If you take a stroll through your local lumberyard or the lumber department of your big box home center, you'll find a few rows of dimensional SPF Lumber. Most of the construction material (from 2 x 4's up to 2 x 12 planks), as well as some of the one-by material (1 x 2's up to 1 x 12's), are harvested from the various softwoods that make up the SPF Lumber class.
SPF is an acronym that stands for spruce, pine, and fir. These softwoods are often harvested from trees grown on tree farms, where the species grow very quickly but whose trunks are straight and tall. These trees yield softwoods with fewer knots, but whose growth rings may be spaced farther apart than older trees harvested from old-growth forests.
Most softwoods are graded into four different grades, labeled A through D. There are sub standards of each grade, with categories such as Supreme, Choice, Quality, Construction, Standard and Utility. Much of the higher-graded material ends up being sold to specific buyers, while the grades determined best for structural uses end up in the lumberyard.
Softwood trees such as the spruces, pines, and firs farmed for lumber are harvested in a methodical fashion. The trees are felled, the upper branches are lopped off and the trunks are inspected and separated into anticipated grades before being loaded and hauled off to the mill. Once at the mill, these trees are milled into the most efficient sizes possible based on the tree grades, sizes, and requirements of the mill contracts. Any scraps left over by the milling process are captured and used in other processes to create manufactured wood products such as plywood, medium-density fiberboard (MDF) and particle board.
Note that dimensional SPF lumber is most often sold in pre-determined dimensions, such as 2 x 4's -- this differs from hardwoods which are typically graded and sold by the board-foot.
Because it is harvested and milled "wet" (without having had sufficient time to dry and shrink), the actual sizes of the wood as it is being cut is larger than the eventual size the lumber is expected to be when it has cured.
To get the wood closer to a usable state, most of this wood is put into large dryers for a period of "kiln-drying" which reduces the moisture content to an acceptable level for use. This kiln-drying produces a more consistent material than if the wood had been simply allowed to air dry naturally. By reducing the moisture content, it also reduces the weight of the wood, lowering the cost of shipping the materials to market.
One side note: most of the pressure-treated lumber you find in the home centers started off as SPF lumber.
Use In Fine Woodworking
Can SPF Lumber be utilized in fine woodworking projects? The answer is absolutely, although there may be better choices, particularly if you intend to stain your woodworking project.
Why would you want to use SPF lumber on a fine woodworking project? In a word, because SPF lumber is cheap.
Keep in mind that, when you buy SPF lumber, you do not know with certainty which variety of spruce, pine or fir you're buying. As such, if you intend to build a fine woodworking project out of SPF lumber and intend to stain the project, you should choose all of your lumber from a single stack in the yard. This will greatly increase the likelihood that all of the materials are of the same species.
Why does this matter? Because each species takes stain a little bit differently, and if you use spruce on part of the project and pine on another part, the stain jobs will likely not match. Therefore, try to select all of your materials from the same stack of lumber to reduce the possibility that you're working with different species.
When choosing SPF lumber for your project, try to select boards that are as straight as possible (watching for warping, cupping, twisting and bowing), and choose boards with as few knots as possible. This may be easier said than done, and the time spent going through two or three stacks of wood to find ten acceptable pieces of stock may not be as productive as if you had simply spent a little bit more to use a higher-graded pine, poplar or hardwood on your project.
However, if you must use SPF lumber, look closely at the end grains of the wood. Try to find material whose grain patterns are tight, with the line of the grain cutting between the narrow sides of the boards (which is most indicative of quarter-sawn lumber). You're not likely to find much quarter-sawn lumber in the stacks of the home center, as this material would probably have been graded out a bit higher and sold to a different customer.
When using SPF lumber for your projects, buy about 25% more material than you need, and take the wood back to your shop and allow it to acclimatize to the local surroundings.
The time for wood to reach a state of equilibrium with the local environment will vary depending on the species of wood, the starting moisture content of the wood and the typical humidity of the local environment. However, a few weeks getting acclimated to the environment in which the project will be used will make the material more stable to use.
Also, be advised that SPF lumber, particularly the grades found in home centers, will occasionally have pockets of sap (called pitch) in the fibers that can be damaging to your blades and bits. Allowing the wood to acclimate will reduce these pockets of pitch, but if you encounter excess pitch in a stick of wood, you may want to replace that piece of material with one of the extras from your stack.