1. Introduction
Wood exposed outside to the weather erodes, cracks and becomes grey in color [
1]. These adverse effects of the weather on the surface properties of wood do not penetrate deeply into wood and are distinct from fungal decay, which under favorable conditions can penetrate into and affect the strength of large wooden structures [
1,
2]. Fungal decay can be prevented by pressure-treating wood with solutions of chemical preservatives [
2]. Wood preservatives, however, are less effective at protecting wood surfaces from the adverse effects of the weather, and as a result, treated wood used outdoors is often finished with coatings to maintain its appearance and prevent the wood from cracking (checking) [
3]. The checking of wood used outdoors can also be restricted by machining micro-grooves into the surface of wood [
4,
5,
6]. Micro-grooving, hereafter referred to as profiling, is commonly applied to deckboards manufactured in Asia, Australia, Europe and New Zealand, but it is uncommon in North America [
7,
8]. The decking market in North America is valued at
$US 7 billion per annum, and wood products command 84% of the market [
9]. However, wood is rapidly losing market share to wood plastic composites that are less susceptible to checking and require less maintenance than wood decking [
10]. For example, demand for wood plastic decking in North America is growing at 5% per annum compared to 3% per annum for wood decking [
9]. The same trend is occurring in other countries [
11]. As a result of the success of wood plastic decking, there has been significant interest in improving the resistance of wooden deckboards to weathering and in particular checking. This interest explains the recent attention in North America to optimizing profiling to make it better at reducing the checking of deckboards exposed to the weather [
12,
13,
14,
15,
16,
17,
18,
19]. It also accounts for increasing interest in Europe and elsewhere in deckboards made from tropical wood species or thermally or chemically modified woods that are less susceptible to checking and cupping than preservative-treated deckboards [
20,
21,
22].
Research on optimizing surface profiling to reduce the checking of decking has focused on a limited number of wood species. The focus of research in Canada has been on the amabilis fir (
Abies amabilis, (Dougl.) ex J. Forbe) because it is susceptible to checking and, as a result, is under-utilized for decking, even though it is easier to treat with wood preservatives than most other Canadian wood species [
23,
24]. The checking of amabilis fir can be reduced significantly by profiling, and profiles with narrow grooves (rib profile) are more effective at reducing checking than wavy profiles (ribble or ripple) with wider grooves [
13,
15]. In addition, rib profiles with deeper grooves appear to be more effective at reducing checking of amabilis fir than profiles with shallower grooves [
19]. These findings on the greater effectiveness of rib profiles compared to wavy profiles at reducing the checking of wooden deckboards are only relevant to boards made from amabilis fir because two previous studies showed that wavy profiles were more effective than rib profiles at reducing the checking of southern pine (
Pinus spp.) decking [
13,
15]. Hence, further research is needed to optimize profiling for commercially important North American wood species that are used to manufacture decking.
In this paper, we examine the effects of surface profiling on the checking and cupping of wood decking exposed outdoors to the weather. We selected three important commercial wood species for our research: Douglas fir (
Pseudotsuga menziesii (Mirb.) Franco); western hemlock (
Tsuga heterophylla (Raf.) Sarg.); and white spruce (
Picea glauca (Moench) Voss). Douglas fir, hemlock and spruce comprise over 50% of the 27.5 billion cubic meters of wood growing in non-protected land in Canada [
25]. Wood from these species is used to manufacture decking [
23], and pre-commercial trials of profiled Douglas fir decking have commenced in North America [
26]. This study was carried out to determine the profiles that are most effective at reducing the checking of decking made from the three different wood species. We hypothesize that the geometry of profiles will influence checking of profiled decking. Our results show that rib profiles with narrow and deep grooves were better than profiles with wide or shallow grooves at restricting checking of all three species. Furthermore, our numerical model explains why a rib profile with deep grooves was more effective than a rib profile with shallow grooves at restricting checking of profiled spruce deckboards.
4. Discussion
Profiling of deckboards is a good way of reducing the negative effect of surface checking on the appearance of deckboards exposed outdoors, and profiled deckboards are common in Asia, Australia, Europe and New Zealand, as mentioned above [
7,
8]. They are not common in North America. Interest in manufacturing profiled wooden deckboards in North America is increasing, but profiling has only been tested on a handful of wood species [
12,
13,
14,
15,
16,
17,
18,
19]. Previous results suggested that rib profiles with narrow grooves were better than profiles with wider grooves (ribble or ripple profiles) at reducing the checking of amabilis fir [
19], but studies have also shown that the effectiveness of different profiles varies with wood species, as mentioned in the introduction [
13,
15]. Therefore, it has been difficult to recommend the best profile for the manufacture of profiled decking from North American softwood species, apart from amabilis fir or southern pine. Softwoods such as Douglas fir, western hemlock and white spruce are more commercially important than amabilis fir [
36], and results here suggest that a profile with narrow grooves (rib profile) is better than wavy profiles (ribble or ripple) at reducing the checking of these three species. This finding accords with previous research that optimized surface profiling for amabilis fir deckboards [
13,
15,
19]. Our finding that tensile stresses are greater in grooves of rib samples also accords with findings that tensile strains during drying are highest in the grooves of deckboards with wavy profiles [
37], and observations that checks are mainly located in grooves of profiled deckboards [
4,
12,
13].
