Designing for Durability

Strategies for achieving maximum durability with wood-framed construction
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Code Requirements

To avoid decay caused by fungi and termite infestation, it is critical to separate untreated wood from the ground and other moisture sources, including concrete less than 6 inches above finished soil level. Such separations are required by current codes, including Section 2304.11 of the International Building Code (IBC). This section addresses protection against decay and termites, and sets out requirements for non-residential construction applications, and for wood used above ground for framing, decks, stairs, and similar features.

To protect framing at the connections to foundation, 2012 IBC Section 2304.11 requires that specific wood framing members of a building be protected against decay and termites. Protection can be provided by using either naturally durable or preservative-treated wood.

Western red cedar’s unique properties make it ideal for weather-resistant applications, including roof shingles, decking, exterior siding, and exterior cladding. At the Cascades Academy of Central Oregon, western red cedar cladding is arranged in vertical sections separated by glazing to reflect the rhythm of scattered junipers and pines.

Photo: 2014 WoodWorks Wood Design Award, Hennebery Eddy Architects, Inc.; Josh Partee Photography

Western red cedar’s unique properties make it ideal for weather-resistant applications, including roof shingles, decking, exterior siding, and exterior cladding. At the Cascades Academy of Central Oregon, western red cedar cladding is arranged in vertical sections separated by glazing to reflect the rhythm of scattered junipers and pines.

Wood that is naturally durable for decay and termites, as defined in IBC Chapter 2, includes the heartwood of redwood, Alaska yellow cedar, eastern red cedar, and western red cedar. Preservative-treated wood typically needs to comply with IBC 2303.1.8. Preservative-treated southern pine is distributed throughout much of the United States, while preservative-treated Douglas fir and hem-fir are also commonly distributed in the western states. Preservative treated Douglas fir and hem-fir are often incised to improve their treatment absorption. Because the incising process cuts a small percentage of wood fibers in the lumber, for structural applications, the reduction in strength must be accounted for using the incising factor prescribed by the referenced design standard, the National Design Specification® (NDS) for Wood Construction, available from the American Wood Council.

The required use of treated wood or a naturally durable species is dictated by the proximity of the wood to earth or to concrete in contact with earth.

The IBC requires specific wood framing members of a building to be protected against decay and termites. Protection can be provided by using either naturally durable or preservative-treated wood.

Photo courtesy of BS&S Treated Lumber

The IBC requires specific wood framing members of a building to be protected against decay and termites. Protection can be provided by using either naturally durable or preservative-treated wood.

Sill plates and wood sleepers on concrete in direct contact with the earth, such as a foundation wall or slab-on-grade, require protection. Sills can be protected from moisture migration from the concrete by installation of a sill sealer between the concrete and sill. Such sill sealers are often self-adhesive closed cell foam ribbons installed on the concrete before placing the sill. Not only do sill sealers act as a capillary break for moisture migration, they also serve as an air seal, eliminating potential gaps beneath the sill, and as a small thermal break between the concrete and framing.

In raised floors, wood joists and underlayment are best constructed at least 18 inches from exposed ground and wood girders at least 12 inches from exposed ground. With less clearance, the IBC requires the members to be naturally durable or preservative-treated wood.

Height above ground of non-treated wood elements. Based on the U.S. model building codes and American Wood Council recommendations.

Courtesy of the American Wood Council

Height above ground of non-treated wood elements. Based on the U.S. model building codes and American Wood Council recommendations.

At the exterior of the buildings, it is good practice to provide vertical clearance between the ground and any wood members. A stem wall in a raised floor or a curb made of concrete or masonry on a slab-on-grade foundation can provide the required clearance. Alternatively, the top of a foundation slab is located at an elevation to provide the desired vertical clearance at the exterior edges. Wood framing members, such as studs and wood structural sheathing, are best located at least 8 inches above earth. Below that, IBC 2304.11.2.2 requires wood framing on foundation walls to be naturally durable or preservative-treated wood.

Once delivered to the construction site, wood should be stored and wrapped properly.

Photo courtesy of KK Law

Once delivered to the construction site, wood should be stored and wrapped properly.

 

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Originally published in Architectural Record

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