by Professor Gene Wengert
Q.
Our company is designing a line of medium to high-end outdoor furniture. What species should we use? We are considering mahogany and teak. Can we offer one "native" species? We expect that customers will leave the furniture unfinished, plus we need moderate strength and "user friendly" surfaces.
A.
When choosing a wood species, it is critical that you define the major constraints, as you have done. (For example, you do not list price as a constraint, but do list strength.) You also should probably list a few other characteristics or requirements, including a moderate to high natural resistance to decay, good natural color, and resistance to checking and splitting.
Your choice of teak is very good as it offers low shrinking and swelling, good color with age, natural decay resistance, natural oils that repel water, and good strength. Honduran mahogany is also very good, as it provides most of the needed properties (plus most Honduran mahogany is from plantations, not old forests, so it is more environmentally acceptable). It does shrink and swell with wetting and drying more than teak, however, so some checking and splitting can be expected. Note: There are three wood species (not related and with considerably different properties) that we call mahogany--African, Honduran, and Philippine mahogany.
Another good choice is a dense species of eucalyptus (mainly from Australia). Common names for two species of lumber are karri and jarrah. Dense red maerranti is another possible choice (Shorea species).
Of our native species, we don't have one that stands out as being fully acceptable. Most softwoods are not suitable due to resin exudation or their low strength. Old growth cypress is one possibility. Many hardwoods are prone to checking or are very low in strength. Woods like black locust or Osage orange, which are good candidates, are hard to find, machine with some difficulty, and have high enough shrinkage to cause a little concern about splits. Walnut is a strong possibility, but when wet it might leach a little, causing a person's white pants to become discolored if they sat on wet furniture; same problem with Osage orange. (American chestnut would be a good candidate too; lumber is still available.)
Have you considered redesigning the furniture so that you can use a weaker species? Make the pieces under stress a little larger; increase the size of fasteners, etc. If you can do this, then several cedars look like good possibilities, and maybe even redwood.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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Comment from contributor A:
Why not consider redwood for outdoor furniture? Clear grades, virtually all heart and flat or vertical grain, is quite naturally disease and decay resistant, plus the natural red color makes for exciting designs.
