2015 International Residential Code Brings Big Changes to Decking

The 2015 edition of the International Residential Code brings the biggest update of deck codes the IRC has ever seen. We asked Glenn Mathewson, a municipal building inspector and deck construction expert based in Colorado, to provide an overview of some of the biggest changes:

Largely based on prescriptive tables from the American Wood Council’s DCA6 document, many of the new code provisions may look familiar.

The 2009 IRC saw deck ledgers addressed, the 2012 set up a section in the IRC specific for decks, and the 2015 brings joist and beam span tables, as well as post sizing and foundation provisions. The still ambiguous lateral load provisions for decks have not been completely put to rest, and the notorious anchor detail remains. However, a second “permitted” lateral load anchor detail has been included.

Joist and Beam Span Tables
The joist span tables that have long been in the IRC are not based on a wet-use environment or for incised, treated lumber, both conditions that reduce the maximum allowable spans. The 2015 code adds a new joist span table that accounts for these conditions and includes new species and sizes, such as redwood and cedar. The new table also reflects the design value changes to Southern pine published in 2012. Many builders may find spans reduced from what they are accustomed to; however, the maximum allowable cantilever for deck joists is also clearly defined, and the limit of 1/4 the joist backspan is a generous design option.

Most helpful to the industry, a beam span table is now included, and it’s also based on all the proper conditions and design properties. It includes 2-ply and 3-ply beams and heavy timber Douglas fir beams. Creating a pre-engineered beam span table is no easy feat, and many design assumptions have to be made. The table is based on joist span, as that reflects the load the beam carries; however, it is also based on the joists cantilevering their maximum allowable distance beyond the beam. When there is no joist cantilever, the maximum beam spans are very conservative. Similar to joists, beams are allowed to cantilever beyond the end post by up to 1/4 the adjacent span of the beam.

Sizing Provisions for Posts
Sizing provisions for posts are included, appropriately limiting their length based on their size. 4×4 and 4×6 posts are permitted up to 8′ in length, and 6x6s up to 14′.

Lateral Load Connection
For those seeking relief from the invasive lateral load detail that requires access inside the floor cavity, a second “permitted” detail is included that is installed entirely from the outside of the building.

Foundation Details
Foundations details are also provided, allowing for various design approaches, such as embedding posts into pier foundations and how deep the embedment must be.

While the foundation provisions aren’t complete, they are a great start to the subject.

Stairway Lighting
Requirements for exterior stairway lighting were revised to be more understandable and clearly require a light at the top landing.

These are just some of the highlights of new provisions coming in the 2015 IRC that tighten the reins on the decking industry, something that’s been a long time coming. However, they are still a work in progress. Negotiations and research are already underway for 2018 IRC proposals (due in 2016), and you are welcome to contribute.

Glenn Mathewson is a municipal building inspector and deck construction expert in Colorado. He also advises the North American Deck and Railing Association on code issues and provides free education online at buildingcodecollege.com.

Melinda Stiefel
Melinda Stiefel