Deck Joist Span Calculator

Reading the joist span table

IRC Table R507.6 gives the longest span allowed for each joist size and spacing, for three species groups. The span is measured from the face of the ledger (or the centerline of the supporting beam) to the centerline of the beam at the far end. It doesn’t include the cantilever past the beam. The table assumes:

• 40 psf live load plus 10 psf dead load, which is a normal residential deck with no hot tub and ground snow load of 40 psf or less;
• No. 2 grade lumber, adjusted for wet service, with an incising factor for treated southern pine;
• a deflection limit of L/360 on the main span and L/180 on the cantilever, which is also checked with a 220 lb load at its tip.

The 2021 IRC adds rows for 50, 60 and 70 psf ground snow loads, and limits cantilevers by the joist’s back span rather than by spacing. Choose the edition your building department uses and your snow load. The calculator reads the matching official table.

Where reproductions of the table disagree

Published copies of this table don’t all match. Three 2x6 cells differ between sources, for example Douglas fir 2x6 at 16″ shows as 8′-4″ in some and 8′-8″ in others. In each case we use the shorter span. The methodology lists every cell and source, and the full tables are on the deck span tables page.

Span isn’t the only limit

Decking manufacturers set their own maximum joist spacing, commonly 16″ on center for composite boards and 12″ for diagonal layouts, and that is usually stricter than the framing table. A deck that is code-legal at 24″ spacing may void a board warranty. Check the board spec sheet before choosing spacing. See deck joist spacing for more.