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.
Navigating the Maximum Deck Beam Span Table
Table R507.5(1) “Maximum Deck Beam Span - 40 PSF Live Load” in the 2021 IRC applies to decks built for 40-psf live loads and is similar to the tables in the previous codes. Now, with new tables for 50-psf, 60-psf, and 70-psf snow loads, deck builders in high snowfall regions around the Great Lakes, northern New England, and mountain areas can size beams without consulting an engineer or working through the engineering calculations themselves. The tables are numbered and titled successively: Table R507.5(2) “Maximum Deck Beam Span - 50 PSF Snow Load,” and so on for 60-psf and 70-psf snow loads. As logic would reason, the increase in snow load results in shorter maximum deck beam spans in the corresponding table.
In this article, I’ll use the 40-psf table for beam-sizing examples, but the same process would apply using the other three tables.
Maximum beam span. The maximum beam span lengths listed in the field of the beam table are controlled by three factors: the species of wood, the size of the beam (depth and number of plies), and the span of the deck joists. Though it may seem odd to factor in the joist span for deck beams, the joist span is a simple proxy for the tributary load that is borne by the beam. These factors become the rows and columns that feed into the field of the table where the beam span lengths are listed.
Effective joist span length. The major column, “Deck Joist Span,” in the previous versions of the beam table was retitled “Effective Deck Joist Span Length” in the 2021 IRC. The change may seem minor, but the new title corresponds to a new table, R507.5(5) “Joist Span Factors for Calculating Effective Deck Joist Span”.
As in previous code versions, the beam span lengths listed in the field of the table are based on the joist span plus the maximum cantilever listed in Table R507.6 “Maximum Deck Joist Spans,” which was the focus of my article, mentioned earlier, in the March issue.
When you size a beam directly from the joist spans listed on the table, the beam will support the maximum span of a given joist depth and the maximum joist cantilever allowed. But if your deck design has a flush rim beam without any cantilever, or if the cantilever is less than the maximum allowed, then you can employ the new “joist span factors for calculating effective deck joist span” to right-size the beam for your deck design and span greater distances between posts.
Shown at the end of the article are three examples to demonstrate how the cantilever distance influences the beam span. To keep things simple, snow loads for these examples are less than 40 psf, so I’ll use Table R507.5(1).
Navigating Table R507.5(5) “Joist Span Factors for Calculating Effective Deck Joist Span”
Table R507.5(5) is the engine behind the effective deck joist span length. It’s a workaround used in conjunction with the beam tables that adjusts for the lighter load a beam has on it when there is no joist cantilever or when a joist cantilever is shorter than the maximum allowed.
The left column of Table R507.5(5) is a series of fractions. The column is labeled “C/J,” with C being the actual joist cantilever length and J being the actual joist span length.
The right-hand column lists joist span factors. These are the numbers you multiply the actual joist span by to determine the effective deck joist span length on the beam table. You’ll notice that most of the joist span factors are numbers less than 1. So when you multiply the actual joist span length by a number less than 1, the resulting effective deck joist span length measurement will be shorter. When that measurement is applied to the beam table, it will result in a greater beam span than you get when you use the actual joist span length.
It’ll make your head spin the first few times you use it. I found the trick is to avoid confusing the decimal values in the left and right columns.Below, you’ll find an example of the steps involved in solving an “Effective Deck Joist Span Length” problem. The deck will be built with 2x10 SYP deck joists with a 14-foot span between the beam and ledger, and a 1-foot-1 1/2‑inch cantilever (the fact that the joists are 2x10 SYP is irrelevant to the calculation).
After determining the joist span factor and calculating the effective deck joist span length following step 1, refer to the beam span table to find the maximum beam span allowed. In this example, the effective joist span length is 10 feet 15/16 inches, which I rounded up to 10 feet 1 inch.
Unless you’re lucky, the effective joist span calculated won’t match one of the column lengths listed, in which case you have a few options. The simplest is to round up to the column with the next longer length, in this case from 10 feet 1 inch to 12 feet, then read the maximum deck beam span length associated with the beam size you’re planning. For double 2x10 southern pine beams in this example, that measurement is 7 feet 4 inches, as shown (Option A below).
Interpolation. If you want a more precise measurement of the beam span, you can interpolate between deck-beam-span values that fall between the next lower and next higher columns associated with the effective deck joist span length you came up with. In this example, again, you would be using the 10- and 12-foot joist-span columns.
In cases where the effective joist span falls less than halfway between the 10-foot and 12-foot joist-span columns, you can perform an analog interpolation (Option B below). This is a bit like rounding up, but rounding up to a column that could be in the table if it were graduated in 1-foot increments instead of 2-foot increments. An analog interpolation could even divide the values between the two joist-span columns by quarters rather than halves pretty easily.
In this example, the effective joist span (10 feet 1 inch) is rounded up to the next foot length (11 feet), which is halfway between the 10- and 12‑foot joist-span columns. Then split the beam length halfway between the spans listed (8 feet 0 inches and 7 feet 4 inches). The result is a beam span of 7 feet 8 inches.
For the mathematically inclined, you can actually calculate the interpolated value following the formula in the example shown here (Option C below). In this case, the calculated maximum beam span is 7 feet 11 11/16 inches.
As you can see, the difference between the estimated maximum beam span (7 feet 4 inches), the analog interpolation of the beam span (7 feet 8 inches), and the calculated interpolation of the beam span (7 feet 11 11/16 inches) isn’t huge. But the ability to eke out a few extra inches of span (and to be able to explain to your inspector how you did it) could make a difference in your next project.
Photo by Mike Guertin; illustrations by Tim Healey
