Shrinkage in Wood
Wood shrinks as it loses moisture. This simple fact is the cause for a wide range of potential problems for wood users, including warping and splitting in lumber, squeaking wood floors, and checking and "settling" in house logs. On the other hand, an understanding of wood moisture relations is the key to preventing nearly all problems related to shrinkage. This article explains the relationship between moisture changes and dimensional changes in wood, and shows how those who manufacture wood products can minimize shrinkage-related problems in their products.
To understand how wood shrinks, we need to understand its structure. At the microscopic level, wood has the appearance of bundles of soda straws, in which the fibers (usually referred to as wood cells) are long, hollow, and oriented along the direction of the trunk. In the living tree, these hollow cells are filled with liquid water. The walls of the cells also contain water, though that water is bound molecularly to the cellulose molecules that make up the cell wall material. Whenever liquid water is present in the hollow cells, the cell walls are also saturated with water.
After a tree dies or is harvested, the liquid water in the hollow portion of the cells is slowly lost to evaporation. The point at which all liquid water has evaporated, but the fiber walls are still fully-swollen and saturated with water, is called the fiber saturation point. This is an important condition, because even though a significant amount of moisture has been lost, no shrinkage has taken place, since the cell walls are still fully swollen. In most wood species, the fiber saturation point is around 28-30% moisture content.
As wood continues to dry below the fiber saturation point, it begins to shrink, since moisture is being lost from the cell walls. The amount of moisture that leaves the cell walls depends on the relative humidity of the surroundings, since wood moisture content eventually reaches an equilibrium point with the relative humidity. The drier the air, the greater the moisture loss, and, consequently, the greater the shrinkage. Wood used indoors, such as in furniture, cabinets, and wood floors, eventually reaches an equilibrium moisture content around 8%. So, manufacturers of these products should make sure that the wood they use has already been dried to that level. If they do not, the products will shrink and could cause problems after they are placed in service.
The surface of a piece of wood reaches equilibrium with its surroundings fairly quickly. The inner part of the wood takes a bit longer, since it takes a while for the moisture to migrate out of the piece. We refer to this moisture difference within a piece of wood as a moisture gradient. In extreme cases, such as when green, fully swollen wood is placed in a very dry environment, the outer shell dries quickly before the inner portion begins to dry at all, and damage can occur to the wood. In these cases, the outer shell shrinks and squeezes the wet inner wood. If the squeezing, or compression, of the inner wood becomes high enough to exceed the compression strength of the wood itself, the inner cells may collapse. Kind of like a flattened soda straw. Improper kiln drying can cause collapse of the inner wood cells, but good kiln operators are aware of this and use drying schedules that prevent this type of damage from occurring.
Logs used for house logs should be dried prior to being placed in service, and a reasonable target moisture content is 15%. That way, most of the shrinkage has already taken place. Building with "green" logs, which are those harvested from living trees, will result in considerable shrinkage in walls of log homes and requires specialized construction techniques to allow for shrinking around doors, windows, interior walls, and stairs. The actual final moisture level of logs in log home walls will vary by climate zone, but a typical range is between 10 to 14% moisture content. One industry standard considers logs to be "dry" as long as the moisture content is no greater than 19% at a depth of 1 1/2 inches, but it is clear that additional drying, and therefore, shrinking of walls, should be expected in the completed home. So, pre-drying to the 15% target moisture content has some real benefits.
The amount of shrinkage that takes place in wood depends on several factors, including: the amount of moisture loss; tree species, and grain orientation. We have already seen that pre-drying can limit the amount of moisture loss (and, therefore, shrinkage) that occurs after a home or piece of furniture is built. Some tree species shrink (and swell) more than others. However, as long as pre-shrinking has taken place, the individual differences become less of a consideration, unless the wood continues to dry once it is converted to the final product.
An interesting phenomenon with wood, however, is that shrinkage varies according to the grain (or, fiber) orientation. Along the grain, wood typically shrinks very little, so length change in logs or lumber is quite small. Most shrinkage occurs across the width of logs or lumber, and even then, there are differences. Think back to some geometry terms. A line drawn from the center of a circle to the outside is called a radius.
A line drawn such that it only touches the edge of a circle is called a tangent. Boards cut from logs such that the exposed face is oriented in a tangential direction will shrink twice as much as boards cut in a radial direction. In lumber this shrinkage difference can cause distortion in the form of warping or cupping. In logs, the shrinkage difference results in checking.
Pre-drying round logs can take a considerable amount of time, since they contain lots of water and their volume is large. Log home producers specializing in round logs typically must wait over a year for their logs to dry to 15% or less moisture content. Nonetheless, this drying period is essential for pre-shrinking the logs and minimizing (or eliminating) further shrinkage after the walls are built. Even "standing dead" trees can contain lots of water so drying for a period of time may still necessary. It is also important to note that even though the outside shell of a log may be dry, the interior may remain at a high moisture content, and additional shrinkage will occur.
One advantage in laminating kiln-dried lumber to produce house logs is that each board has already been dried to the appropriate moisture content. That way, the laminated product has a uniform moisture content that is quite close to the final equilibrium moisture content it will achieve in the log home. Another advantage is that checking is essentially eliminated, since checking is nearly eliminated by converting logs to lumber. The result is a stable, check-free log.
"Settling" of log home walls often occurs after construction. The amount of settling varies considerably from one home to another. The major causes of settling include: settling of the foundation into the soil; settling of the layers of logs, and additional shrinkage taking place. It is important that concrete foundations be poured onto undisturbed soil, or subsequent compaction will take place. If foundation settling occurs unevenly, cracks in the concrete can form and the home may tilt slightly. Good builders usually can prevent this type of settling from occurring. Log layers may compact on each other slightly, taking up the slack between logs as the weight of the building (and snow) presses down. Compression of the logs does not occur, since the strength of wood in compression far exceeds the loads involved. Additional shrinkage, on the other hand, is usually where the most "settling" takes place. A home built with "green" logs can have as much as 2 inches of shrinkage or more in an 8-foot wall! Log home builders who specialize in green log construction will allow for this shrinkage around doors and windows, so as to prevent damage. However, they must also take care in the placement of interior walls and stairs, so that the log walls can shrink around them. Building walls with logs that are pre-dried to an equilibrium moisture content of 10 to 14% will minimize the amount of shrinkage-related "settling" in walls.
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