Learn which hardwood types are the most resistant to wood rot.
Dan Cassens, professor of forestry and natural resources at Purdue University, has recently developed a CD to help woodworkers and sawyers better understand the characteristics of hardwoods. Lumber from Hardwood Trees includes many “hard-to-find” facts about 35 species of hardwood lumber. Cassens based the information on 30 years of research and personal experience as the owner of a hardwood forest and operator of a Wood-Mizer sawmill.
People frequently say that locust and cedar trees produce decay-resistant hardwood lumber. What is the relative decay resistance of species such as Osage orange, locust, cedar, oak, black walnut and others?
Decay resistance is difficult to address because the resistance of a piece of wood depends on not only the natural resistance of the wood (which can vary) or the effectiveness of a preservative treatment, but also on the exposure conditions of the wood and geographic location. Fence posts are good examples. They often fail due to decay at the ground line. Conditions for wood rot are ideal at this location, but the top part of the post may remain perfectly sound. I have had several 6-inch-diameter black locust posts in place for 25 years in the central Midwest. Two have completely rotted off and some of the others are beginning to show signs of deterioration. If these posts were installed in the Deep South, with warm temperatures and abundant moisture most of the year, I think all of the posts would have failed before now. If they had been installed in the cold North, perhaps none would have failed by now. If installed in the dry Southwest, little evidence of decay would be noted.
Performance expectations are another important factor. If only a year or two of service life is expected, just about any wood can be used. If failure of the wood isn’t life-threatening, standards are usually somewhat more relaxed. The fence posts might be a good example. However, if we were evaluating longer poles that support a deck hanging over a deep, rocky ravine, we have an entirely different issue. Failure could result in injury, and the deck is also subject to building code restrictions, which will likely require pressure-treated lumber.
We can further increase expectations by considering a cross-country power line with wood poles. These lines are expensive to build and failure of just a couple of poles could result in serious outages of power to a large number of people. In these applications, standards for treatment are carefully followed and quality control checks are performed. Expectations are for decades of service.
The Wood Handbook uses three categories to rank the natural decay resistance of heartwood. These are (1) resistant or very resistant, (2) moderately resistant and (3) slightly or nonresistant. Assuming preservative treatment is not acceptable, choose only wood from the resistant or very resistant category if the wood is to be used where a decay hazard exists. These include the heartwood of all commercial cedar, black and honey locust, osage orange, white oak, black walnut and others. Black locust and osage orange have a particularly high resistance to decay.
Are there decay-resistance differences between heartwood and sapwood?
Absolutely. Of the resistant species, only the heartwood is naturally durable. The sapwood of any species has no resistance. Unfortunately, the amount of sapwood in young-growth hardwood timber is usually greater than in old-growth. Young hardwood trees are more vigorous and faster growing, thus they typically have more sapwood. When selecting wood for natural durability, the sapwood should be sorted out and used where a decay hazard does not exist.
“Old-growth” and “young-growth” brings us to another issue. There have been three technical papers published several years ago in regards to the natural decay resistance of old-growth and second or young-growth bald cypress, redwood and western red cedar, all classified as softwoods. In each case, it was shown that the young-growth did not have the decay resistance of the old-growth, but it still had resistance. No comparable studies exist for the hardwood types.
For someone cutting hardwood lumber from his or her own woodlot, what are some of species that are most resistant to wood rot?
In the eastern hardwood region, the most decay-resistant and available species include white oak, black locust and osage orange. Other more valuable — but resistant — species include cherry, walnut and sassafras. The durability of these and other species can be checked in the woods. Naturally durable species will often have the sapwood or outer band of lighter colored wood completely rotted and mostly gone, while the heartwood is still intact.
Among the eastern softwoods, the cedars are resistant, as is the heartwood of old-growth bald cypress. In addition, the heartwood of old growth longleaf ash and eastern white pine show moderate resistance.
In the western United States and Canada, cedars and redwood are the most resistant. Douglas fir, western larch and young-growth redwood are moderately resistant.
Should the lumber cut from certain species of trees never be considered for outdoor projects?
With the exception of some minor species rated as resistant or very resistant and the ones discussed above, others should not be used where they are subject to wetting (continuous or periodic) and more than a couple years of service life are expected.
Many people suggest do-it-yourself methods for improving the decay-resistance of wood (charring the ends of fence posts, dipping wood in creosote solutions, or coating wood with turpentine, paraffin or linseed oil). Do any of these methods really work?
At one time, easily treated species and particularly air-dried sapwood of southern pine could be cold soaked for 24 hours or more in the appropriate preservatives and the service life extended substantially. Other processes also existed. The chemicals used for treatment are no longer available.
Brushing, spraying and dipping will add little to the life of wood where a rot (decay) hazard exists. These are surface treatments only, and they do not protect the interior portion of wood. With chemical treatment, it is important to get adequate penetration (usually at least 2 inches on thick stock) and retention of an adequate amount of chemical.
Borate treatments may be an exception. In addition to pressure treatment and soaking, they are sometimes sprayed on the wood surface, or the wood is bored and rods inserted and sealed off. The borate diffuses into the wet wood and protects it. However, the borate is water-soluble, so it can leach away in the presence of free water. Care must be taken as to where and how these treatments are applied.
Charring brings up an interesting new development in wood preservation. Charcoal is often found in archeological digs, and it seems to have resisted decay. Some have tried to char wood to prevent decay. This will likely be a surface treatment as well, because wood does not transfer heat readily. Thermal treating of wood has been commercialized in Europe because chemical wood preservatives have been eliminated. This process is now being developed in the United States. The wood is heated to high temperatures in a controlled atmosphere. The wood turns brown, becomes more stable and is said to be acceptable for aboveground applications. There are some trade-offs between durability and strength. The higher the temperatures used, the better the decay resistance — but greater reductions in strength can occur. Available data is limited.
When buying hardwood lumber, what are your top species recommendations for (1) fence posts, (2) inside DIY projects and (3) outside DIY projects?
For fence posts, I would suggest pressure-preservative-treated wood, followed by naturally durable cedars, black locust or osage orange. These woods have good track records. Locust and osage are hard, so it’s difficult to drive nails into them. Cedars are relatively weak, so don’t expect small posts to hold up with heavy use, such as confined cattle. Southern pine is easily treated and strong.
Virtually anything can be used for an inside project because decay is not an issue. Your choice is going to depend on whether the project is functional, such as framing for walls. In that case, cost will probably determine the choice. If aesthetics are important, it will depend on personal preference to a large degree. Strength can sometimes be a factor.
For outside projects, one should select treated wood or naturally durable wood. Keep in mind that the cedars, redwood, cypress and pine heartwood are relatively easy to work with as compared to dense hardwood types such as black locust, osage and white oak. Sassafras, however, has good resistance and is relatively easy to work with, but isn’t available in commercial retail outlets.
What are the best sources for information related to decay-resistance, etc.?
Information about decay resistance and other wood properties, including color photographs of various quality levels of wood from many hardwood species, is available in the Purdue University CD Lumber From Hardwood Trees. The Wood Handbook (U.S. Forest Products Laboratory in Madison, Wisc.) is a more technical and detailed reference for both hardwoods and softwoods. Note table 3-10, “Decay Resistance,” and chapters 13 and 14, “Biodeterioration of Wood and Wood Preservation.”
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