DECK STABILITY

The requirement for diagonal bracing when a deck is greater than 2 feet above grade is applicable to both free- standing decks and decks supported by a ledger. If the deck is attached to the house an alternative to knee braces for deck stability is installation of decking at a 45 degree angle to the deck joists. Decks are stiffer laterally with diagonally oriented lumber decking versus decking installed perpendicular to joists.  With respect to hold down tension devices, IRC R502.2.2 requires anchorage to the primary structure for both vertical and lateral loads as applicable. Further, the IRC includes hold down tension devices as a prescriptive means to achieve compliance with the lateral load connection requirements without requiring engineering. was taken directly from the FEMA Home Builder’s Guide to Seismic Resistant Construction (FEMA 232) which refers to it as an “above-code recommendation.” In lieu of the prescriptive hold down tension device specified, an alternate engineered connection detail is permitted or the deck can be designed to be free-standing. While Item 7 of DCA 6 Minimum Requirements states that the document does not address wind or seismic design issues, some interpret R502.2.2 to be applicable in all cases since lateral loads can be developed by other sources including people moving or dancing on a deck. Another interpretation is that the term “as applicable” in R502.2.2 means the provision is only required for code prescribed loads. The only code prescribed lateral loads are wind and seismic. Where deck joists are perpendicular to the house floor joists, blocking between house joists and boundary nailing of the house floor diaphragm to the blocking is required.

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