Q. My clients—who are on a fixed income—would like a small deck off the side entrance to their house. I’d like to build it as efficiently as possible to keep it affordable, and hope to use a simple single-ply 2-by beam supported by a pair of 4x4 posts. The problem is, I can’t find any span tables for single-ply beams or appropriate post-to-beam connection details in DCA6 to show my inspector.
A Glenn Mathewson, a building code consultant and former inspector and plan reviewer in Westminster, Colo., responds: Here’s a case where I think (at least for now) you’ll have better luck if you check with your code book rather than DCA6, and here’s why: Starting in 2015, prescriptive deck codes finally began to appear in the IRC—the mainstream model building code. While this may sound like bad news for an industry that sometimes views building codes negatively, it’s actually quite the opposite. That’s because prescriptive deck codes spell out uniform design methods (like maximum joist and beam spans and connection details) that allow contractors to know what to expect from building departments. Prescriptive codes also allow building departments and inspectors to be more confident in approving decks without requiring review by a design professional or engineer.
The 2018 edition of the IRC built on this momentum by adding some flexibility with single-ply beams in Table R507.5 Deck Beam Span Lengths. This key beam-span table (which will appear in the yet-to-be released 2018 edition of DCA6) has five new rows for 2x6 through 2x12 single-ply beams. The spans aren’t huge, but they help the IRC be what it is supposed to be—a minimum standard.
Intermediate stair landings are a prime example of a place where a single-ply beam can be an appropriate framing solution, even for above-code builders. In most cases, a two-ply beam at the front of a stair landing isn’t structurally necessary, because (according to Table R507.5) a single-ply 4-foot-11-inch 2x6 beam can support joist spans up to 6 feet.
This prescriptive solution is also good news for folks who are living on a budget and might prefer a small deck instead of a cheap concrete paver patio. The single-ply beam spans from the IRC’s prescriptive table allow for a simple 7x7 deck or 8x8 deck with cantilevered joists using only single members, a ledger connection, and two footers. That’s an economical solution.
If you’re going to have single-ply beams for budget decks, you might as well have a notched and bolted post-to-beam connection, especially since hardware manufacturers don’t offer much in the way of hardware that is designed for single-ply beams. The problem with that plan is that the 2015 IRC only offers a notched post-to-beam connection detail for a 6x6 post to a two-ply beam, which calls for a 2 1/2-inch flange beside the notch to bolt the beam to. However, only a 2-inch flange is left when a 4x4 is notched for a single-ply beam. Fortunately, the 2018 IRC takes care of this problem by including a new detail for notched 4x4 posts and single-ply beams, which specifically allows for the narrower, 2-inch flange.
