All about the strong, durable and beautiful
building material that is the basis for log homes.
Story by Edwin J. Burke
No particular species of wood is best for general use in a log home. There are several that work quite well in general practice and others that are less desirable when used in certain applications or in certain physical settings. In reality, log builders often employ more than one kind of tree species in their homes. This decision to favor one particular kind of wood over another is usually made for one or more of the following reasons; strength, appearance, durability, treatability, size, availability and cost. Let's examine each of these properties.
Strength.Certainly, strength is a primary consideration. I believe that all log homes need to be designed by a qualified architect or engineer and that they should be produced by a company that adheres to some recognized log-grading program, such as Timber Products Inc. or the Log Homes Council.
Wood scientists and building engineers have long since determined how different sizes and grades of different wood species behave when placed under stress by wind loads against walls, snow loads on a roof or just the weight of people and furniture on a floor. In other words, they know how much weight a wooden building member will carry without breaking or bending excessively.
Some species of wood are naturally stronger than others, making the decision to use them in certain applications much easier. Say, for example, that a design includes a long roof overhang at the gable end. Obviously, the weight of the roof must be supported by strong timbers. Or suppose the design includes a broad prow front - very popular today among log-home buyers. The roof and the glass must be supported by logs that are strong enough to accept the weight of wind stresses without bending. Even logs that span window and door openings are under more stress than those in a stacked wall.
Two major factors affect the strength characteristics of a tree: density and knots. Density is the weight of the wood per unit of volume when it is dry. Knots weaken a piece of wood, so the fewer they are and the smaller they are, the stronger the building member. This is as true of a 2-by-4 inch stud as it is of a log. Ponderosa pine, lodgepole pine, red pine, jack pine, Southern Yellow pine, Douglas fir, Western larch, and tamarack are trees with dense wood and relatively small knots, so they are often employed to carry heavy loads.
That does not preclude a weaker species from being employed. You just need a bigger log. For example, an 8-inch-diameter spruce or pine rafter can do the work of a 7-inch fir rafter. In fact, you could use a balsa tree if you wanted to, but you might need a 20-inch-diameter tree, and that is not economically feasible. The final equation is usually a ratio of strength to cost.
Appearance. This is probably the key factor in a homeowner's choice of log species. What does he or she find visually pleasing?
If dark woods are preferred, then the choice might be Western red cedar or incense cedar. Intermediate tones can be found in species like tamarack, Southern yellow pine or Douglas fir. Light woods are the pines and true firs, like Fraser fir or subalpine fir.
The next consideration is knots. These affect the appearance of wood, as well as the strength. Some people want a tree with large knots for its character, while others want a smoother, clear wood appearance. The latter could be found in the spruce family or several species of the Western pines.
It would also be good for log-home buyers to remember that logs will change color over time. So, consumers would be wise to visit homes made of a particular species that have been standing for several years.
Other factors in the equation are straightness, taper and the number or size of checks. Many types of log building, such as the handcrafted style, require long, straight logs with minimal taper. Within itself, this requirement will limit the builder to certain species, like Eastern white pine, the Western pines or Western larch.
Checking is a natural phenomenon that is present in all logs. As a log seasons, it loses moisture and begins to shrink in circumference. Natural stresses in the tree cause checks (vertical cracks along the grain) to appear. These don't adversely affect the strength of a timber, and many people enjoy them for the rustic flavor they add to the decor. But others find them unsightly. Higher density woods tend to have more checks. In the Eastern half of the United States, species that would be likely to show large checks include yellow poplar, oak, birch and Southern yellow pine. In the West, species like Douglas fir, Western larch and lodgepole or Ponderosa pine would experience more checking.
This tendency of wood to check is one of the reasons that some log-home producers prefer standing-dead trees. These are trees that have been killed by fire or insects and have stood in the woods until seasoned. Standing-dead trees usually have only one or two large checks, and producers or builders will attempt to position these in such a way that the checks will not be visible. If green, or freshly cut, logs are being air dried, manufacturers sometimes cut a deep saw kerf into the center of the timber. As drying takes place, this relieves stress and tends to minimize checking. When the log is milled for use in a wall, the kerf is positioned on the underside where it isn't visible. This is a fairly successful practice for keeping checks off exposed log faces.
While checks are not a structural defect, there is one fact that log-home owners should keep in mind. Upward-facing checks exposed to weather can collect moisture, which can lead to wood decay deep inside the log. Upward-facing checks should be protected by a wide roof overhang, a minimum of 24 inches, and a good coat of water-repellent wood finish that will keep moisture out. Large, deep checks should be sealed with caulk and treated. This is especially true in areas of the country where wood decay is a problem, the Southeast and Northwest.
