Pressure-treated Wood: A new look at a traditional product

Photo courtesy Timber Specialties Co.
Photo courtesy Timber Specialties Co.

By Henry Walthert, CAE
It may seem pressure-treated wood has been around forever. Some may view it as an old product with very little, if any, innovation. While the former statement may have some element of truth, the latter is far from accurate.

Treated wood products, in some form or fashion, have been around for millenia. In the 1925 publication, The Preservation of Wood, A.J. Wallis-Taylor reported wood preservation by chemical means could be traced back more than 4000 years, to the time when the Egyptians apparently used bitumen to treat wooden dowel-pins in the stonework of temples. The Roman Empire saw tar, linseed oil, cedar oil, and mixtures of garlic and vinegar used for the preservation of wooden structures. (For more, see the 1991 “United States Wood Preservation Patents in the 19th Century” by J.P. Hosli).

In the Middle Ages, charring of wood surfaces and soaking in brine, alum, arsenic, or copper salts were common methods of protecting wood from decay organisms. Investigations to develop alternative wood preservation agents were reported in the late 1600s. With the coming of the 1800s, economic development and population growth drove the need for durable wood products, including materials for ships, railway ties, and trestles. (For more, see B. Fuller’s “The Analysis of Existing Wood-preserving Techniques and Possible Alternatives” from 1977 in a technical report by Mitre. Also see A.J. Wallis-Taylor’s The Preservation of Wood from 1925).

Treated wood products have, and continue to be, part of the backbone of this country’s development and expansion. In its early history, Canada’s expansion rested on the steel rails held together by treated wood cross-ties. As its population grew and expanded, the need for power was met by generating stations that distributed electricity using power lines suspended on treated wood utility poles. Similarly, the telegraph lines used to communicate were hung on these same poles. Bridges for roads and railways were constructed using treated wood products. Many of these applications are still used in practice today.

Creosote-treated marine pilings support a waterside pedestrian walkway. Photos courtesy Wood Preservation Canada
Creosote-treated marine pilings support a waterside
pedestrian walkway.
Photos courtesy Wood Preservation Canada

Pressure-treating with preservatives
Wood exposed to the elements eventually falls prey to insects or fungal organisms that cause decay through a natural process. For this reason, it is important to protect wood and make it less desirable to decay organisms. This is done through the process of pressure-treating wood with preservatives. To facilitate the production of pressure-treated wood products, the preservatives are manufactured using either oil or water as solvents to assist with penetration into the wood. It is through this the preservative is able to form a protective barrier resistant to insect and fungal attack that would cause the structure’s decay and deterioration.

Oilborne preservatives
Oilborne preservatives include creosote—a byproduct of the coking process in steel production first introduced in the early 1800s. Creosote-treated wood is primarily used for railway cross-ties, but can also be employed for marine/land piling, heavy timber, and bridge construction. Another oilborne preservative, pentachlorophenol (PCP), was introduced in Canada in the 1950s and is primarily used for utility poles and cross-arms but can be specified to treat timber for construction. The oily nature of these preservatives helps stabilize the wood and protects it from moisture and wood-destroying organisms.

Chromate chopper arsenate (CCA) continues to be used in industrial and non-residential applications, such as highway guardrails.
Chromate chopper arsenate (CCA) continues to be used in industrial and non-residential applications, such as highway guardrails.

Waterborne preservatives
Developed in 1945, the waterborne preservative, chromated copper arsenate (CCA), was first used for industrial/commercial/agricultural uses. It was introduced to the residential construction market in the 1970s to provide an alternative to more costly naturally durable wood species.

As a competitively priced alternative with similar service life, CCA pressure-treated wood products in Canada allow for the use of more plentiful Canadian species such as spruce, pine, and fir. CCA’s ability to chemically bind to the wood fibres through a process called fixation ensures a long service life.

Canadians’ desire to increase their living space by building decks, gazebos, and play structures around their homes has been the driving force behind the need for outdoor wood building materials. Thedo-it-yourself market requires products that are simple to work with and easy to install. It was CCA that initially assisted in the popularity of pressure-treated wood in outdoor residential applications and grew the market for future generations of treated wood products.

Permanent wood foundations using CCA pressure-treated wood were also introduced in the 1970s as an alternative to concrete and block foundations. Wood foundations provide for easy construction, especially in areas where other foundation materials are difficult to obtain or construct. They provide a warm, dry, and easy-to-finish basement or crawl space. Permanent wood foundations are built in accordance with Canadian Standard Association (CAN/CSA) S406-92 (R2008), Construction of Preserved Wood Foundations. The National Building Code of Canada (NBC) and various provincial codes include permanent wood foundations as a standard form of residential foundation.

Currently, CCA is primarily used in industrial, commercial, and agricultural applications. These include:

  • plywood;
  • marine applications;
  • highway construction;
  • fence posts and poles for agricultural use;
  • round poles and posts used in building construction;
  • poles for highway and utility uses;
  • sawn cross-arms;
  • laminations before gluing;
  • cooling towers;
  • shakes and shingles; and
  • land, freshwater, foundation, and marine piles.

Ammoniacal copper zinc arsenate (ACZA) was introduced in Canada in the 1970s for the treatment of wood used in marine structures and construction timbers. For a short time it was also used to treat utility poles.

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