Properties

The properties of lumber are the result of many different factors.  Lumber products are cut from logs and logs are produced from different species of trees that grow in various different environments.  Weather patterns and soil types can affect the rate of growth of trees which, in turn, can have an effect on the strength of the lumber which is yielded from the trees.

Every tree is made up of three major components; the root system, the crown, and the tree trunk.  The roots are required for anchoring the tree in an upright position.  They are also critical in absorbing water and mineral salts which results in the production of sap that flows up the tree trunk to feed the foliage held in the crown.  The crown of softwood trees is made up of branches whose foliage is made up of long thin leaves, aptly called needles.  The needles produce food for the tree that is distributed throughout the tree, some to the buds, some to the roots, and for storage, and most importantly, the majority of the tree food going to the growth layer where it is formed into new wood cells in the trunk.

The main portion of a tree from which lumber is obtained is the trunk.  The quality of lumber produced depends on the part of the trunk from which it was cut.  The trunk consists of several parts; the Pith, the Heartwood (both inner and outer), the Sapwood, the Cadmium, and the Bark.

The cadmium is a layer between the bark and the sapwood. It is here from which new wood cells are produced, where as the bark provides protection against fire and extreme climate conditions.   The pith is the heart of the tree, the initial growth of that part of the log’s cross section.

As far as production of lumber is concerned, our focus is on the heartwood and sapwood.

The properties of lumber depend on what part the log it was cut from.  Lumber cut from the heartwood, close to the pith, will contain more knots.  Lumber cut from the outer portion of the log will generally contain a high percentage of clear wood, free of knots.  The frequency of knots and quality of knots in a piece of lumber are characteristics that are considered when grading that piece of lumber.

The grain of lumber is also determined from the part of the log it was cut from.  Grain is reflective of how the annual rings are arranged in a piece of lumber and comes under consideration when determining the grade of lumber.  The arrangement or direction of the annual rings (vertical grain, flat grain, cross grain…) and the relative width of the annual rings (coarse grain, close grain…) are considerations when choosing lumber for the user’s application as well as when grading the piece of lumber.

• Vertical Grain or Edge Grain lumber is a piece of wood sawn at near right angles to the annual rings so that the rings form an angle of 45° or more with the surface of the piece.

• Flat Grain lumber is sawn approximately parallel to the annual rings so that most of the rings form an angle of less than 45° with the surface of the piece.

• Mixed Grain is any mixture of both vertical and flat grain lumber in a shipment.  Any piece of lumber that has both vertical grain and flat grain is categorized as flat grain.

• Cross Grain is a generic term that describes any fluctuation of the wood elements from a line parallel to the axis of the piece.  The amount of cross grain for grading purposes is determined by measuring the slope of grain.

• Spiral Grain is a deviation in the direction of the grain caused when the fibres in a tree take a spiral path around the trunk of the tree instead of the normal vertical course up the trunk of the tree.

• Diagonal Grain is a slope of Grain through the thickness of a piece of sawn lumber.  It is usually a result of sawing logs with a flared butt or other growth abnormalities in the tree.

• Dark Grain is exactly that.  Grain that is darker than the others.

• Medium Grain & Close Grain are tied to the rate of growth of the tree.  The faster a tree grows, the wider the width of the annual rings and, conversely, the slower the tree grows, the thinner the width of the annual rings.  Wide annual rings lead to coarse grain and thin annual rings lead to close grain.