Q Can you suggest a species, or what to look for in a species of wood that will be used for outdoor furniture. The furniture will be protected from the rain.
A: In general, outdoor furniture does not do well when exposed to direct water wetting, including rain, hose spray, and so on. Such wetting can cause swelling, grain raising, grain shelling (which is ugly and can cause slivers and uneven surfaces), graying in color. In the extreme, prolonged high moisture in wood can cause decay and insect damage.
Next, a good wood species is one that does not move a lot when the humidity changes. For example, teak shrinks and swells with humidity changes about one-half as much as red oak with the same humidity change. Although the average outside humidity for much of the USA is 65 percent RH (12 percent EMC), furniture exposed to direct sunlight will cause drying; furniture in a foggy air condition will increase its moisture. So, some cycling can be expected. You would probably like a species that is not prone to developing shelling or extremely uneven surfaces when exposed.
Of course, a strong wood is desirable, but even weaker woods can be used with proper designing, including proper fasteners.
You may also want a wood that has some reasonable degree of natural decay and insect resistance.
Even though your furniture will be protected from rain, it would be prudent to see if you can use a water-repellant finish to keep an accidental water wetting from getting to the wood.
So, with these qualities in mind, past woodworkers have used redwood and western red cedar. In areas where these species were not available or were too expensive, some of the other North American cedars were used...Alaska yellow cedar is one that I really like. In recent years, foreign wood species have come onto the lumber market including ipe (my choice in many cases), Spanish cedar (not a true cedar, but a hardwood) sapele, and karri. In all cases, avoid the white-colored sapwood; always use heartwood.
A good source of information on wood is the The Wood Handbook from the U.S. Forest Products Lab. Online copies (15 meg file size, pdf) are at
www.fpl.fs.fed.us/documnts/fplgtr/fpl_gtr190.pdf. For example, see table 4-3 for shrinkage and table 14-1 for decay resistance. Want a copu? Many copy centers (like the UPS Store) will make a three-ring copy of this book for you if you give them the Internet address.