Thermally modified decking and siding is catching fire in North America, says PROSALES contributor Charles Wardell. Don't worry, not literally. But the wood is gaining traction among many builders for its durability and other properties. The wood is decay resistant and non-toxic because no chemicals are used in the process. The wood is placed in a kiln where temperatures are slowly raised to about 400 degrees F or more, then held for several hours. Because the temperatures in the kiln exceed the wood’s combustion temperature, air is evacuated and replaced with steam.
The treatment leaves no food for bacteria or mold. “Because we cook all the sap and resin out of the wood, there’s no nutritional value,” says Igor Danchenko, founder and president of the Westwood Timber Group, which owns a kiln in Macon, Ga. The process also alters the wood cell structure, making it moisture-resistant and dimensionally stable.
Appearance is what initially attracts most architects, builders, and homeowners to thermally treated wood. The ash has a rich brown color, which many see as a warm complement to cold modern architecture. But it won’t stay that way without some sort of coating—sun and rain cause the wood to patina to a silvery gray. You have to use an oil-based product because the wood doesn’t absorb water. In addition to color and weather resistance, builders also like the material’s workability. “It’s easy to work with when compared to exotic species like ipe,” says Toronto contractor Rosario Ungaro. “Cutting is extremely easy because there are no resins or sugars in the wood. It leaves tools and blades clean with no gumming.” The wood also comes out of the kiln about 30% lighter than when it went in, making it easy to handle.