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Naturally Rot-Resistant Woods (page 2 of 3)

by Alex Wilson

Tropical Species

Exceptionally resistant: angelique, azobe, balata, goncalo alves, greenheart, ipe (iapacho), jarrah, lignumvitae, purpleheart, and old-growth teak.

Resistant or very resistant: aftomosia (kokrodua), apamate (roble), balau, Spanish cedar, courbaril, determa, iroko, kapur, karri, kempas, American mahogany, manni, secupira, and wallaba.

Moderately Resistant: andiroba, avodire, benge, bubinga, ehie, ekop, keruing, African mahogany, dark red meranti, mersawa, sapele, second-growth teak, and tornillo.

Resistance Levels

Why is it that some pieces of redwood will last very well on an outdoor deck, while others have a much shorter life? There are two reasons. First, the sapwood (wood closest to the bark that tends to be lighter in color) of most trees, even highly decay-resistant species, offers very little decay resistance. If a piece of lumber has much sapwood, it won't last as long outdoors. As an illustration of this, when I pulled up some of those locust posts around my garden after 15 or 16 years in the ground, all of the sapwood up to ground level was totally gone--eaten away by decay--while the heartwood remained.

Second, the amount of extractives deposited in the heartwood depends on how the tree was grown--the more rapid the growth, the fewer extractives in the heartwood and the lower the decay resistance. The FPL defines old-growth bald cypress and old-growth redwood as resistant or very resistant, for example, while second-growth lumber from those species is defined as only moderately resistant.

Finding Naturally Resistant Woods

Unfortunately, the most resistant woods are not widely available. Hardly anybody raises black locust as a timber crop, and some sawmills are loathe to mill it, fearing it will dull the saw blades. While black locust trees often grow straight and tall, lumber from this species tends to twist and check as it dries. It is not an easy wood to work with. The other domestic tree species that offer exceptional resistance tend to be small, twisted trees that offer very little commercial lumber potential--though cutting fence posts from these trees is still possible.

As for domestic woods that can be milled easily into lumber, old-growth trees are the best, but old-growth forests are almost gone. Commercially available old-growth redwood is quickly disappearing; most harvesting today is done in second-growth forests. Old-growth western red cedar is still being harvested extensively, but once this supply is gone, in another few decades or so, clear western red cedar will be much harder to come by: it does not regenerate as readily as redwood, and the species is rarely planted. Virtually all cypress harvested today is second-growth, and there hasn't been any old-growth longleaf yellow pine--a moderately resistant species--for close to a hundred years.

For many species of wood today, the only way to get highly resistant, old-growth wood is to salvage it from structures that are being torn down or from rivers and lakes where it sank a century ago. (Underwater, where there is very little oxygen, wood lasts for decades, or even centuries, with little decay.) Dozens of companies specialize in salvaged wood: redwood from wine vats in California, longleaf yellow pine from bridges, cypress from timber buildings in the Southeast. Wood is even being salvaged from a timber-crib dam currently being dismantled in Maine. You can find a list of companies that supply salvaged wood at the Web site of Environmental Building News:

If salvaged wood isn't an option, you can get new wood from a well-managed forest by insisting on certification from the Forest Stewardship Council (FSC), an international organization that has developed standards for responsible forest management. FSC accredits companies or nonprofit organizations, which then carry out the actual forest certification and "chain-of-custody" certification, so you can be sure the wood you buy as certified actually came from a certified forest. Choosing only FSC-certified wood is especially important with tropical timber, such as teak and ipe, because many tropical forests are poorly managed. The Certified Forest Products Council, can provide you with names of companies that supply FSC-certified wood.

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