Wood Grain Orientation and Lumber Cutting Methods
Grain orientation may be a boring topic to many. But it’s something many woodworkers are aware of and must plan for in their designs as it affects the longevity of the piece. For non-woodworkers it may be of interest if you’re gauging the structural integrity of furniture or other wood crafted goods.
Grain orientation is best understood visually. Take a piece of lumber like the ones sold at your local lumberyard or Home Depot (see below). The widest side of a piece of lumber is referred to as the wood’s face grain; the parallel, thinner side, of this same piece of lumber is termed as the edge grain; and the end of this piece of lumber is called the end grain. From the perspective of looking at a tree, the face grain and edge grain can be found by theoretically slicing a tree open vertically along its length. By contrast, if one were to cut down a tree in the traditional (horizontal) manner, the top surface of the resulting tree trunk would be classified as the tree’s end grain. Going back to the piece of lumber, you can observe how the end grain is just a portion of a tree’s growth rings.
So why does any of this matter? The answer is primarily wood movement. Wood holds moisture and as this moisture content increases or decreases, the wood expands or shrinks, respectively. Moisture will move into and out of the wood to reach an equilibrium where the moisture content within the wood matches that of the surrounding air, the wood can eventually bow, warp, and even crack. End grain, being very porous by nature, absorbs and expels moisture at a rate far greater than that of flat grain (face grain or edge grain). In other words, wood moves primarily across the width of a piece of dimensional lumber rather than the length.
The curvature of the tree rings in end grain play a huge factor in wood movement. Wood moves more tangentially, along the growth rings, rather than radially, perpendicular to the growth rings. Therefore, lumber cutting methods directly influences wood movement! This was my ‘aha moment’ as an inexperienced woodworker. The methods of lumber cutting results in growth ring properties which can be categorized as Flat/Plain Sawn, Quarter Sawn, or Rift Sawn. Flat sawn lumber is the easiest to mill into lumber and results in the least amount of waste which is why it is cheaper than lumber which is quarter sawn and rift sawn. The old adage “you get what you pay for” certainly applies here. Despite its abundance, flat or plain sawn lumber is more prone to tangential wood movement which often leads to cupping when exposed to moisture due to the growth rings being roughly 30 degrees or less to the face of a dimensional piece of wood. Quarter sawn lumber has four cut directions which depending on the cut can result in lumber with growth rings that are 60 to 90 degrees to the face of dimensional lumber. Rift sawn lumber is the most dimensionally stable because the lumber is milled perpendicular to the growth rings where tangential wood has little to no effect on a dimensional piece of lumber. This type of lumber takes the most amount of effort to mill and is not as widely available. Due to its general scarcity, it’s pricier than either flat sawn or quarter sawn lumber.
You can use these visual cues the next time you pick out lumber for a project or are searching for a quality piece of solid wood furniture. For example, if you come across a solid wood table top with exposed end grain (as opposed to having a border around the slats), observe whether the slats are plain sawn, quarter sawn, or rift sawn by inspecting the end grain. If it is plain sawn, each plank will likely have alternating up and down tangential grain to minimize the chance of the whole table-top cupping tangentially. If there is a frame surrounding the perimeter of the table top and the end grain is not visible, you may still be able to identify the type of lumber cut by inspecting the face grain which also takes on a visible distinction depending on lumber cutting method (see below). Of course, a plywood and veneer approach to furniture design will circumvent any chance of wood movement as plywood is completely stable and unreactive to moisture. I prefer to work with hardwoods. This is a lost artform, after all.
Plain Sawn Quarter Sawn Rift Sawn
Since most lumber is plain sawn and most people live in locations where seasonal wood movement is prevalent, woodworkers must factor in tolerances for movement if they want their pieces to last and to avoid customer dissatisfaction. I was fortunate to learn woodworking in San Diego where the climate is, for the most part, moderate in terms of moisture (humidity) changes. However, as I sold more items out of state and abroad, I found that grain orientation and wood movement played an integral part in my designs.
To round out your understanding, here are some examples of furniture construction where wood movement is accounted for. The construction of table tops generally incorporate a tongue and groove or panel-in frame-approach. Table legs may be attached using mortises and tenons or drilled holes with hardware specifically designed to permit wood movement. Of course, outdoor decks incorporate slight gaps between the planks to allow for wood movement. Finishing methods are also crucial for prolonging the longevity of the piece. Oil-based polyurethane, shellac, varnish, lacquer and other hard finishes are used to seal furniture surfaces which will slow the effects of wood movement. This will not prevent wood movement over time, however. The best preventative measure is always in construction. By contrast, cutting boards cannot be treated with these hard finishes which form a toxic film on the surface and when scratched can be accidentally ingested. This is why all cutting boards, and especially end grain cutting boards, should be treated every so often with a liberal amount of mineral oil to fully saturate the grain, thereby preventing external moisture from further penetrating the wood. See Wooden Cutting Board Care Instructions and Tips for more information on this.