Since the depletion of the old growth forests, it has become more economical to use softwood harvested from farmed forests for construction framing. Eighty percent of the timber being logged today is used for framing. Hardwood is scarce and is typically used for decorative construction, such as interior finishes and furniture.
Hardwood and Soft wood
Softwood is mostly derived from conifer trees such as Pine, Douglas fir, Cedar and Cypress. Examples of hardwood are: Oak, Ash, Aspen, Cherry, Ebony, Mahogany, Maple, Walnut, Beech and Birch. It should be noted that some of the softwoods are actually harder wood than some of the hardwood from hardwood trees.
What is wood stability?
Wood is a natural, organic product, and its form is subject to change. Wood stability is a reference to its resilience to losing its form after harvest, especially after it has been cut and fit into a structure or furniture. The stability of any piece of wood is very much dependent on the specie of the tree as well as the way the log was milled, dried and stored.
Milled lumber can lose its form due to shrinkage as well as twisting, and bowing. Old growth timber with tight grain has much more resilience to losing its form than farmed trees that are cut down at a much younger age. A tree is composed of cellular structure that, at the time of harvest, is filled with water. After a logged tree is milled, it will begin the process of losing the water stored in its cells, which will result in the shrinkage of the wood.
Why is wood stability important?
Anything that is built out of wood has joints – places where one section of wood is joined to another section. If milled wood goes through the drying process after it has been joined to other pieces of wood, as the wood shrinks the joints will open. If the method used to mill the wood was “Plain Sawn”, which is a common method used on most logs, the milled wood will also be having a tendency to twist and bow.
Most lumber used for rough framing a building is Plain Sawn and not dried. Therefore, the wood will do additional drying after the rough framing has been completed, and the framing will adjust itself to movements caused by the drying process. Historically this has been an accepted outcome in the construction industry.
The importance of how a log is sawn
Plain Sawn is the most common and economical way of sawing a log. The log travels across the saw blade, and the blade slices the log in a flat form. With this method, every slice of the log inherits the form of the grains at the location the board is sliced. The closer the board gets to the core of the log, the more unstable it will potentially be during drying time. A professional framer should avoid using a piece of timber with the core “bull’s eye” showing at the cross section.
Quarter Sawn wood is far more stable than Plain Sawn. The timber is initially radially cut into four quarters, and every slice is cut at a 90 degrees angle to the rings. This method of cutting produces beautiful parallel grain. The parallel nature of the grain produces superior stability to the board during and after the drying period.
Rift Sawn also referred to as Rift Cut, is a process where the saw blade cuts the log at 45 degrees to the tree rings. This process develops boards with exquisite grain that are the most stable.
With the advent of modern technology, such as structural industrial glues, the lumber industry has begun producing engineered lumber that is far more stable than logged dimensional lumber. The basic principal used in the engineered lumber is the use of chips or laminates that are glued together with industrial adhesives that can be tested for strength and stability under laboratory conditions. Today, engineered lumber is practically replacing the use of dimensional lumber in framing buildings.
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