Specifiers and installers of decks continue to rely on pressure-treated wood, but often not the same treated wood as in the past.

Preserved wood is pressure-treated for protection against termites and fungal decay, the two primary groups of wood-destroying organisms. This wood resists deterioration in such structures as decks, walkways, fences, retaining walls, and gazebos.

For the past 40 years, the predominant chemical for preserving dimension lumber was CCA, chromated copper arsenate. While this preservative remains the usual choice for heavy duty construction, newer alternatives have replaced CCA for decks and non-industrial applications.

Primary among these are copper azole and alkaline copper quaternary. Both are combinations of copper and a carbon-based co-biocide. In North America, wood treated with copper azole is called Wolmanized® Outdoor® wood. Lumber treated with alkaline copper quaternary is sold under several brands.

These preservatives mark a trend to treatments involving less chemical and, notably, less metal. Farthest ahead of this movement is Wolmanized® L³ wood. Suitable for out-of-ground applications, it is the first non-metallic preservative for residential uses.

Copper-based preservatives are slightly more corrosive to hardware than CCA, so more care must be taken in choosing fasteners and connectors. The preservative manufacturers recommend hot-dipped galvanized fasteners (meeting ASTM A 153) and connectors (ASTM A 653 Class G185 sheet), or better. Aluminum should not be used in direct contact with this wood.

Non-metallic preservatives do not react with hardware, so they are no more corrosive than untreated wood. However, most building codes do not distinguish among types of treated wood. As a result, similar fasteners and connectors may be needed regardless of preservative.

In the East, where pressure-treated lumber has been used for decades, southern pine is the most commonly treated species. It is strong and readily accepts treatment. Treated wood in western markets is made from other species. While treatable, these species require incising (surface slits cut by mechanized knife blades) to facilitate preservative penetration. In areas where Douglas fir is its principal species, treated wood is used mainly for posts and joists rather than decking.

There are several reasons why preserved wood is preferred for decks. One feature is its price; treated wood is usually the least expensive alternative.

Often, wood is chosen for its appearance. Wood is a natural material and looks good in natural settings. Consequently, manufacturers of other deck materials try to imitate the appearance of wood.

More and more, environmental considerations influence the choice of materials. Wood decking is made from plentiful species of a renewable resource grown on managed timberlands. It requires less energy to produce than alternatives. Growing trees absorb carbon dioxide and wood products sequester carbon, so greenhouse gases are reduced with wood. And, treated wood lasts a long time, thereby extending forest resources.

When properly specified and installed, preserved wood serves a practical function in structures exposed to termites or fungal decay while offering natural beauty and meeting environmental demands.