An Indepth Article On the Requirements and Process a Timber Goes Through to Make the Grade

The grade categories for wall logs are:
1) beam (this grade meets the highest structural stress standards);
2) header (this grade along with the beam grade are designed for uses where timbers with the highest strength properties are required, such as headers/lintels, beams, joists, etc.);
3) wall (this grade is designed for uses where high bending strength is not required, but where values for compression and elasticity strength are of primary concern, can be used either vertically or horizontally for any walls);
4) utility (this grade is designed for non-habitable buildings and where economy is preferred);
5) non-grade (this grade was established to encompass the timbers that did not meet any above grade standards).

Essential background knowledge for grading log home timbers
Visually grading the structural integrity of house logs and timbers requires a familiarity with wood growth and the structure of a tree. At the core of any tree is a pith, a remnant of the original shoot which gave the tree its height. As the trunk and branches thicken with growth and age, a series of concentric rings surround the pith. These, usually obvious, concentric rings are often referred to as growth rings and are defined by latewood growth which is produced in the summer months and is denser, heavier, and darker than the growth of earlywood which is produced in the spring months. The color of the heartwood comes from resins, minerals and other compounds being deposited in the cells as they are decommissioned from its role as sapwood. The resin-saturated heartwood contributes to the durability and strength of the timber. This feature also has the added benefit of making it almost impervious to rot and termites. All wood is first formed as sapwood, its main function is to aid in storing and distributing its needed nutrients. As the tree increases in age and diameter an inner portion becomes inactive and finally ceases to function—it is then relegated to heartwood status. The cambium is a thin layer of cells that divide to produce both the bark cells and the wood cells. Most of the wood cells hollow out to form the sapwood through which minerals and water travel from the roots to the leaves. Sugars produced in the leaves travel down the phloem also known as the inner bark to feed the cambium and ultimately the roots. Other wood cells store food such as starch which is produced by a process known as photosynthesis .

There are two main types of trees:
1) the hardwood tree which is distinguishable by its being a flower-bearing broad-leaved tree; and
2) the softwood trees which are cone-bearing conifer trees.
The most common log homes are built from coniferous or the softwood species of trees, the reason being that the majority of the wood cells are of one variety, called tracheids, which do not generate pores common and predominant to many hardwood species. So, as a result the wood texture is more uniform. In some coniferous species like the white pine there is not much visual contrast in the growing seasons so the wood is more uniform in color and texture, its consistency makes it easy to work with, while harder pines present a dense and deep colored contrast.

Natural Timber Characteristics - How they influence grading log home timbers
There are many factors that can reduce the strength and integrity of a log or timber, such as slope of grain, knots, splits, checks, shakes, and saw cuts. Therefore it is important to have a comprehensive understanding of the terms and definitions. Mark Elliott , vice chair of the Log Grading Committee for the Log Homes Council and vice president of Coventry Log Homes said, “Properly assessed and utilized timbers can turn what could have been considered defects into desired natural characteristics, that can be appreciated for their own unique beauty. “

Download the Wall Grade Chart

Slope of Grain
—The measure of the angle of the wood grain from a line parallel to the edges of the piece. There are three types of log grains: the straight grain; the spiral grain; and diagonal grain. The diagonal grain is accentuated by such factors as the taper of the tree, crooks, twists, swelling and whether the timber is either quarter or edge sawn. The specific acceptable range for the beam grade timber, for example, is no more than a one inch drop in a 14-inch span.

