inches in span length. The 2x6 joists made from Western Cedar, spaced at the same intervals, should not exceed 8 feet 7 inches in span length.

Deck Beam Span

• The span and frequency of the beams that support the joists depend on the spacing of the joists as well as the size of the beam and type of wood it's made from. The deck beam span will also affect how many posts you will need to support the structure. A solid 4x6 Redwood beam supporting a joist span of 10 feet or less can be no longer that 4 feet 11 inches, which will affect the number of posts supporting the beam according to the size of the deck.

Factors Affecting Beam and Joist Span

• Although the IRC recommendations and requirements are the guideline for deck construction, your local building codes may have variations that affect the span of the beams and joist. Additionally, the IRC only provides a bare minimum of recommendations for deck construction that can allow alternative approaches, according to Glenn Mathewson of Deck Magazine.

  Mr. Mathewson suggests that the Prescriptive Residential Deck Construction Guide, written by the American Forest and Paper Association, is but one reference the IRC uses for deck construction requirements. He also states that this deck construction guide contains elements that differ from or expand on the basic IRC deck codes. This opinion, coupled with the variance in local codes, would suggest that you should always consult your local code enforcement agency before starting your deck project.