What is the Value of Log Grading
Monday, December 2nd, 2013 | Uncategorized | No Comments
In my previous post I made reference to grading and grading agencies. This is a subject that is often not considered, let alone, understood by the log home community. The grading system was established by ASTM (American Society for Testing and Materials) and administered by approved grading agencies. In the log home industry Timber Products, Inc., and the NAHB’s Log Home Council are the approved grading agencies.
The grades established define the design strength of a wall log to resist vertical, horizontal and other inherent forces. The grades are not a measure of the cosmetic characteristics of the log. The system starts with a list of potential defects that might be found in any given log. There are approximately 25 defects that are measured, e.g. burl, checks, decay, grain, pitch, etc. An inspector inspects the log to determine how many of the defects are present and the degree of the defect. Each grade has set limits that are allowed to meet the criteria for that grade. Put simply, the more defects and the higher the degree, the lower the grade. The grades are, from highest to lowest, Premium, Select, Rustic, Wall Log 40, Wall Log 30 and Wall Log 27.
Like moisture content, the lower the moisture the more predictable the long term performance of the log, the lower the number and degree of defects the more predictable the performance of the log. To provide some examples let’s look at a couple listed defects that are measured.
Slope of grain, which is a measure of how much the grain visible on the log’s surface deviates from the axis line of the log, is one of the defects. The slope is recorded by measuring how many inches the slope deviates from the axis in twelve inches. For example, a slope of 1 in 12, is very low and a slope of 5 in 12, is quite high. The risk of a log with a slope of 1/12, twisting after it has been installed in the house wall is very low. The risk of a log with a slope of 5/12 twisting in the house wall is relatively high. If that log happens to span over a window and it twists it could rack the window frame making it impossible to open the window.
Knots are another characteristic that determine the grade of a log. Size, location, soundness, decay present in the knot, number of and other characteristics of the knots are all reviewed to determine their affect on the grade of the log.
So, you can see, having a moisture content near the average moisture content of the environment where the log is used, and having high grade logs improves the owner’s probability of not having problems develop from the natural phenomena that occur with large timbers.