Knots
— “A portion of a branch or limb that has been surrounded by a subsequent growth of the wood of the tree. As a knot appears on the cut surface it is merely a section of the entire knot, its shape depends upon the direction of the cut.” —American Society for Testing and Materials, 1992. Standard terminology referring to wood. Standard No. D-9-87 [Reapproved 1982]. Philadelphia, PA
Knot types are as follows:
Black knot: Also known as an encased knot, the annual growth rings have not grown along with the surrounding wood. It’s found to have bark or pitch pockets separating the dead limb-wood from the stem wood.
Branch knots: Two or more spike knots diverging from a common point at or near the pith. These knots are limited only by size, assuming that they are sound and tight.
Center line knot: Also referred to as a center knot whose center lies at the midline of any lumber face.
Cluster knot: Two or more knots, extending to the pith and that are not part of the same limb, which are grouped together and are defined by the local deviation of wood fibers such that the group creates the deflection rather than an individual knot. Knot clusters are not permitted in beam and header grades of wall logs since no reliable measure can be established. A group of single knots is not a knot cluster. In practical terms, longitudinally oriented stem wood exists between the knots, they are single knots, and not a knot cluster. If one of the knots, usually markedly smaller in diameter and containing fewer growth rings than another larger knot, lies next to the larger, it is more likely a branch from the branch making up the larger knot, and does not constitute a knot cluster.
Corner knot: A knot caused by sawing and is located at the intersection of adjacent surfaces. It is measured using the method (s) of each of the faces it is found on, with the measurement most-closely displaying the true diameter of the branch causing the knot used for grading purposes.
Decayed knot: A knot that, due to advanced decay, is softer than the surrounding wood. These knots are not allowed in beam and header grades of wall logs.
Edge knot: A knot located at the edge of the face or whose perimeter falls within one-sixth of the knot width from the edge of the piece of lumber.
Elsewhere knot: A knot that is not a center line, edge, or corner knot.
Encased knot:  Also known as a black knot whose rings of annual growth are not inter-grown with those of the surrounding wood.
Firm knot: A knot that is solid across its face, but which contains incipient decay.
Fixed knot: A knot that will hold its place in dry lumber under ordinary conditions, but can be moved under pressure, although not easily pushed out.
Flush knot: A knot that is smoothly cut close to the bole of a full-round house log or sawn round timber. It is specified in header and beam grades of minimally-machined logs.
Group knots: Two or more single knots grouped together, but with fibers of the wood present between the knots.
Hollow knot: An apparently sound knot containing a hole more than 1/4-inch in diameter.
Intergrown knot: Also known as a red knot whose rings of annual growth are completely intergrown with those of the surrounding wood.
Large knot: A knot larger than 1 1/2-inch in diameter.
Loose knot:  A knot that is not held firmly in place by growth or position and that cannot be relied upon to remain in place. In house logs, these are generally graded as “holes.”
Medium knot: A knot between 3/4-inch and 1 1/2-inch in diameter.
Oval knot: A knot cut at from 45° to 90° to the long axis of the branch.
Pin knot: A knot not greater than 1/2-inch in diameter.
Pith Knot: A knot having a pith hole not over 1/4-inch in diameter.
Red knot: Also known as an intergrown knot whose rings of annual growth are completely intergrown with those of the surrounding wood.
Round knot: A knot that is cut at approximately 90° to the long axis of the branch.
Single knot: A knot having adjoining wood fibers deflected around it alone and not around another knot.
Small knots:  A knot between 1/2-inch and 3/4-inch in diameter.
Sound knot: A knot of good quality which is solid across its face, is as hard as the wood that surrounds it and shows no indication of decay.
Spike knot: A knot cut at 0 to 45° to the long axis of the limb.
Star-checked knot: A knot having radial checks.
Tight knot: A knot so fixed by growth or position that it will firmly retain its place in the piece.
Unsound knot: A knot which is not solid across the face as a result of decay and is not as hard as the surrounding wood.
Watertight knot: A knot having sound and watertight wood completely intergrown with the surrounding wood on one surface or on the entire projection of one end of the knot.
Well-scattered knots: knots that are not in clusters or groups, and each knot is separated from any other by a distance at least equal to the diameter of the smaller of the two.
Well-spaced knots: The sum of the sizes of all knots in any 6-inch of length of a piece must not exceed twice the size of the largest knot permitted. More than one knot of maximum permissible size must not be in same 6-inch of length and the combination of knots must not be serious.
 
Shakes
—A naturally occurring feature that involves the separation of wood fibers due to mechanical stressors during the growth of the tree or the drying and subsequent shrinkage of the wood prior to or after processing. Shakes indicate an inherent weakness in the cross-section of the stem along the length of the stem. There are two distinctly different types of shakes.
Shake types are as follows:
Heart shake: Also known as heart crack and rift crack this shake is recognized by the widest separation of fibers originating at or near the heart or pith of the timber which narrows to a hairline crack as it extends radially toward, but not to, the surface.
Ring shake: Also known as cup shake, this shake occurs in the plane of the growth ring (tangentially) either partially or entirely encircling the pith and occasionally moving radially to a nearby ring. This phenomena occurs in the living tree whose roots are planted in the ground and not limited to the living part of the tree which is the sapwood area.

