• Structural protection. For many decades, preservative-treated framing has been successfully used to deter termites. These days, wood products can be treated with borate (disodium octoborate, sometimes expressed by its oxide B2O3), a water-soluble chemical that is benign to humans, but kills insects that feed on it. Borates are used in many common household products, from eye drops to washing powder. Borate-treated products—from dimension lumber to plywood, OSB and cellulose insulation—are enabling termite-resistant structures. While suitable for framing and sheathing, borate-treated wood should not be used for outdoor applications such as decks and porches since the chemicals are water-soluble. A good quality coating, such as a three-coat film-forming finish, prevents the borate from moving out of the wood. Architects should consult the American Wood Protection Association (AWPA) standards for borate and other preservative treatments under specific conditions and types of termites.¹⁰

Preservative treatments can either be pressurized or non-pressurized. With pressure treatments, combinations of pressure and vacuum achieve a deep, thorough chemical penetration by infusing the wood cells with preservatives. Pressure-treatment preservatives consist of chemicals in either a water or an oil solvent: odorless water-borne options are more popular as they achieve a paintable or stainable clean wood surface. Depending on local codes, several types of wood preservatives may be used. With non-pressure treatments, the preservative can be applied via brushing, spraying or dipping of the wood member. Spray application of borates at the framing stage is recognized as an effective termite protection measure by some codes. For information on preservative-treated and naturally durable species, see the section below on code requirements.

• Surveillance and remediation. To ensure the effectiveness of the management strategies discussed above, regular inspections are necessary. Baiting may be used to detect the presence of termites. Tubes or traps holding pieces of untreated lumber, or “bait” are placed around the building. Termites feed on the bait, which can then be replaced with treated lumber that contains a slow-acting chemical that the insects bring back to the colony. Over a period of time, the colony is destroyed. The original untreated bait can then be restored to the traps to determine whether there are any remaining termites. Alternatively, the infested building can undergo either chemical fumigation or heat treatment, the former having the obvious disadvantage of toxicity and the necessity of evacuation for several days.

Both types of remediation must be done by licensed contractors and, while they will eradicate the termites, they do not protect against re-infestation. Vigilant monitoring is key to identifying sources of food and moisture, including leaks in downspouts or gutters, air conditioners, pipes, window or door seals, and poorly ventilated bathrooms. The type of action taken against termite colonies will vary widely depending on the type of termite, location and condition of the building.

Height above ground of non-treated wood elements. Based on the U.S. model building codes and American Forest & Paper Association recommendations. Chart courtesy of American Wood Council

Western red cedar's unique properties make it ideal for weather-resistant applications including roof shingles, exterior siding, exterior cladding, and decking. Shown here, the Tamarack Ski Lodge Heavenly Lake Tahoe Ski Resort, South Lake Tahoe, California, designed by Collaborative Design Studio. Photo courtesy of Carrie Compton

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), which 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.