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Cut-and-Dried
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| A Look at Log Moisture
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| By Jim Cooper
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All logs shrink as they dry and shrinkage can cause settlement. If you and your log home company don't plan for shrinkage, you could end up with doors that bind and windows that
jam--or worse. Many people dream of settling down in a log home, but they don't always understand that, beyond their dream, settling is an important reality of log home living. Because of the potentially serious consequences of settlement of log systems, a whole mythology has grown up around it. Indeed , settlement, shrinkage and log moisture content stir impassioned debate among log home shoppers, sales reps and manufacturers.
To sort settlement myths from reality and to provide information on which to base decisions, we need to define a few terms. Let's start with log moisture content, a topic salespeople often use to promote a particular manufacturer's product (or attack a competitor's).
A living tree consists of several types of cells, including the woody fibers responsible for the log's engineering characteristics. The root system continually absorbs moisture from the soil. That moisture is transported through the tree as long as it lives. A freshly cut log contains a large volume of water, found both between the cells (free water) and within the cells (bound water). Moisture content is a measure of the total amount of both types of water in a log in relation to the weight of solid wood.
The amount of moisture in a log is expressed as the percentage of its total weight that is water. Thus, a
40-pound piece of freshly cut (green) wood may be placed in an oven and baked until all traces of moisture are removed. If its
oven-dry weight is 20 pounds, we say than the green log has a moisture content o 100 percent. If its final weight were 30 pounds, its original moisture content would be 50 percent. The moisture content of wood is expressed by the formula Water Weight/
Oven-Dry Weight x 100 = % moisture content where water weight equals starting weigh minus oven-dry weight. There are ways to measure the amount of water in wood without baking it. Most log companies, woodworkers and wood technologists use electronic moisture meters that pass an electric current between metal probes inserted in the sample.
Now let's turn to shrinkage. When tree is cut, its uptake of water stops and it begins to lose moisture to the atmosphere, or dry. As moisture leaves, wood fibers contract to fill the voids once filled by water. Fibers bind more closely, and cell volume decreases. Free water leaves the drying log first. Then, when all free water is gone, bound water will start to leave. The moisture content of the log at the point where all free water has been removed and only bound water remains is called the fiber saturation point
(FSP).
FSP varies among wood species but most of the woods used in log home building reach fiber saturation at about 28 percent moisture content. In understanding the behavior of log systems it is important to recognize that wood does not show substantial shrinkage until it reaches the fiber saturation point. No matter what the starting moisture content of the log, it does not start shrinking until it reaches its fiber saturation point.
Once the log reaches its FSP, shrinkage begins and will continue until the moisture level of the log is in equilibrium with its environment (equilibrium moisture content, or EMC). Once at EMC, the log dimensions may vary slightly with changing environmental conditions, but for practical purposes the log is stable and further changes will be minor.
Equilibrium moisture content is determined by the environment rather than wood species, meaning that all logs, regardless of species, should wind up with approximately the same EMC. A rule of thumb says that where relative humidity averages 50 percent, the EMC will be approximately 9 percent; 25 percent relative humidity yields about 5 percent EMC; 75 percent relative humidity results in about 14 percent EMC.
So a log starts shrinking when it reaches FSP (about 28 percent moisture content) and stops when it reaches EMC (5 to 14 percent moisture content). How long does this take? It depends on the wood species and the size of the log. Hardwoods and dense softwoods take longer than lighter or more porous softwoods. Larger logs will naturally take longer than smaller ones.
I've measured logs more than three years old that were still several points above EMC, meaning they were still shrinking.
Shrinkage affects all dimensions of a log. Shrinkage along the length of the log is almost negligible regardless of the starting moisture content of the log. Of most concern in log construction is reduction in cross section, or radial shrinkage. This varies among wood species, but all species used in log home construction range from 2 to 6 percent of the log's radius. Thus an
8-inch round log (4-inch radius) with a radial shrinkage of 5 percent will lose 0.4 inches or slightly more than 3/8 inch in cross section as it goes from fiber saturation to equilibrium moisture content.
Finally, settlement is the reduction in height of a log wall resulting from several factors. Compression of wood from the weight of logs above it, compression and shrinkage of sealants and pressure on fasteners all contribute to overall settlement, but shrinkage of wood is potentially the greatest contributor.
How do these factors apply to log homeowners? Is all this information really important? Consider
air-dried 8-inch logs with moisture content of 25 percent. We know that over their first several years these logs will dry to a moisture content of about 9
percent-a loss of 16 percent. The shrinkage that accompanies this moisture loss can range from 1/8 to 1/4 inch. So, in a log wall containing 16 courses, potential settlement can range from 2 to 4 inches.
If door openings reach the 14th course, that means leaving a settling space of 1-1/2 inches to prevent logs,
second-story floor system and roof from bearing on doors and windows.
Many manufacturers and many experienced log builders allow as much or more over doors and windows as standard building procedure, but what about interior walls? If an exterior wall settles 2 inches and there is no provision for settlement on interior walls, the result can be sloping second floors (possibly causing
second-story doors to bind or not close), and bowed or cracked drywall as
second-story floor system and roof bear down on interior framing. This is why a number of manufacturers now use adjustable supports for the center girder and include a settling space, concealed behind trim for interior partitions. By adjusting the supports periodically through the log settlement period
and a short time after the logs have reached EMC, the second floor and roof systems can be kept plumb and level.
Will settlement of this magnitude always occur? Not always, but it's impossible to say with certainty when it will be less. A number of factors can influence settling. These include the type of sealants and coatings placed on the logs, the type of wall fasteners used and the logs' initial moisture content. The last is affected by the amount and kind of drying used, as well as storage conditions for logs on the job site.
Consider a kiln-dried log with a typical moisture content of 19 percent. It is still 8 to 12 percent above EMC in most areas of the country. Thus it is still capable of shrinking. According to the formula, our
8-inch log can still shrink as much as 1/16 inch for a total potential settlement of 1 inch in a
16-course wall.
Fasteners won't affect shrinkage, but they can have an impact on settlement. Driving nails through door and window frames into log openings instead of slotting nail holes, for example, or failing to countersink lag screws or spikes may prevent logs from settling. If logs shrink 1/4 inch each in cross section, the space between logs will widen by 1/2 inch, enough to permit air infiltration.
Sealants and finishes usually affect the rate of drying rather than amount of drying of logs. Completely sealing logs to prevent moisture loss introduces the potential for decay. Usually, breathable sealants are used outside, and surfaces within the wall aren't sealed. As a result, the logs will eventually reach the same EMC as if left unsealed.
The bottom line for log home shoppers is to accept some amount of shrinkage and settlement as a natural characteristic of log construction regardless of the type or amount of log drying involved. Look at the construction system used to see how settlement is handled. Settlement details included in blueprints and construction manuals should assure you that the manufacturer is aware of these factors and has taken steps to control them. |
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