Deck beam span and cantilever — posts set the span; the beam overhangs the post by ¼ of it
Posts set the beam span; the beam may cantilever past the end post up to ¼ of that span (IRC R507.5.1). This is a separate limit from the joist cantilever (¼ of the joist span, past the beam) — don't mix the two up.
What this diagram shows
An elevation of a deck beam carried on two posts down to footings, with joists bearing on top of the beam. The distance between the two posts is the beam span. Past the end post the beam cantilevers, limited to one-fourth of the beam span (IRC R507.5.1). A callout stresses that this is a separate limit from the joist cantilever, which is one-fourth of the joist span past the beam — the two cantilevers are independent and must not be confused.
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Deck Beam Span Calculator
How far can a deck beam span? Free calculator for beam size, ply, post spacing, footing & cantilever from the joist span it carries — per IRC R507.5 / DCA 6.
