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Pressure Treating 101 - The Basics

• Natural enemies of wood
• A brief history of treated wood
• The production process
• Major preservatives
• Typical uses
• Heartwood & sapwood

Wood has long been used as a building material. It is strong, easy to cut and fasten, attractive, and economical. It is also the only major building product that is naturally renewable. But the life of a timber structure can be shortened by wood-destroying organisms. Pressure treating wood with an appropriate solution provides protection against this deterioration. 

Natural enemies of wood

Various living organisms can harm wood when they use it as a source of food, for shelter, or as a place for incubation of their young. These organisms include decay fungi, insects, and marine borers. Fire and moisture also can damage wood.

Fungi are non-photosynthesizing plants which use wood as food. Roots of their spores penetrate into damp wood, breaking down the structure of the wood.

Among the insects which destroy wood are termites, powder post beetles, carpenter ants, and carpenter bees. Termites, the most destructive of the insects, consume wood as food. The others burrow into wood but do not eat it.

Wood is most vulnerable in marine applications. Some marine borers, such as the shipworm Toredo, construct elaborate tunnels within wood, while others (including the crustacean Limnoria) attack the surface of the wood.  

A brief history of treated wood

Inventive people have tried a variety of methods to prolong the life of wood.

In centuries long past, ancient builders coated wood with pitch and charred the surface of wood in attempts to thwart hungry pests. In England in 1716, a patent was granted for "oyle or spirit of tarr" to protect ship planks against marine borers.

The building of American railroads was a significant stimulus to wood preservation. Wood ties were more resilient than the stone ties that had been used, but wood was subject to decay and termites. In 1875, the first commercial pressure treating operation was established for the preservation of ties using creosote.

Then followed the development of other chemical preservatives Ò a procession that continues today. Oil-borne preservatives remain in common use for some applications; waterborne preservatives have assumed most of the residential treated wood market.  

The production process

The essentials of the treating process are rather simple, but there are many details that a treating plant operator must consider in order to preserve wood effectively and efficiently.

The lumber, timbers, plywood, poles, posts, or piling that is to be treated is loaded onto trams and pushed into a large horizontal cylinder. This pressure vessel, also called an autoclave and a retort, may be 5ft - 8ft in diameter and 40 ft - 120 ft in length. Not all species of wood can be successfully treated; principal woods are southern pine, ponderosa pine, red pine, hem-fir, and Douglas fir, depending on locally available stock. This wood must be dry before treatment or it will not be able to accept an adequate amount of the liquid preservative solution.

Once loaded, the cylinder door is sealed, and a vacuum is applied to remove most of the air from the cylinder and wood cells.

Then the preservative, carried usually in an oil or water or ammonia solution, is pumped into the cylinder. After the liquid fills the cylinder, the pressure pumps drive the preservative into the wood.

When the desired treatment has been reached, remaining preservative is pumped out of the cylinder and back to a storage tank for later reuse. A final vacuum removes excess liquid from wood cells. The wood is then pulled from the cylinder and allowed to sit on a sloped concrete pad where any drips of liquid flow to a filter and can be reused.

To check the retention (amount of preservative retained in the wood) and penetration of the treatment, sample borings are taken from the wood and analyzed. The quality control of responsible plants is monitored by an independent inspection agency accredited by the American Lumber Standard Committee.

For some applications, the treated wood may be dried after treatment - in a kiln or in the air - prior to shipment.

Standards for treated wood are set by the American Wood-Preservers' Association. Compliance with AWPA standards is indicated on the wood - either on an ink stamp or end tag. If the wood does not meet standards, it is marked "Treated to Refusal," meaning that the wood would not accept sufficient preservative. There are several reasons why this might occur: the species may be difficult to penetrate by the treatment used, there might be a high percentage of hard-to-treat heartwood compared to the more easily treated sapwood, or the wood may have been too wet before treatment.  

Major preservatives

Around the world there are many wood-treatment preservatives approved for particular applications. In the United States, several are in common use.

CREOSOTE. Produced by the distillation of coal tar, creosote was the first preservative applied by pressure treatment (1875). It remains the principal choice for railroad ties, and is also used to treat utility poles, marine piling, and foundation piling.

PENTACHLOROPHENOL. This oil-borne preservative, developed in the 1930s, is used for utility poles, fence posts, and glue-laminated timbers.

CCA (CHROMATED COPPER ARSENATE). Invented in 1933, waterborne CCA has become the predominate choice for treating lumber intended for residential use because it leaves wood clean to the touch. It is also used for a wide range of nonresidential applications.

ACZA (AMMONIACAL COPPER ZINC ARSENATE). Carried in an ammonia solution, ACZA is especially valuable for treating otherwise hard-to-treat species like Douglas fir. ACZA-treated wood is used primarily for utility and building poles, and is also used for some lumber applications.

ACQ-B (AMMONIACAL COPPER QUATERNARY). ACQ is marketed for those applications where there may be environmental concerns regarding ingredients of the other preservatives. Like ACZA, ACQ-B is carried in an ammonia solution and can be used for effective treatment of Douglas fir.

CBA (COPPER AZOLE). Popular in Japan and areas with restrictions on other preservatives, wood treated with copper azole is effective in a range of applications.

Typical uses

Pressure-treated wood has been used in numerous residential, industrial, commercial, agricultural, marine, and institutional applications. Among the many uses are:

• Decks
• Landscaping
• Fences
• Gazebos
• Storage sheds
• Dog and bird houses
• Screens and covers
• Play structures
• Edging
• Plywood siding
• Shakes and shingles
• Wet floors and walls
• Soffit and fascia
• Sill plate
• Permanent Wood Foundations
• Swimming pool sidewalls
• Pole buildings
• Corrals
• Tomato stakes
• Arbors
• General construction
• Bridges
• Walkways
• Marine and foundation piling
• Bulkheads
• Docks
• Guardrail and sign posts
• Retaining walls
• Sound barriers
• Utility poles

Heartwood & Sapwood

When specifying a particular species of wood, it's important to note that for some applications, the portion of the tree from which the lumber is cut may also be an important consideration.

Typically, the heartwood - or center part of the tree - may be quite dense and less porous that the sapwood - the younger, outer portion of the tree. The heartwood is naturally more resistant to attack by pests, but it is also less accepting of preservative. Therefore, in marine conditions, for example, where wood-destroying organisms are a threat, it is wise to specify wood with a minimum of heartwood exposed (seawall grade) to be assured of adequate preservative protection.

Likewise, even with "naturally durable" species like redwood, cedar and cypress, heartwood grade must be specified for construction where decay and insect attack are likely. If sapwood grades of redwood or cedar are selected for such applications, they should be pressure-treated since the sapwood does not have the natural protection of the heartwood.