November 11, 2019

Should pressure-treated wood deck structures be flashed to prevent rot?

Patio deck with stairs

Pressure treated wood continues to be a popular material for deck building in the United States, primarily due to its perceived cost and availability. A recent study of professional users of decking products (made up mostly of repair and remodeling professionals and deck specialists) was done to survey the perception of decking materials including naturally durable softwood, wood-plastic composite (WPC), pressure treated lumber, tropical hardwood, and thermally-modified wood (TMW). The survey data revealed that pressure treated wood was the top choice for budget-friendly decking projects costing less than $5,000, wood-plastic composites was the top choice for decking projects between $5,000 and $15,000, and tropical hardwood was the top choice for decking projects over $15,000. Durability and aesthetics were the most important perceived attributes among respondents when designing, constructing, or remodeling a deck.1

Pressure-treated wood deck framing

Is pressure treated lumber susceptible to wood rot and decay? As shown in the following photo, the answer is yes. Pressure treated wood can soak and lose moisture, resulting in the wood moving, cracking, twisting, bending, and virtually tearing itself apart.2 The knife-markings where the treating takes place and the holes created by decking screws create an entry point for water to penetrate into the timber. A compromised structural integrity from rotted pressure treated wood can lead to the risk of a deck collapse.

Pressure-treated wood rot

The lifespan of pressure treated deck structures can be extended by flashing the joists and beams with joist tape. Imus Seal® Butyl Joist Tape diverts water away from joists and beams, seal decking screws and fasteners, is compatible with common decking materials, and can be used to prevent corrosion of metal deck hardware by providing a barrier between the metal and treated wood.

Deck with pressure-treated wood structure

1 Professional Consumer Perceptions of Thermally-Modified Wood | Forest Products Management Development Institute | University of Minnesota
2 Wood Myths: Facts and Fictions About Wood | Building and Construction Technology | UMass Amherst