Another aspect of appearance is the ability of a log to twist. More common in green logs than dry logs, twisting is most often caused by spiral grain in the tree, and it can cause structural problems. A twisting timber may open gaps between the log courses, permitting air infiltration. All trees exhibit twisting, but pines and firs have a greater propensity than cedars or spruce.
The Log Home Council's grading program for logs includes twisting as one of the considerations. Different grades, based upon strength, allow different deviations of the wood's grain direction relative to the long axis of the log. This deviation, called slope of grain, can adversely affect the bending strength of the log if it exceeds laboratory-determined values. If the house were mine and I were able to choose the logs used in structural applications where bending strength and stiffness were of great importance (such as headers above doorways and windows, roof rafters and floor joists), I would pick logs with a slope of grain no greater than 1-inch deviation in 10 inches (1:10). For logs stacked in the walls, a 1:6 slope of grain would be the maximum I would allow. These values correspond to those contained in the Log Home Council's grading rules and are based on American Society for Testing and Materials (ASTM) grading standards.
Finally while aroma is not a matter of appearance, it can certainly affect a homeowner's love affair with a house. Some cedars give off an aromatic smell that many people like, although some don't. A handful of the population is even allergic to the natural chemicals in wood that produce these smells. It would be best, therefore, if buyers took the time to acclimate themselves to the aroma of these tree species and see if they really enjoy them.
Durability. Durability of a log component is a house depends more upon home design and maintenance than it does upon tree species. Yes, some tree species do show more natural resistance to insects and decay than others - redwood, the cedars, baldcypress, black locust and white oak, to name a few. But you are throwing your money away if you buy a cedar home simply because you think it will never weather and rot. All logs will rot under certain conditions. Proper design is the most important factor in extending the life of a log house. Good practices include proper roof overhangs, keeping logs at least a foot above grade, not allowing rain, lawn sprinklers or water run-off to have regular contact with wood and porches to protect homes on the rain-driven sides.
Treatability. Log homes are usually protected by periodic applications or water-repellant wood finishes. They allow moisture from within the log to escape while keeping rain from soaking into the wood. For these finishes to work well, they need good penetration into the wood. Most of the pines allow for good penetration by preservatives. Spruce, hemlock, the cedars and true firs permit moderate penetration. Douglas fir, larch, tamarack and Southern yellow pine absorb these chemicals poorly because natural extractives in their wood cells tend to block the process.
Size. A homebuyer that wants 30-inch diameter wall logs will be limited to a few species, like larch, Western red cedar, hemlock or spruce. Handcrafted log-home builders prefer these larger timbers for their homes. The problem is that large logs are in great demand by a variety of industries. They are worth two to three times as much as the small diameter logs that might be employed by the milled-log industry. They are also more expensive to ship and erect.
Availability. Some species are simply more available on the national market than others. For example, Western red cedar is harder to find than species like the pines or firs. Naturally, a more abundant species tends to be lower in cost per lineal foot.
Modern forestry practices strive to provide raw materials for a variety of products, including house logs. With the nationwide reassessment of ecosystem management, some species, such as Western red cedar, may well become less available or less cost effective for use as house logs. However, the tree species commonly used for house logs in America are in absolutely no danger of becoming extinct, regardless of what some media reports lead people to believe.
Dying and standing-dead trees have historically been a significant source of raw material for the log-home industry. These trees are often unsuitable for other wood products, such as paper, plywood, and lumber because they have died on the stump. Manufacturing these trees into house logs is an excellent use for these trees, considering that many of the most desirable traits for a house log were found in these pre-seasoned logs. Better forest-protection practices have barely kept pace with the fire, disease and insects that kill large acreages of trees, but environmental concerns (some of which are justified) for other resources in these areas have significantly reduced the salvage of these trees. Good standing-dead house logs are still available, mostly in the West, but are substantially more expensive than those harvested as recently as five years ago.
Cost. Availability and demand are the driving forces behind the cost of wood. Demand for a particular type of tree is usually related to how many products it might be used for, such as furniture, dimensional lumber, paper, fuel, plywood, railroad ties and telephone poles. House logs, therefore, are just one of many uses. Several species, such as lodgepole pine, are increasingly being employed for a variety of products. This increased competition affects both the availability and the price. Even with increased competition for raw materials, however, most manufacturers are not having significant supply problems for most common species and sizes.
In conclusion, I believe that buyers want four things from their homes: a comfortable living environment, longevity, reasonable maintenance requirements and affordability. Tree species is only one part of the equation. Far more important are factors like house design, engineering, regional climate and site conditions.