Trees are made up of tiny straw like cells, think of millions of tiny straws throughout the tree, that carry water and nutrients from the root system all the way to the top of the tree. When trees are “green” these cells are filled with sap or water. The ratio of water to dry wood substance is known as moisture content. The moisture content of wood affects its physical and mechanical properties.
Wood can dry down to approximately 30% moisture content before the physical and mechanical properties of wood begin to change, this is because it is simply removing the water from within the cells. However, as moisture content drops below 30% it begins to remove water from the cell wall which changes the physical and mechanical properties of the wood. So why do we dry wood if it is going to change its physical properties?
First, wood with over 20% moisture content is susceptible to bio-deterioration, also known as rotting. Second, wood will naturally acclimatize to the relative humidity of its environment meaning it will dry whether we want it to or not. The purpose of drying wood is to control the physical and mechanical changes in a way to minimize its affects on the final product. This is accomplished by drying wood prior to final manufacturing.
Due to the fact the perimeter of wood will dry before the core of the wood, all wood experiences checks or splits in the surface as it dries. While these checks have little effect on how the wood performs they do allow exposure to the elements where water can be reintroduced creating other issues. A good way to control the physical and mechanical affects of drying is to use a “Kerf” (a manufactured cut made along a log’s length into the center of the log) which allows the log to dry more evenly as well as to relieve the stress of the uneven drying that naturally occurs in wood. Other mechanical and physical effects of drying include cupping, bowing, twisting, and warping. In most cases these issues are addressed through the final manufacture of the material.
To verify you are getting a “dry” log the moisture check, performed by a moisture meter, should be performed with the meter needles penetrating the center of the log. Be aware of how the moisture is being measured by your suppliers. Many industry standards only require the log be measured at 1” depths within so many feet of the end of the log, which in effect is the driest part of the log. Using logs dried at or below industry standards throughout your entire log package means you are buying a more stable and consistent product.
What does all this mean for you and your log home? You need to research your supplier and ensure your full log wall system has moisture content at or below industry standards as measured at the core of the log. Also ensure your log walls are “kerfed” to reduce checking, warping and twisting giving you the most consistent and stable natural product available to you.