Most deck builders probably follow the IRC’s familiar notching and drilling guidelines—no notches in the middle third of a joist and no holes closer than 2 inches from the top or bottom edge of a joist, for example (see 2012 IRC, Figure R502.8). But what most deck builders don’t realize is that those prescriptive guidelines are intended for framing in dry locations only. The IRC is silent on this topic in the deck section, and the Prescriptive Residential Wood Deck Construction Guide, DCA 6, only addresses the notching of guard posts.
For any outdoor, uncovered application such as a deck, almost all lumber and connection design values, which are based on the National Design Specification (NDS) for Wood Construction, are subject to wet-service adjustments (penalties). Prescriptive documents such as DCA 6 already account for those wet-service adjustments in their span tables and background calculations for fasteners, but not so for notches and holes. Joists and beams that are subject to repeated wetting and drying should not be modified unless it is part of an engineered design.
John “Buddy” Showalter
American Wood Council