Log Shrinkage

The CHECKS, The CRACKS and Their CRITICALS

It is important to understand how checks are formed and what their impact is on the logs you choose for your log home.  As discussed in more detail in the article KILN DRYING and WOOD MOISTURE CONTENT, the departure of bound water originating within the walls of the wood cells is responsible for wood shrinkage.  This shrinkage begins at a moisture content of approximately 30 percent.  As the cell walls dry, their thickness decreases, thus causing mechanical forces to occur in the log in the form of drying stress.  The forces build up until the crack develops and opens, relieving the stress.

More specifically, the tangential shrinkage (the direction of the circumference of the log) is 1.5 to 2 times greater than the shrinkage in diameter.  Since the circumference of a circle is slightly more than 3 times the diameter, for every 1 inch of diameter shrinkage in a large log, the circumference should decrease 3 or more inches.  In reality, the wood shrinks only about 2/3 of that amount, so the crack develops to allow the residual stress to be relieved.  Checks therefore are inevitable and indicate the wood has been drying. 

The presence of checks is a good thing relative to log strength.  Contrary to a common perception, the dried, checked log is actually stronger and stiffer than it was when it was saturated with water.  But other than the common sales talk that “Checking adds character,” this is where the optimistic effect of checking ends.

On the more important, critical side of checking, drying checks play a pivotal role in the exclusion of air and water from your home.  Apart from the startling noise that can occur as checks form, checks that extend into the corners, checks on the outside walls that open upward and checks that face upward and spiral into a joint or even into the interior — should be identified and treated in order to prevent problems with energy usage, water stains and decay.

When checks, particularly wide checks, occur in exterior logs lacking a good finish, liquid moisture from rain can enter the check and accumulate in its bottom at the center of the tree (the pith).  With enough precipitation or errant landscape irrigation, liquid water can move via wind or gravity along the length of the check and follow the course of the spiral check into the interior of the house.  If the spiral is severe enough and the log of sufficient length, the water may flow out of the down-turned portion of the spiral check and run down an interior wall many feet from where it actually entered the check.  Of even more concern is the water that eventually migrates away from the reservoir and forms at the bottom of the check.   This water seeps into the walls and interior spaces of the surrounding wood cells creating an ideal environment for multiplying decay fungi, whose spores are present in abundance in the air around and on the surface of your house.

While checking is normal, checks should be managed through a program of identification, remediation and periodic maintenance.  As one might expect, the most significant checks are usually found on the log walls facing south and west, where full exposure to the sun generally dries the wood to a lower moisture content.  These sides of the structure allow wider and more numerous checks to form, thus increasing the threat of decay and incursion of air and water.  On the other hand, the higher temperatures on these walls tend to dry the wood more quickly than what is found on the more sun-protected sides and areas of the structure’s walls.  In warm dry climates, as found in several locations in the western United States, decay in the logs is a lesser problem because the larger and more numerous checks can dry more quickly than in warm moist climates in the Midwest and East, as well as coastal area. That is not to say logs cannot decay in Southern California, but the water in the bottom of the checks must be replenished on a regular basis, as might occur when above-ground irrigation hits the walls of the structure.  Furthermore, this program becomes more critical over the first 5-10 years (based on log diameters) as logs continue to check as the moisture content equalizes with its environment.

I am told small checks less than 1/16th of an inch in width, generally will not let liquid water into their opening if, and only if, a quality water-repellant exterior finish is in place.  The theory suggests water is naturally repelled from the surface of the finish and forms droplets that grow in size as moisture accumulates, until they are too large to fit into the checks on the wood’s surface. Therefore, these small checks do not pose a threat to the wood’s integrity; so long as the surface finish is in place and working.  However many new homeowners skimp on the finish, whether it be the use of inferior materials, poor preparation and/or cheap labor, because it is one of the last remaining tasks to accomplish and the budget has already been exceeded.  This common error is something a log home dweller is sure to regret.  Consequently the right log finish may be the single biggest factor that determines your overall satisfaction log home living should bring.

Now that you know that checks are normal and expected in conventional logs, be well aware before your log home purchase of the need to manage their impact on the structural and functional integrity of a conventional log home.  If the inherent phenomena and required maintenance of log checking concerns you, the near elimination of checking inherent in a quality laminated log is worth your strong consideration.

 

 

References:  Edwin J. Burke Ph.D. Professor of wood science, University of Montana