As wood is one of the most widely-used materials in the world, architects are accustomed to being able to easily obtain sawn wood at a nearby store. However, many of us know little about its manufacturing process and all the operations that determine its appearance, dimensions, and other important aspects of its performance.
The lumber we use to build is extracted from the trunks of more than 2000 tree species worldwide, each with different densities and humidity levels. In addition to these factors, the way in which the trunk is cut establishes the functionality and final characteristics of each wood section. Let's review the most-used cuts.
Parts of a Trunk
A trunk is composed mainly of cellulose fibers joined by lignin. From the outside to the inside, we can identify the following parts:
- Bark: irregular layer composed of dead cells that protect the inner layers.
- Cambium: the layer next to the bark, where new cells are generated that increase trunk diameter each year.
- Sapwood: young, clearer and growing wood, with high water content and little lignin.
- Heartwood: adult, dark wood, more rigid and hard because of its high lignin content.
- Pith: central part of the trunk, very rigid and cohesive, without humidity.
When classifying woods for building according to their hardness—for both soft or hardwoods—it's fundamental to define the proportion of Sapwood to Heartwood inside the trunk. Softwoods (from fast-growing trees) are usually cheaper and easier to handle but are less resistant, while hardwoods (extracted from slow-growing trees) typically have greater strength but are more expensive and delicate.
The growth rings, which tell us the age of the tree, and the medullary rays, which move the sap along the tree vertically, will also make a difference in the aesthetics and characteristics of the resulting wooden board.
The Many Ways to Saw a Trunk and Their Results
Although these techniques and their denominations may differ depending on the country and the use required, there are three main ways to cut a log into boards: Rift, Quarter and Flat Sawn– and a series of variations that arise from them:
This cut is made perpendicular to the growth rings. It keeps the wood grain visible and avoids warping (deformations in the shape of the board) or fissures (longitudinal cracks), but wastes more material than other cut types.
The trunk is cut into quarters, obtaining pieces that are not too prone to warping with a large number of visible rings.
Flat Sawn/Live Sawn
This is a widely used method, although the resulting pieces are not of the best quality since most include a certain percentage of the Sapwood and the Heartwood. The centerpiece that coincides with the core can tend to break, while the remaining pieces are prone to warp and curl.
This is similar to the Flat Sawn system, but in this case, pieces of a smaller section are obtained, leaving fewer problems with warping.
This method allows for wide boards without major waste, while also eliminating the core of the trunk.
Quarter Sawn (Alternative Method)
The trunk is cut into four quarters to extract pieces of good quality in terms of strength and appearance.
In this case, the trunk is used to its maximum potential, eliminating the bark to obtain a single square log.
This cut focuses on obtaining very resistant pieces, using the heartwood of the trunk to the full. From the remaining sapwood, smaller pieces are cut.
The boards intersecting the core of the trunk are cut first. The remaining wood provides boards that are thinner but quite resistant to deformation.
When wood—which has a high moisture content as it grows—dries, different contractions occur depending on the cut made and the resulting arrangement of the growth rings. Although this varies in different species, the deformation is always greater in the direction tangential to the rings than in the radial direction.
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This article was originally published on May 21, 2018.