Pressure-treated lumber is engineered to last in the outdoor environment, but it also raises several concerns in arbor design. American pressure-treated lumber manufacturers stopped using arsenic in their chemical treatments in 2003, so pressure-treated lumber is less toxic today. But the new pressure-treated lumber may have corrosion problems where it comes into contact with standard metal nails and fasteners.
Utah building inspector Michael Levitt has assembled a comprehensive website with information on deck collapses which have occurred due to use of new pressure-treated lumber with inappropriate fasteners. The site includes information on using safe fastening mechanisms. Any pressure-treated wood arbor which is intended as a place for sitting, or under which children may play, should follow safe construction methods.
Additionally, even the new pressure-treated wood raises concerns about food safety if you plan to grow fruits or vegetables for human consumption on the arbor. Commercial pressure-treated lumber may be best used as a support for ornamentals such as roses or clematis rather than for grapes, beans, or other food plants.
Nontreated Lumber and Alternatives
Many nonpressure-treated types of lumber last a long time in the outdoors, especially if kept painted or coated with a waterproof sealant. Douglas fir, hard spruce, cedar and, when available from sustainably-harvested sources, redwood have long been popular outdoor building materials and are appropriate for arbor construction. Green building suppliers also offer lumber treated with MicroPro, a certified Environmentally Preferable Product which preserves lumber without many of the dangers associated with pressure-treated wood. Although alternative building lumber may be expensive, an arbor is a relatively small project, and using long-lasting materials ensures that vines planted on it will be able to grow beautifully to maturity instead of needing replacement every few years along with the boards comprising the arbor.
Natural and Found Materials
While smooth-finished, painted lumber makes a tidy arbor appropriate to complementing an elegant or modern home, natural materials can also be long-lasting and work well with a cottage, farmhouse, or other country-style home. Natural cedar posts--the full trunks of small cedar trees with the bark still on--can be purchased from some lumber yards as well as farm-supply outlets, or they can be salvaged, often for free, when a landowner is clearing a field in which cedar has sprouted. Cedar is a dense wood, and cedar oil acts as a natural preservative. Driftwood is another found-material arbor construction alternative. Because driftwood is well-seasoned, it also tends to last a long time, especially when gathered from salt-water environments, since the salt acts as a wood preservative. Natural materials do not come in standard dimensions or with known strength ratings, however, so extra planning and care should be used to ensure that a natural-material arbor is sturdy enough for the desired use.