What are Log Rules ?
General Features of Log Rules
Since the first sawmill was built in the United States, over 100 log rules have been
developed, using a variety of methods. Some were based upon the lumber tallies of
individual mills, others were developed by diagramming the cross-section of boards in the
ends of logs, while still others were developed using mathematical formulas. In general,
log rules must account for the taper that exists in all logs, saw kerf (or the loss of
wood as sawdust), and a fixed procedure for removing wood on the outside of the logs for
slabs. The Doyle, Scribner, and International log rules are probably the most widely used
rules in the eastern United States.
Doyle Log Rule
The Doyle Log Rule, developed around 1825, is based on a mathematical formula and is
widely used throughout the southern United States. This rule allows for a saw kerf of 5/16
inch and a slabbing allowance of 4 inches, which is about twice the normal amount. Because
of this, the Doyle Rule is somewhat inconsistent; it underestimates small logs and
overestimates large logs. As a seller of timber, you must be aware that for smaller logs
the Doyle Rule will underestimate the actual volume of wood that you have in your trees.
Scribner Log Rule
The Scribner Log Rule, developed around 1846, is a good example of a diagram rule. It was
created by drawing the cross-sections of 1-inch boards within circles representing the end
view of logs. A space of 1/4 inch was left between the boards to account for saw kerf. The
Scribner Rule does not have an allowance for log taper and typically underestimates logs,
particularly if the log length is long. The Scribner Decimal C is a different form of the
Scribner Rule; it rounds the volumes to the nearest 10 board feet. For example, 392 board
feet on the Scribner is equivalent to 390 board feet on the Scribner Decimal C scale.
International 1/4-Inch Log Rule
This rule was developed in 1906 and is based on a reasonably accurate mathematical
formula. The rule allows for a 1/4-inch saw kerf and a fixed taper allowance of 1/2 inch
per 4 feet of log length. Deductions are also allowed for shrinkage of boards and a slab
thickness that varies with the log diameter. Overall, the International 1/4-Inch Log Rule
is the most consistent and is often used as a basis of comparison for log rules. |