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 U.S., while Douglas fir and hem-fir are also commonly distributed in the western states as preservative-treated. 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 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 Jerry Parks

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.

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.

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.

Construction: Keeping Wood Durable

Informed design is essential to durability, but encouraging proper material handling during construction will greatly improve the long-term performance of wood material in the building envelope. “While an architect's role typically does not extend beyond specification of wood to materials handling and construction practices, specifications can state S-DRY or KD lumber as well as 19 percent maximum moisture content at time of enclosure,” says Paul Morris, research leader, durability and building enclosure at FPInnovations. “It would also be ideal if architects provided recommendations on how wood should be treated during construction. Even something as elementary as requiring field treatment of saw cuts in treated wood to close the opening would help protect the structure's long-term durability.”

Weather Protection

Advanced planning to protect wood during construction can contribute to long-term durability. Material delivery should be timed as close to the installation date as possible. Too often wood construction products, even for interior use, arrive at the construction site and are left outside and exposed to the elements, absorbing moisture from rain, snow, puddles or soil. If wood does need to be stored for any length of time, a clean well-drained material storage area is critical. Upon arrival, wood should be inspected to see that wrappers are intact—faulty ones should be replaced with heavy tarp—and then stored 6-8 inches off the ground away from ponding water. If the soil is wet or poorly drained, a polyethylene tarp or a temporary gravel pad should be placed on the ground below as supports for the materials. The storage area should also be free of vegetation which can harbor insects, hamper ventilation and maintain moisture in the air.

After installation, the structure should be protected from rain and other sources of bulk water by installing roof coverings and moisture barriers on the exterior of the building envelope as soon as practicable. If elevated moisture levels exist, materials inside the building envelope should be adequately dried before installing insulation and other materials. It is best to avoid the premature application of interior membranes like gypsum wallboard. Allowing the wood framing to acclimate and lower its moisture content to the extent reasonable is highly desirable.

If lumber has been significantly wetted during construction, schedules should allow for drying of framing and sheathing materials to an MC of 19 percent or below. Moisture barriers, installed soon after assemblies are framed, will reduce exposure to weather. If needed, artificial heat and/or dehumidification can accelerate drying.

Once delivered to the construction site, wood should be stored and wrapped properly. Photo courtesy of KK Law

Buildings That Work

Over the years, research and experience have identified several key characteristics of wood buildings that last.

Dry Buildings

As has been discussed, controlling moisture is a key factor in ensuring durability. Wood meant for a dry structure must start out dry and stay dry. Buy dry wood, store it properly, design the building to keep the wood dry and observe proper maintenance practices. This is fundamental to an approach called “durability by design.”

For example, if wood is in direct contact with concrete outdoors, moisture can accumulate unless the concrete is detailed to drain away from the wood. Moisture can be a problem in basements because they are surrounded by soil and basement walls are subject to penetration by tiny amounts of water through concrete foundation walls. This can be prevented by use of an effective moisture barrier.