Calculating Joist Spans for a Deck

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Posts, beams, joists, and decking comprise the structural members of a deck. Their spacing and sizing are critical to ensuring a deck is safe and secure. Building codes vary, and it is up to you to contact your local building department for guidance in designing your deck.

When calculating allowable spans be aware that specifications vary depending on the type of wood you use. Also, spans often change for a deck that is higher than 12 feet. And when you change the size or spacing of one structural member, it can affect the size and spacing of others. For example, you could use fewer posts, spaced farther apart, if you use a larger beam.

The following tables list spans typically allowed for pressure-treated (pt) Southern yellow pine. The calculations that produce these spans assume that the deck must support a load of 50 pounds per square foot. That figure breaks down into 10 pounds per square foot of "dead weight" (the weight of the construction materials) and 40 pounds per square foot of "live weight" (the weight of people and objects on the deck).

Continued on page 2:  Chart: Deck Lumber Spans


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