In the mid – 1970’s, Timber Products Inspection, Inc., was asked by the log home industry to establish grade-stamping standards for the industry. In response, TP developed and provides a nationwide grading and grade-stamping program to log home manufacturers. The TPI grading rules and design values are based on the nationally recognized standard ASTM D3957 "Standard Methods for Establishing Stress Grades for Structural Members Used in Log Buildings".
In order to design with logs, one must have log design values. To obtain safe design values, logs must be structurally graded. TPI’s log grading program is, and always has been the leader in the industry, accepted by all National Building Codes and by the US Dept. of Housing and Urban Development.
Under current TPI Log Grading Program, the manufacturer’s personnel are trained and qualified to grade and grade-stamp their logs. The manufacturer must maintain a performance conformance rating of 95% or better when monitored by inspections by TPI personnel.
The TPI log grade-stamp provides the buyer with the assurance of receiving building logs from a quality-oriented company who is manufacturing a product under a meaningful quality-auditing program.
In years gone by, the pioneer and his family would select and cut the logs for his home. If a log was not acceptable to the owner, it could be cut into firewood and another log selected. Today, we have to be concerned with the cost of the log, diminishing log supplies and waste removal. The structures built by the pioneers were generally simple two or three room, one story structures. Log homes today are no longer simple one story, two room cabins. Instead, they are demanding large, multistory homes with elaborate roof systems and breathtaking views through oversize windows.
With that thought of the changing log home industry, let’s discuss the what, why and how of log grading.
In its simplest terms, a log grade is a set of maximum allowable characteristics that are permitted to be present in the log.
*In a structural log grade, the characteristics that are limited are those that affect the strength and use of the log. Such as, knots, slope of grain and compression.
*Manufacturing characteristics such as sawing and finished size tolerances may be limited.
*Naturally induced defects such as shakes, checks, decay and insect damage may be limited.
The simple answer is that building codes and financial institutions require structural logs to be graded and grade marked. However, log grading also provides a layer of protection for the manufacturer and the home owner by establishing a definable level of quality for the products being sold and a prescribed procedure for handling complaints on the quality of the logs.
Most importantly the grading helps to determine the design values for the log. The allowable characteristics permitted along with the size, shape and species of wood, help to determine the safe structural capacity for each log (i.e. allowable span and loads). Most logs in a log home are required to carry a structural load.
Being able to determine the structural capacity of each log allows an engineer or architect to properly design the structure to eliminate sagging rafters, bouncy floors and other types of structural failures.
Logs are normally grade during the manufacturing process by "approved graders" who work for the manufacturer. These "approved graders" must be trained and certified by an independent grading agency that monitors their conformance by making periodic unannounced inspections of graded material.