When is a board foot not a board foot? Most of the time, as it turns out. You probably already know that a 2×4 is a nominal (meaning “by name”) measurement for a board 1-3/4” by 3-1/2”. But what about determining the amount of lumber in logs when your cutting them on a portable sawmill? Whether purchasing logs or making an estimate for a customer, measuring the volume of lumber in a log is an important skill for the sawyer. The problem is, there are several lumber scales, which give very different answers when you plug in the numbers. The two most common are the International 1/4” scale and the Doyle scale.
The International 1/4” scale is accurate for a saw blade with a 1/4” kerf. Narrow kerf bandsaw mills (1/8” kerf) get about 15% better yield—the equivalent of an extra board for every seven you cut. If a customer needs a bid for milling a stack of logs, it is fairly quick and easy (not to mention professional) to whip out a tape measure and a log scale chart and give a very accurate estimate of how much lumber you expect to get out of a pile of logs. This is especially useful if you charge by the board foot. Crooked logs and logs with rot or split, however, will naturally yield less lumber. With practice, you will be able to determine how much to deduct for these types of defects.
The Doyle scale is the standard by which hardwood logs are bought and sold. It underestimates the volume of smaller logs and overestimates the volume of larger logs The smaller the log, the greater the penalty. For example, a 14” diameter, 12’ log scales out 75 board feet on the Doyle scale, 98 board feet on the International 1/4” scale and will yield about 113 board feet from a bandsaw mill. It was designed to compensate for the extra time and expense it takes to mill a given amount of wood from small logs. Most hardwood mills buy lumber on the Doyle scale. If you buy small logs on the Doyle scale, you get the best deal. For example if you buy 1,000 board feet of 14” diameter logs on the Doyle scale, your pile of lumber will come to about 1,500 board feet after milling. But remember, small logs require more log loading and turning for every board foot you get out of them and generally have lower quality wood; hence the Doyle penalty.
A third called the Scribner scale is most common in the Pacific North West. Like the Doyle scale, it penalizes for smaller logs, but not quite as much. There are a number of other scales, less commonly used. The “cedar” scale, for example is occasionally used for Eastern Red Cedar.
Even wood off the mill can have some variation. Typically, even if a customer asks for 3/4” thick boards, most sawyers will scale them as though they were 1” thick. After all, the amount of time it takes to make a cut is the same, no matter how thick the wood is. Make sure your customer, the logger and you all have the same understanding of how the logs are being scaled and are in agreement before the sawmill blade ever touches the wood!