If you’re using the beam sizing tables in older versions of the International Residential Code (IRC), you’re probably oversizing your deck beams. In this article, I’ll focus on how to read the new maximum deck beam span tables in the 2021 IRC, and explain how to use the code’s new “joist span factors” table to determine the “effective deck joist span length” so you can get the greatest span out of a beam (don’t worry, it’ll all make sense shortly). I’ll refer to my previous article, “Right-Size Your Deck Joists” (JLC, Mar/23), which contains information about measuring joist span. That article also explains the differences between dead, live, and snow loads, which are the different loads the code tables are adapted for.
A Work in Progress
The 2015 IRC simplified sizing deck beams with the introduction of Table R507.6 “Deck Beam Span Length.” That table presented the maximum spans for a series of multi-ply, built-up-dimensional-lumber beams from a double 2x6 through a triple 2x12, with beam spans based on the span of the deck joists from the ledger to the beam. Joist span is a proxy for the tributary area of deck that is supported by a beam and—in turn—the load on the beam.
In the 2018 edition of the IRC, more beam options were added to the southern pine species group: single-ply 2x6, 2x8, 2x10, and 2x12 beams. While most deck builders don’t think of a single two-by as a “beam,” singles can indeed act as beams and may be a good option, especially for small decks and landings (see “Single-Ply Beam Solution,” February 11, 2019).
Limitations. One of the problems with the 2015 and 2018 IRC beam tables is that they can be applied only to decks with up to 10-psf dead loads and 40-psf live loads. Anyone building decks in areas with a snow load greater than 40 psf can’t use the tables to accurately size beams.Another issue is that interpolation isn’t explicitly permitted between field values in the 2015 and 2018 tables. That means that when the actual joist span falls between two joist-span columns, you have to round up to the next longest joist span, resulting in a shorter beam span than is structurally necessary for a given size beam.
In addition, those beam tables address only two species groups: southern pine in one group and everything else in the other. Grouping pressure-treated Doug-fir, hem-fir, and SPF; redwood; western cedars; ponderosa pine; and red pine all in the same category penalizes the stronger species in the group by limiting the beam spans to those of the weaker species.
Finally, the beam tables in both the 2015 and 2018 IRC presume that the deck is designed with a dropped beam and the maximum joist cantilever allowed. This results in oversizing beams, sometimes substantially, when a deck design has a shorter cantilever or no cantilever at all, as when you’re framing a deck with a flush rim beam.
Expanded 2021 IRC beam tables. Some of the shortcomings that are outlined above are addressed in the beam tables in the 2021 IRC, making it possible to size beams more precisely to use lumber efficiently and to minimize the number of footings. The improvements include:
· Three new beam span tables for snow loads greater than 40 psf. · Three beam species groups instead of just two.
· A series of rows for single-ply beams added to each species group (not just for southern pine).
· Interpolation is explicitly permitted and can be used to refine the beam span when a joist span length falls between the columns listed on the beam table.
· A new adjustment table alleviates oversizing beams on decks that don’t have the maximum joist cantilever.
