What’s in a Log Rule?

Although cubic volume scaling based on the metric system is almost universally practised in forestry around the world (including Crown timber in Ontario), in many parts of southern Ontario the forest sector (e.g., logging, sawmills) harvesting and purchasing wood from private land continues to use the traditional method of measuring log volumes by the board foot scale. 

This method of measuring log volume is based on a simple assumption about how much of the log’s volume can be recovered through the sawing process (lumber yield).  Even though there has been an accepted unit of measure (board foot) used for over 150 years, the means or method in determining (scaling) the volume of a sawlog still varies according to local needs and traditions.  Measuring logs is unregulated, and the logger or sawmill may use the method of their choice. 

Developing a log rule

There are three basic methods in the development of a log rule, and it may involve utilizing more than one of the procedures:

1. Based on lumber tallies (e.g., a measurement of the lumber produced from sawlogs of a given diameter and length).  Data is collected until such time that all sizes of logs have been sampled;

2. Developed by diagramming the cross-section of boards in the ends of logs based on applicable conditions (e.g., saw kerf, slabs, board thickness and width, taper assumptions, etc.); and

3. Using mathematical formulas based on an assumed geometric solid form and adjusting for losses due to saw kerf, slabs, etc.






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Log rules – there are many

Over the past 150 years, more than 95 different log rules (used under at least 185 different names) have been developed and used in North America to measure sawlogs.  As part of our rich logging history there have been a number of different log rules used by the forest sector in eastern Canada.  Some of the more obscure rules used at one time include the Canadian Twenty-One-Inch Standard Rule, the Twenty-Four-Inch Standard Rule, and the Canadian Twenty-Two-Inch Standard Rule.

However, only a couple of rules have survived the test of time and have gained any recognition in Ontario.  They include the Ontario Log Rule, Doyle Rule, Scribner Rule and to a much lesser degree the International Rule.  Here is a brief description of these four main log rules.

Ontario Log Rule (OLR) – this log rule was first adopted for use in Ontario in 1952 replacing the Doyle Rule.  It applies to hardwood logs 4 to 40 inches in diameter and 8 to 18 feet in length.  Logs measuring over 18 feet must be scaled using another method. 

The OLR is based on a general lumber recovery formula (Chapman, 1921) and takes into account several factors influencing the amount of lumber that can be sawn from the log.  For example, it allows 30% loss for kerf (thickness of the saw blade) and 1.2 inches for slab thickness (loss of slabs from four sides of the log). The accepted formula for the ORL is outlined in Table 1.

In 1985, the Ontario government replaced the OLR for the measurement of Crown timber by a cubic volume scaling (cube-grade method) based on the metric system.  Today, all timber harvested from Crown land is scaled using the metric system.  However, the OLR is still used extensively by the forest sector throughout many parts of Ontario to scale hardwood logs harvested from private woodlots.

Doyle Rule – this log rule is one of the oldest and widely used rules in North America.  It was first published in 1825 by E. Doyle and is based on a mathematical formula.  The formula was based on the following presumptions – a 5/16 inch saw kerf, 2-inch slab allowance, and no allowances were made for taper.

There are several weaknesses in this log rule.  The slab allowance is approximately twice the norm and is excessive for small logs, the kerf allowance is inadequate for larger logs, and the formula doesn’t account for log taper.  Some say using the Doyle Rule will under scale average sized logs by up to 30% (mainly due to the low volumes for small diameter logs).  The formula for the Doyle Rule is outlined in Table 2.

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The Doyle Rule is still used in many parts of southwestern Ontario and is widely used in the eastern United States.

Scribner Rule – first published in 1846 by J.M. Scribner this rule was designed using a diagram technique for 1-inch lumber (in board widths of 4, 6 and 8 inches).  The technique used in developing this rule entailed drawing a cross-section of 1-inch boards within circles (diameters 12 to 44 inches) representing the end view of logs.  The rule is based on a ¼ inch kerf, doesn’t account for log taper in its volume calculations and generally underestimating volumes of longer log lengths.

Since 1846 there have been a number of revisions to the Scribner Rule.  In 1900 the Lufkin Rule Company modified the rule to create the Scribner Dec. C Log Rule.  In this modified version the board foot volume recorded is taken at the closest 10 board feet mark and the last digit is dropped.

In 1950, Bruce and Schumacher proposed modifying the rule’s formula (refer to Table 3).  The modifications would correct the irregular increases in board feet volume from one diameter class to another, as a result of the diagramming process used to create the rule and to adjust for the modifications made by Lufkin in the Dec. C. version of the log rule.  This revised formula resulted in providing fairly consistent volumes for 16-foot logs or shorter (under 28 inches in diameter).

Today, the Dec. C Log Rule is the most widely used version of the Scribner Rule and is used in many parts of the eastern United States.

International 1/4 Inch Kerf Rule – originally developed in 1906 by J.F. Clark, and modified in 1917 to allow for a saw kerf of 1/4 inch, this log rule is based on a mathematical formula and an analysis of the losses occurring during the sawing of logs to lumber.  Deductions for shrinkage and slab thickness vary with log diameter, and allowance for log taper has been incorporated at ½ inch per 4 feet of log length.  Table 4 provides the formula(s) for various log lengths.

Over the years other modifications have been made to this rule including the development of an integrated formula to accommodate a variety of log lengths (Grosenbaugh, 1952).   Although many experts consider this log rule (Table 4) as being the most consistent and accurate it is not widely used.

How these other do rules compare against the Ontario Log Rule?

Log volumes will differ (in some cases substantially) depending on which log rule is used.  The differences are due to the fact that when each log rule was developed different assumptions were used – e.g., allowances made for saw kerf, taper, slab size, edgings and shrinkage.

Suggested reading

If you are interested in learning more about the different log rules used in North America over the past century you may want to read A Collection of Log Rules. Frank Freese. U.S. Forest Service. 66-pages. 
A copy of this publication is available on the Internet at <http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr01.pdf>.

Additional information on how to scale logs using the Ontario Log Rule and the Doyle Rule (including volume tables) can be found on the association’s sister website at <www.ontariowoodlot.com>.

This article was featured in a past edition of the S&W Report, the newsletter of the Ontario Woodlot Association. 

© Ontario Woodlot Association, 2010

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