Extracting greater value from subalpine fir: Profiled decking

Forest Products Journal, Mar 2009 by McFarling, S M, Morris, P I, Knudson, R M

Abstract

Subalpine fir (Abies lasiocarpa (Hook.) Nutt.) is the fir species in the spruce-pine-fir (SPF) group harvested in British Columbia and Western Alberta. Due to preferential logging of lodgepole pine, the availability of subalpine fir is projected to increase significantly, in some regions to as high as 30 percent of the SPF mix. Examination of the properties of subalpine fir suggested opportunities for new higher margin products in applications where durability and dimensional stability are important such as decking. An experiment was designed to compare the dimensional stability of f�at decking to ribbed decking using three copper-based preservatives: alkaline copper quat (ACQ-D), copper azole (CA), and chromated copper arsenate (CCA). Preservative treated and non-treated decks were installed at Forintek's western test site. After 17 months of exposure, the preservative-treated ribbed decks looked significantly better than the preservative-treated flat decking in terms of size of checks and visibility of checks.

Oubalpine fir (Abies lasiocarpa (Hook) Nutt.) is one of four species harvested together by British Columbia and Western Alberta lumber manufacturers as spruce-pine-fir (SPF); the other species are white spruce (Picea glauca (Moench) Voss), Engelmann spruce (Picea engelmannii Parry), and lodgepole pine (Pinus contorta Dougl). Of the four species, subalpine fir is considered the most difficult to process and market because of its relatively low wood density, high moisture content (MC), high occurrence of moisture pockets, and lower drying rate compared to the other species (Jessome 1986, Josza 1991, USDA 1999). The primary utilization practice in British Columbia has been to process the four SPF species together for dimension lumber, with the residuals used for pulp and paper.

Examination of the properties of subalpine fir suggested opportunities for new higher margin products in applications where durability is important such as decking (Knudson et al. 2008). Subalpine fir is classed as a moderately difficult to treat wood species, but it has the most treatable heartwood of the species within the western SPF mix. Incising the lumber substantially improves treatment (Morris 1991). The preservatives that have replaced chromated copper arsenate (CCA) for most residential uses since January 1 , 2004, can provide better penetration of Canadian species than CCA (Morris et al. 2002).

Five-quarter-inch flat decking is becoming increasingly popular for its appearance. It may be less susceptible to warping and may also dry out faster after rain events. Furthermore, even with a relatively shallow treatment, a considerable portion of the cross section will be treated. It is not possible to treat most Canadian wood species (including subalpine fir) to meet the 1 0 mm penetration requirements in the Canadian and U.S. standards for industrial lumber. At the time this work was initiated, the CSA 080.32 decking standard (5-mm penetration) (CSA 1 997) had been developed with no incising requirement. Since that time, a process specification has been developed for small dimension and profiled residential wood products that cannot be incised due to excessive damage to the appearance and cannot be bored or cut for penetration measurements without making pieces unsaleable. This standard, now published (CSA 2008), applies to nominal 2-inch and 5/4-inch flat decking.

Durability can be predicted based on the performance of CCA with a thin shell of treatment (Choi et al. 2004, Morris et al. 2004) and the known performance of the alternative preservatives compared to CCA. Long-term field tests, however, are needed to confirm this. Surface profiling for enhanced appearance was also examined. Profiled decking is extremely popular in the United Kingdom and Australia, but has not had a major impact on the market in North America.

Materials and methods

Subalpine fir (Abies lasiocarpa (Hook.) Nutt.) logs and lumber used for this study were supplied by Canadian Forest Products Ltd. (Canfor) from the Fort St. James region of British Columbia. The logs came from five subalpine fir trees, one dominant, one codominant, and three intermediate. These growth classifications were assigned by Canfor foresters based on the trees' growth positions in the forest canopy. The trees were considered representative of the site type and age class expected to be harvested over the next period of years. Canfor Ltd. in New Westminster, British Columbia, manufactured decking boards from the rough sawn, kilndried lumber cut from these logs. Half of the boards were manufactured as flat surfaced, radius edge decking using a molder. The other half were provided with a ribbed surface profile based on product from Australia (Fig. 1). Fifty boards of flat decking (26 mm by 133 mm by 2.43 m) and 50 boards of profiled/ribbed decking (26 mm by 131 mm by 2.43 m) were selected based on visual criteria (suitable grade for deck surface boards and no initial checking). The moisture content (MC) of the decking material was found to range from 15 to 18 percent. Ten boards from each group (flat and profile/ribbed decking) were set aside as untreated control specimens. These boards were cut into two end-matched samples 0.6 m long. Each of the remaining 40 boards per group were then cross-cut into three end-matched 0.8-mlong samples and labeled. Each of these three end-matched samples were treated with a different preservative. These samples were then end-sealed with three coats of a two-part epoxy resin (Intergard 740, International Paint LLC).