Inside a sawmill
A sawmill turns logs into boards, and the central challenge is getting as much useful lumber from each log as possible. The usual sequence is straightforward.
- Sorting and debarking. Logs are sorted by species and size, then the bark is removed. Bark is typically sold as mulch or used as fuel.
- Scanning and planning. Modern mills often scan each log and use software to decide how to cut it, since a small change in the cutting plan can change the value of the output.
- Breakdown. Large saws, most commonly band saws or circular saws, cut the log into slabs, boards and timbers. The outer curved pieces are trimmed or chipped.
- Edging and trimming. Boards are cut to a consistent width and length, and defects may be trimmed out.
- Drying. Boards are dried to a target moisture level.
- Grading and finishing. Boards are sorted by quality, often planed smooth, and bundled for sale.
A single saw cut removes a strip of wood as sawdust, called the kerf. Thin saw blades waste less, which adds up over thousands of logs. The mill’s chips and sawdust are not generally wasted, since they go to paper mills, panel plants or to fuel the mill’s own dryers.
How a log is cut matters
The way a board is cut from the log affects how it looks and behaves.
- Plainsawn (or flat-sawn) boards are cut roughly tangent to the growth rings. This is efficient and produces the familiar cathedral patterns on the face. These boards tend to shrink and cup more across their width.
- Quartersawn boards are cut with the growth rings running closer to perpendicular to the face. They are generally more stable and show straighter grain, but the method yields less wood from a log and costs more.
- Riftsawn boards fall between the two and are used where a consistent straight grain is wanted, as in some furniture.
The visible lines in a cut board are the annual growth rings, and their spacing and pattern relate to the conditions in which the tree grew, a subject treated in the article on tree rings. Fast-growing wood and slow-growing wood have different properties, and species differ, so the same cut can produce different results.
Why drying is part of the process
Green lumber is full of water and changes shape as it dries. Boards are therefore dried before sale, either by stacking them in the open air for a period or by heating them in a kiln, which is faster and gives better control. The target moisture content depends on the use: lumber for indoor furniture is usually dried more than lumber for outdoor framing. Poor drying leads to warping, splitting and checks, which also affect grade.
What grading means
Grading sorts boards by quality so that buyers know what they are getting. Grade rules are published by industry or national bodies, differ between countries and between softwood and hardwood, and are updated over time. The principles are common, even if the names differ.
Softwood dimension lumber, such as the framing boards used in houses, is typically graded on features that affect strength and appearance:
- size and location of knots;
- slope of grain, meaning how much the fibers run off-axis;
- wane, where bark or missing wood leaves a rounded edge;
- checks and splits;
- warp, including bow, crook, twist and cup.
Grades might be labeled with names such as Select, Number 1, Number 2 and Number 3 in North America, with higher grades having fewer and smaller defects. Structural boards may also be graded by machine, which measures stiffness to estimate strength. Other regions use different grade names or strength classes, so the labels from one place do not translate exactly to another.
Hardwood lumber is graded differently in many markets. In the United States, for example, a common system looks at how much clear, defect-free wood can be cut from each board, with grades often called FAS, Select, and Number 1 and 2 Common. A top grade does not mean the board is flawless, only that a large share of it yields clear pieces. Furniture makers often accept a lower grade for the lower cost, cutting around defects.
Grade, species and size together help set the price, and a better grade does not always mean a better choice. For painted trim or rough framing, a lower grade may serve perfectly well.
Nominal and actual sizes
Lumber sizes can be confusing. A softwood board sold as a two-by-four in North America is nominally 2 by 4 inches, but after drying and planing it actually measures about 1.5 by 3.5 inches. The nominal name reflects the rough size before drying and finishing. Other countries use metric dimensions, and sold sizes differ.
Species and wood type
The kind of tree influences how a log is sawn and graded. Softwoods and hardwoods are processed with different priorities, and the terms are explained in the article on hardwood and softwood. Large knots result from branches, so how a tree was grown affects the clear wood within. Managed stands where lower branches are pruned off can produce a clear outer zone, a practice discussed in silviculture. Some of the best-grade lumber comes from slower-growing trees with few branches low on the trunk.
From boards to bigger products
Lower-grade boards and offcuts still have uses. They can be finger-jointed, laminated or chipped. Engineered products such as those in mass timber start with sorted boards, which are glued to produce large beams and panels. Smaller logs and residues may go to pulpwood for paper.
A note for buyers
Understanding grades helps in choosing wood for a job: structural grade for load-bearing use, appearance grade where the surface shows, and the lowest suitable grade where it does not. The label stamped on lumber also often identifies the mill, species group, moisture level and grading agency, a piece of traceability that connects the board back to a source and the larger system described in the article on harvested wood.