Decks in Washington must now be built to support live loads of 60 pounds per square foot, thanks to an amendment to the Washington State Building Code (SBCC) that took effect last fall. However, many of the state's builders and homeowners may be unaware of this new requirement, because it’s buried in the SBCC’s modified version of IRC Table R301.5, a list of minimum uniformly distributed live load values for various building components. The minimum live load for decks and balconies in the 2015 IRC, DCA6-12, and most other state and local building codes remains at 40 psf.
But that may change, at least in some states. Prior to 2009, decks and balconies occupied separate lines in Table R301.5, and had different live load requirements: 40 psf for decks, and 60 psf for balconies. In the 2009 IRC, decks and balconies were combined, with a 40-psf live load requirement for both. Two code changes (RB26-16 and RB27-16) were proposed last year for the 2018 IRC to modify the "live load" table for a third time, to basically raise decks and balconies to 60-psf live load. According to the author of the proposal, Jonathan Siu, principal engineer with Washington Association of Building Officials Technical Code Development Committee, the amendments' purpose was to align the live load table in the IRC with the live loads in ASCE 7, "Minimum Design Loads for Buildings and Other Structures." “The ASCE determined through their deliberative process that the live load on these structures should be 1.5 times the live load for the area the deck or balcony serves. This more or less corresponds to the 60 psf required in the 2006 and earlier IRC’s for balconies," writes Siu. While RB26-16 and RB27-16 were defeated, code officials in other states have taken notice, perhaps partly in response to the fatal balcony collapse in Berkeley, Calif., in 2015, and may take Washington's lead on this issue.
In its rush to change the live load requirement, the SBCC didn't appear to coordinate Section R507 in the IRC, the 6 ½-page section devoted to decks, which is filled with prescriptive joist and beam span tables based on 40-psf live loads. To address this discrepancy, the state has issued Building Code Interpretation No. 16-11 (available in PDF form as a download from SBCC), which contains revised span and ledger-connection tables based on the new 60-psf live loads. Developed according to the AWC’s National Design Specifications (NDS), the tables show joist spans reduced by about 11%, beam spans that are reduced by about 8%, and of course reduced spacing for ledger bolt connections. Deck-footing values will also need to be adjusted, but this issue isn’t addressed in the SBCC interpretation.
To account for reduced joist and beam spans and increases in the number of fasteners and the size of footings, the SBCC estimated that building a deck to meet the new standard could add as much as $10 per square foot to its cost. In opposing the change to the 2018 IRC, the NAHB estimated that proposals RB26-16 and RB 26-27-16 would raise the cost of a new house by $75 to $154.
Deck builders in the state are understandably concerned about the extra costs that the measure will add to the price of a new deck. Tim Nogler, managing director of Washington State Building Code Council, points out that further proposals to amend the code—including rolling the live load requirement for decks back to 40 psf—can be made at any time (there's an online form for submitting proposals). He says that any new proposals will then be put on the agenda for discussion at the next meeting of the SBCC, which is scheduled for May 12, 2017.