According to a study of deck-related injuries during 2003-2007, failing deck rails (guardrails) were the cause of 33 percent of the structural failures that caused a visit to the emergency room. Failing deck rails caused 26,640 injuries in that 5 year period, nationwide. The Seattle Building Code (SBC) section 1607.8.1 and Seattle Residential Code (SRC) Table R301.5 both require that guards (the code term now used for guardrails) be designed to resist specific forces that simulate an adult falling against or pulling/pushing on a guard. Guards must be able to resist a 50 pounds per linear foot uniform load and, separately, a 200 pound concentrated load. You are required to provide a detailed drawing showing the connection of the guard posts to the structure when you submit your permit application.
Design professionals and building owners should not assume that connections designed in the field will have the required strength. Testing has shown that the ½” bolts typically used as connections usually fail.
For most single-family and duplex buildings, if you plan to connect 4” x 4” guard posts on the outside of the deck rim joist, you can use prescriptive connections as an alternative to an engineered design. These prescriptive connection details are available in the Design for Code Acceptance 6 publication by the American Wood Council (www.awc.org/publications/DCA/DCA6/DCA6-12.pdf). Connecting a guard to the top of a deck typically requires an engineered detail. For larger buildings, we require an engineered connection for guards. For guidance on engineered designs, see the Guards and Connections White Paper at www.seaw.org/assets/docs/WhitePapers/
If you have an existing wood-framed deck, please note that they deteriorate over time. We recommend that you inspect your deck for rotted framing, guards, and connections this summer and every year.