In addition to the effect of groove width on checking (rib vs. ribble or ripple) we also observed that groove depth in boards with narrow grooves (rib profiles) had a significant effect on two measures of checking. Our previous research on the effect of profile geometry on the checking of amabilis fir suggested that groove depth influenced checking, but the relationship between groove depth and check parameters was not strong [
19]. Our current experimental findings are more convincing and our model of the effect of groove depth on checking suggests why ribbed boards with deeper grooves (2 or 2.5 mm) check less than boards with shallower grooves (1.5 mm). In particular, we found that stresses, which cause checking at wood surfaces during drying [
38], were greater in ribbed spruce samples with shallow grooves (1.5 mm deep) compared to samples with deeper grooves (2.5 mm deep). Furthermore, the former samples contained more areas with high stress. Rib samples with shallow grooves dried more rapidly than samples with deeper grooves, which may explain the pattern of stress development in the two types of samples. Accordingly, treatments such as coatings that restrict the rate of drying of profiled deckboards may further reduce checking. In accord with this suggestion, Akhtari and Nicholas [
17] found that coatings containing zinc oxide or titanium dioxide particles reduced the checking of ribbed southern pine deckboards exposed to artificial accelerated weathering by approximately fifty percent [
17].
Ribbed boards with grooves that exceed 2 mm in depth are manufactured commercially [
7]. For example, our survey of the topography of profiled deckboards manufactured around the world found that six of the 19 ribbed boards we analyzed had grooves that were deeper than 2 mm [
7]. Boards with profiles that are very similar to our standard rib profile with a groove depth of 2 mm are manufactured commercially from radiata pine (
Pinus radiata D. Don) or European larch (
Larix decidua Mill.) [
7]. Pre-commercial trials of profiled Douglas fir decking have been established in the USA based on our initial finding that rib profiles were better than wavy profiles at restricting the checking of amabilis fir decking [
19]. The profile that was chosen for these trials was our standard rib profile with a groove depth of 2.0 mm. Results here support the choice of this standard rib profile for the profiled Douglas fir deckboards used for these pre-commercial trials [
26].
The positive effects of profiling on the checking of deckboards exposed to the weather are clouded by results from some previous studies showing that profiling increases the undesirable cupping of amabilis fir deckboards exposed to the weather [
13,
19]. Cupping of profiled plywood siding exposed to the weather is also more pronounced than that of flat plywood siding [
6,
39]. In contrast, other studies have shown that profiling reduces the cupping and distortion of southern pine deckboards exposed to the weather [
16,
17]. Our results here for Douglas fir, western hemlock and white spruce deckboards accord with the results of our previous study that showed that profiling increased the cupping of amabilis fir deckboards exposed to the weather [
19]. Previously we suggested that grooves or saw kerfs that are machined into the undersides of deck or flooring boards might reduce the tendency of profiled deckboards to cup when they are exposed outdoors [
40,
41]. We have carried out a study to examine whether this approach can reduce the cupping of profiled Douglas fir, western hemlock and white spruce boards cut from the same parent boards as those used here. Our results have successfully demonstrated the efficacy of this approach and they will be reported in a separate paper that is being prepared for publication.
In addition to the effects of profiling on the checking and cupping of deckboards, we also observed a significant wood species effect on checking and cupping of deckboards. Our finding that western hemlock deckboards checked more than Douglas fir or white spruce boards accords with the results of two previous studies that compared the checking of decking made from different softwood species [
23,
42]. One of these studies noted that softwood species that checked less than other species were ones that were dimensionally stable or impermeable such as western red cedar (
Thuja plicata Donn ex D.Don), yellow cedar (
Cupressus nootkatensis D.Don 1824) and white spruce [
42]. White spruce deckboards here also cupped less than western hemlock deckboards. Norway spruce (
Picea abies (L.) Karst), which is similar to white spruce, is preferred for exterior house siding in Europe because it cups and checks less than Scots pine (
Pinus sylvestris L.) siding [
43,
44]. This desirable property of spruce has been attributed to its impermeability resulting from high percentages of blocked (aspirated) bordered pits and small proportion of ray tracheids [
44,
45]. Accordingly, it is possible that the lower checking and cupping of white spruce deckboards than western hemlock boards could be due to the lower permeability of the former species compared to western hemlock. Douglas fir is also less permeable than western hemlock, which may account for why Douglas fir deckboards checked and cupped less than western hemlock deckboards. However, Douglas fir deckboards cupped less than spruce deckboards even though they were more permeable than white spruce boards. Hence, differences in the permeability of the three wood species used to make deckboards cannot fully explain the variation in cupping of deckboards exposed to natural weathering.
The Douglas fir, western hemlock and white spruce samples tested here were treated with the preservative chemical ACQ (alkaline copper quaternary). This feature of our experimentation accords with commercial practice, but on the other hand decking boards are expected to maintain their appearance for several years, whereas we assessed checking after only six months’ exposure. Nevertheless, Morris and Ingram [
18] found that a rib profile significantly reduced the surface checking of subalpine fir (
Abies lasiocarpa (Hooker) Nuttall) decking after six, 17, 36, 60, and 120 months’ exposure. The difference in checking of unprofiled and ribbed decking became smaller, particularly after five years’ exposure, but the authors concluded that ‘profiling significantly reduced the checking of deckboards exposed to the weather for 10 years, and ‘checks in ribbed boards were very difficult to see at standing height’ [
18]. Therefore, we suggest that our results on the effectiveness of rib profiles at reducing the checking of Douglas fir, western hemlock and white spruce are promising, but further research, similar to that carried out by Morris and Ingram [
18], is needed to examine the long-term effectiveness of the profiles at reducing the checking of all three species, and to determine whether profiling affects the decay resistance of deckboards [
18,
19].