Wood is susceptible to termites, other insects, and fungal decay. Wood that has contact with the ground is more susceptible to termite and rot damage than wood that remains above ground. The best practice when building outdoor wooden structures or those that will be exposed to moisture is to use pressure-treated wood. Some examples are permanent wood foundations (PWFs), framing against concrete or block basement walls, wooden decks, wood trim, garden structures, mailbox and light posts, and the like.

Pressure treatment is a process that forces chemical preservatives into the wood. The wood is placed inside a cylinder, the cylinder is closed and vacuum-sealed, and the preservatives are forced into the wood via pressure. The preservatives help protect the wood against termites and fungal decay for decades. Some producers of pressure-treated wood even offer warranties that last for the lifetime of the homeowner.

Pressure-treated wood is available in a variety of lumber grades, from knot-free, close-grained grades to grades with more knots, splits, and wane. A full range of sizes is available, from 1x4 boards to 6x6 timbers, in a variety of lengths, plus plywood. Re-dried wood, usually marked KDAT for “Kiln Dried After Treatment” is also available. KDAT wood is lightweight, less prone to warp, and can be painted without a waiting period, unlike non-KDAT wood. Some pressure-treated wood products also include a water repellent to keep the wood looking good longer (note: while the preservative treatment protects wood against termites and rot, it does not prevent moisture damage which can cause warping, cracking, and deterioration of appearance).

Using pressure-treated wood is an environmentally responsible choice, too. The majority of trees used are plentiful and fast-growing, and they are grown on managed timberlands. Further, treated wood requires less energy to produce than alternative building products, and the preservatives are manufactured, in large part, from recycled materials.

Several types of pressure-treated wood are available for use in residential construction. Two main types are borate-treated wood and copper-treated wood, as explained below. One well-known type that’s no longer available for residential construction is CCA (chromium, copper, arsenic). Once the most widely used preservative, CCA use was restricted beginning in 2004 by the EPA, in a voluntary agreement with the pressure-treating industry, to reduce arsenic use and improve environmental safety. CCA is a toxic preservative that must be handled with caution. For CCA structures built prior to 2004, the EPA recommends applying a penetrating oil finish to lessen or eliminate human and animal exposure to the preservative. Today, CCA-treated wood products are used only for some industrial, highway, and agricultural applications.

Borate-treated wood
Borate preservatives are salts such as sodium octaborate, sodium tetraborate, and sodium pentaborate that are dissolved in water, and are effective preservatives against decay, fungi, and insects. Borate-treated lumber has low toxicity levels for people and pets, is odorless and non-volatile (no negative effects on indoor air quality), has fire retardant properties, and is paintable and stainable. Borate pressure-treated wood also has excellent to outstanding corrosion resistance to common metals, according to standards established by the American Wood Preservers’ Association (AWPA), which means no special fasteners or flashings are required. Its uses are limited, however, to applications that are not in contact with rainwater, standing water, or ground contact, because the borate preservative leaches from wood when the wood gets wet. Borate-treated wood can be used, however, in contact with concrete.

Alkaline copper quat (ACQ)-treated wood
One of several recently developed wood preservatives, ACQ contains copper and a quaternary ammonium compound to protect wood against decay, fungi, and insects. Multiple variations of the preservatives are used, including: ACQ-B (ammoniacal copper quat); ACQ-D (amine copper quat); and ACQ-C, formulated with either ammonia or amine and a slightly different quat compound. Currently ACQ-D is the most commonly used. A benefit of ACQ-treated wood over borates is that it can safely be used for applications where the wood is in direct contact with the soil and/or water, such as permanent wood foundations, deck pilings, and deck boards. A drawback to using this material is the fastener requirements, because ACQ is highly corrosive to steel. Triple-galvanized or stainless steel hardware and fasteners must be used with ACQ-treated wood. These fasteners are more expensive than others, and structural failure can occur over time if the incorrect fasteners are used.

Copper azole (CBA)-treated wood
Copper azole is another preservative that relies on amine copper, with co-biocides, to protect wood from decay and insects. The first copper azole formulation developed was CBA-A, which contains copper, boric acid, and tebuconazole. Recently the CA-B formulation was standardized and has largely replaced CBA-A. CA-B does not contain boric acid, but contains more copper and tebuconazole. Copper azole formulations can be used to treat a wide range of wood species used in aboveground or ground-contact applications. Wood treated with copper azole has the same issues with fasteners as ACQ-treated wood—the preservatives are highly corrosive to steel, therefore triple-galvanized or stainless steel hardware and fasteners must be used.

Installation tips
Regardless of which type of pressure-treated wood you’re using, follow these installation tips:

• Select wood with the correct retention rating. Retention levels refer to the amount of preservative remaining in the wood after treatment, and are measured in pounds of preservative per cubic foot of wood. Higher retention levels enable wood to withstand more demanding conditions. Look for the intended use on the wood, e.g., above ground or ground contact.
• Use appropriate fasteners. Triple-galvanized or stainless steel is needed for all copper-treated woods including ACQ-B, ACQ-C, ACQ-D, CBA-A, and CA-B.
• Use hand, eye, and breathing protection. The sawdust from pressure-treated wood can irritate the nose, eyes, and skin, and is harmful to ingest.
• Prevent contact with the skin as much as possible.
• Do not use pressure-treated wood for making cutting boards, or for any food preparation surface.
  Picnic tables made of pressure-treated wood have become common, and are safe to use for serving food, but not for preparing food. These tables should be coated with an oil-based wood preservative to seal in the chemical preservative.
• Never burn pressure-treated wood. It may produce toxic smoke and/or toxic ash. Other wood products on the do-not-burn list are plywood, particleboard, chipboard, and painted or finished wood, for similar reasons.

More Best Practices® for ensuring a quality framing job can be found in the following online training courses from BuildIQ University:

• Framing: Floors
• Framing: Walls
• Framing: Roofs