Designing for Durability

Strategies for achieving maximum durability with wood-framed construction
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Termite Control

Insects, mainly termites, carpenter ants, and powder post beetles, can cause significant property damage, including (though not limited to) wood-frame structures or wood- or cellulose-based components.

They will also attack fixtures and fittings, furniture, cardboard boxes and books. Of the offending insects, subterranean termites cause the most damage, particularly the non-native species known as the Formosan subterranean termite. Though found only in the southern United States, it is responsible for most of the value loss due to insect damage in that area.

The current thinking is that total eradication of termites from certain areas is unfeasible, and a more realistic approach involves containing existing insect populations and limiting risk to buildings. As with the moisture control strategies described above, the strategy for combating termites is based on the assumption that none of the following measures are 100 percent effective on their own, but, in combination, create conditions where the risk is negligible. The six-S strategy includes the following lines of defense:10

Suppression: This involves reducing termite populations in a particular area and preventing their spread to new areas. An “area” can range from an entire state (though usually only if the termite is a recent introduction and still has limited distribution) to a single home. Methods of suppression include locating and destroying termite colonies, burning infested wood, and heat-treating reclaimed lumber.

Site management: Careful site preparation and clean-up helps to reduce the potential for termite infestation. If individual trees, forests, or orchards have been cleared, it is essential to completely remove tree stumps and other buried wood from the site, as well as construction debris that may contain cellulose. Water should be drained away from the site of the building. Wooden formwork for concrete should never be left in place.

Soil barriers: Treating the soil is a popular and cost-effective option for protecting against conditions that lead to termite damage, especially in states like Louisiana and Florida where termites are a major issue. According to the North Carolina Department of Agriculture, the optimum time to protect against termites is during planning and construction, with the key objective of establishing a chemical barrier between the soil, where termites live, and the wood in the building.

Another option, which is sometimes used in California and Hawaii, is a physical barrier, such as precisely sized silica sand, crushed basalt, or granite beneath slab foundations and other locations. Optimum particle size ranges from 1/16 inch to 1/10 inch in diameter, as these particles are too heavy for the termites to move, and the spacing between them too small to move through. A more recent development is termite mesh to wrap the foundation, which protects at-grade or below-grade penetrations of foundations and slabs. Sufficiently fine mesh—1/32 inch—is recommended. Stainless steel mesh installed by licensed contractors will resist corrosion, though it can be costly.

Termiticide-impregnated membranes have also been used as a barrier, mostly in Japan, but don’t have a long track record. They can be expected to have similar characteristics to termite shields in that they require very careful installation, and termites may be able to get around them. Termite bait systems can also be used as a substitute for the soil barrier, but require a long-term commitment to a contract with a professional company.

Slab and foundation details: Slabs and foundation walls should be designed to inhibit the entry of termites into the building and to facilitate inspection for termite shelter tubes. If hollow concrete masonry units (CMUs) or double walls are used in foundations, they must be capped with concrete or masonry to eliminate holes. All exterior slab edges or foundation walls must be kept free of cladding for a height of 6 inches from the finished soil level to allow inspection for shelter tubes. This separation of cladding from the ground must allow for any landscaping to be done immediately after construction or likely in the future.

It’s important to note that there should be no contact between the building woodwork and the soil or fill material. For best results, exterior woodwork should be at least 6 inches above ground and any wood in contact with or adjacent to the soil should be preservative treated.11 (For more details, see the section on code requirements below.)

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.12

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 on code requirements.

Surveillance and remediation: To ensure 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.

Installing termite shields has proven to be an effective means of control.

Image courtesy of the American Wood Council

Installing termite shields has proven to be an effective means of control.

An example of termite control for slab-on-ground construction.

Image courtesy of the American Wood Council

An example of termite control for slab-on-ground construction.


[ Page 7 of 10 ]             
Originally published in Architectural Record
Originally published in December 2015