The Log Cabin

Friday, April 23, 2010

Drying Logs for Log Cabins

The moisture content of logs is crucial to the quality and integrity of log cabin construction. After trees are cut down, they are debarked and cut into rectangular shaped ‘cants.’ Fresh wood is known as ‘green’. Green wood may have up to 50% water content. To prevent shrinking, settling and twisting after construction, the logs must be ‘dried’ to a level that is in equilibrium with the relative humidity of the surrounding air.

Most log manufacturers provide air-dried logs. The cants are stored outside, off the ground and preferably under a roof. The rate of drying is subject to local weather conditions such as humidity and the volume of air movement. It is a cheap but a relatively long process. The moisture content will eventually reach equilibrium at 15-20%. The process takes months to over a year.

Kiln drying is an accelerated but more expensive process. The cants are stacked in a sealed building and the temperature is slowly raised to 170 degrees F. Fans are used to move the air and dehumidifiers remove the moisture from the air. Due to the high temperature, the process kills fungus, mold and insects. It also causes any pitch (wood tar) to crystallize. The rate of drying is controlled by varying heat and humidity to minimize drying defects. Keep in mind that wood dried to a very low level will rehydrate once installed to the equilibrium level. A 19% moisture level is common.

Logs shrink as they lose their water content. Water in wood moves from areas of high moisture content to areas of lower content. The drying occurs on the outside first and then the inside. Splits and cracks result from the outer portion of the log drying while the inner portion remains saturated. Shrinkage of the outer portion is restricted by the wet interior and splitting can result.

There are two primary methods for measuring the dryness of the finished product. A moisture meter measures the resistance of the wood to an electric current. Current is impeded by moisture. This method only measures the outer portion of the wood. The second method determines the overall moisture content by weighing the sample wood before and after drying. The formula for the % moisture content is:

(weight before – weight after) / weight after

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