How Far Can a Deck Joist Span?
I asked my local building official how he checks a deck plan to make sure the joist size and spacing combination will meet the code. He replied he goes to the 40 lb. live load floor joist span tables in chapter 7 of the state residential code (based on the 2012 IRC). And that’s just what many deck builders, designers and inspectors have probably done for years. The problem is that those tables are intended for interior floors, not decks. Decks get wet and may be framed with incised treated lumber so span reductions need to be applied.
Until now, the IRC didn’t have deck joist span tables but the 2015 IRC includes a new one (Table R507.5) that will make it easy for deck builders to spec a joist size for a given span and joist spacing. The table includes spans for common lumber species used for deck framing like southern pine, Douglas fir, hem-fir, redwood, western cedar and others. The table accounts for wet service conditions and an incised lumber reduction for species commonly incised for pressure treatment penetration.
The table has two column sections – one for joists without a cantilever and one for joists with a cantilever. Span entries are listed for 2×6 through 2×12 joists and 12 in., 16 in., and 24 in. on-center spacing.
The illustrations (one for cantilever joists and one for flush rim-beam hung joists) point out where joist span measurements are taken from – the saddle of the ledger mounted joist hanger to the center of a dropped beam or saddle of the hanger on a flush beam.
Section R507.5 Deck Joists of the code text notes that deck joists can cantilever up to 1/4 of the joist span. But you’ll notice when you look at the portion of the span table for joists with cantilevers that the joist span for decks with a cantilever is often less than the span for the same sized joist without cantilevers. The next subsection requires that solid blocking is required over the beam when there is a cantilever.
Even if your local code jurisdiction hasn’t adopted the 2015 IRC, most building officials will look favorably on an ICC sanctioned code provision. So run the table by your building official for approval before using it.
The 2015 IRC includes a new table R507.5 Deck Joist Spans for Common Lumber Species. The table makes it easier for deck builders to accurately size deck joists and for inspectors to check decks for adequate framing.
Figure R507.5 Joists with Dropped Beam shows the optional cantilever and the end points of the joist and cantilever spans.
Figure R507.5 Joists with Flush Beam shows a rim-beam and the end points where the joist span is measured from.