Wood that is not treated is susceptible to damage from moisture in the ground and termites. Lumber that is used to build pole barns can be pressure treated to protect it from rot, fungi, and termites that could potentially shorten the life of the building.
Pressure-treated wood is used mostly for skirtboards, building posts, and laminated columns. Plywood sheathing, girts, and other framing members can be used if there is a threat of termites or concern about fire.
The most commonly used chemical preservatives are chromate copper arsenate (CCA), alkaline copper quaternary (ACQ), and copper azole. Several other treatments are less widely used.
Pressure treatment is the most effective means of chemically treating wood. Before being pressure treated, wood is generally incised to create divots on the surface. This allows the chemicals that are used to penetrate deep into the wood.
After the lumber is incised, it is put onto a flat car that is pushed into a long tube called a retort. A water solution with preservative chemicals is forced into the wood under pressure until the wood is saturated. The treated wood is then removed from the retort and allowed to drip dry. After it has dried, the wood is tested to be sure that the preservatives have penetrated enough into the wood to protect it.
As they grow, trees develop a spiral twist to their grain. Moisture and preservatives are added during the pressure treating process, and then the water exits as the wood dries. The loss of moisture speeds up the twisting process. Twisting can be largely avoided if the wood is used as soon as possible after it is delivered. Bracing the pieces of lumber in a structure reduces the amount of twisting.