Checks & Splits
—Naturally occurring feature that involves the separation of wood fibers due to biological stressors during the growth of the tree or the drying and subsequent shrinkage of the wood prior to or after processing. Splits, also known as through checks, are defined as lengthwise separations of the wood that are parallel to the fiber direction and extending through the piece from one surface to the opposite or adjacent surface, not necessarily through the pith. Splits are not permitted in wall-log category grades. Sawn round timbers are permitted splits if they extend along the length of the piece for a distance no greater than the diameter of the butt. The length of the split should be measured parallel to the line created by the surfaced edge of the sawn timber and perpendicularly from the end (butt or tip) of the timber to the furthest point of the split.

Holes
—A crevice or void in a log caused by a variety of factors during and after the living years of a tree. These factors include damage by insects, birds or other animals as well as the natural development of a tree. One factor includes the process of the lower limbs dying off, this process can take years of continued growth around the dead limb. Therefore, dead branches produce knots which are not attached, as a result the knot stands a chance of coming loose after the tree is sawn into boards.

Wane
—A characteristic which occurs when a milled log retains a certain amount of bark on an edge or corner. In beam or header grades wane and other mill produced scars are limited to 1/4 of any face with slightly more allowed for a short distance.

Scars
—Defects caused by injury to the growing tree is known as catface and include damage by such factors as fire, animal or scraping away of the bark in a localized area. Another type of scarring is called a burl which is an abnormal distortion of the stems growth due to injury or disease in the living tree. Scarring can take place anytime after the tree is harvested as well and can be caused by any penetrating trauma.

Pitch Pockets
—A hollow area in the stem wood enclosed by distorted wood fibers and filled with oleoresin. They are usually small and do not penetrate deeply so therefore they are not limited in wall logs.

Bark Pockets
—An isolated area, causing distortion of the surrounding wood grain, largely made up of bark, consistent with the texture of outer bark, these are usually quite shallow and therefore are not limited in wall logs.

Decay
—”The decomposition of wood substance caused by the action of wood-destroying fungi, resulting in softening, loss of strength and weight, and often in change of texture and color.” —American Society for Testing and Materials D-9. Decay is also known as rot, dry rot, dote and doze and is produced by several types of fungi which feed on cellulose and /or the lignin of the wood cell walls. Decay is limited in grading wood, advanced decay is not allowed in any structural grade of wood.

Sapwood Stain
—Sapstain also referred to by some as mold is caused by a fungus that utilizes the contents of storage cells in the living sapwood for nourishment. The spread of the fungus can occur after an attack of bark beetles that have carried the spores from previously attacked and affected trees. The infected area will take on a bluish to black coloration that will rapidly fade after exposure to sunlight. The wall of the cell structure is not degraded in strength by the sapstain condition, so it is therefore not limited in graded wall logs.

Insect Engravings
—There is a distinction between the two types of damage done by insects who have left tunneling and boring evidence of their presence in a timber. One type is caused by a variety of insects who attack a dead tree leaving engravings in the sapwood and the heartwood which depending on the amount of damage could affect the integrity of the timber. The other is caused by the insects who have fed on the nutritious inner bark and cambium and in the process have killed the tree. In the latter, their presence has only affected the outer surface of the sapwood so the determination is that this timber remains sound in strength as long as the other timber characteristic assessments have deemed it so.

Warp
—A distortion of a timber shape due to a bow, crook, twist, or buckle. Beam, header or wall grade logs are limited to less than 1 1/2” in any 5 foot length.

Out of Round
—The tip and the butt ends have major and minor diameters whose ratios exceed 1.2. This is, of course, only the case with full round and sawn round timbers.

Taper
—For both wall logs and sawn round timber beams, taper is limited to an amount not to exceed 1/8” diameter change per foot of length for the distance between the tip and 3 feet from the butt of the log. The butt circumference cannot be greater than the circumference +8” at 3 feet from the butt end.

Compression or Cross Breaks
—A result of crushing pressure applied to wood which causes a constriction of wood cells and fibers as evidenced by visible compaction and breakage.

—Grading Program Training and Operations Manual, A Guide to the Visual Stress Grading of Round Timbers Used in Solid Wood Structures, 1980 [Revised 2000]. A publication of The Log Homes Council/National Association of Home Builders. Washington, DC