Sawmilling

The process at a glance

Log intake and sorting

Logs arrive graded as sawlogs, meaning they meet minimum diameter, straightness and quality thresholds. Everything below that threshold is pulpwood, and the boundary between the two is commercial rather than botanical: it moves with prices and with what the local mills can use. This is why a single stand of forest supplies both the sawmill and the pulp mill, and why the two industries are linked on the supply side rather than competing cleanly. See fibre types and forests.

Sorting before sawing matters more than it appears. A modern line measures each log in three dimensions and assigns it to a sorting bin, so that logs of similar size run together and the saw pattern can be optimised for the batch rather than for the average.

Breakdown

Breakdown is the first cut, and the choice of machine defines the mill.

• Band saw: a thin continuous blade. The narrow cut wastes little wood and handles large or irregular logs, which is why it dominates where logs are big.
• Circular saw: a thicker blade, faster and more robust, more wood lost to the cut.
• Chipper canter: instead of sawing slabs off the log, the outer wood is chipped away directly and the chips go straight to the fibre stream. Canter and profiling lines are the standard primary breakdown for uniform small and medium logs wherever the supply has that character: the Nordic and Baltic countries, Germany and Austria, parts of Canada, and the US South, where the same principle is known as chip-n-saw. They are not a universal answer: mills handling large, high-value or highly variable logs, and most hardwood mills, stay with band saws.

The width of the cut itself is a real loss. Every millimetre of blade thickness across every cut in every log is wood that becomes sawdust instead of board, which is why blade thickness is a competitive parameter and not a detail.

Drying

Green timber leaves the saw with moisture well above what any end use tolerates. Kiln drying brings it down to a specified target, and the target depends on the destination: construction timber for a heated building is dried further than timber for outdoor use. Moisture content is specified and measured to EN 13183 and is part of what the grade certifies.

Drying is where a sawmill spends most of its energy, and it is normally supplied by burning the mill's own bark and sawdust. That closes a loop: the residues of the process fuel the process. See sawmill byproducts.

Grading

Two grading systems run in parallel and answer different questions.

• Appearance grading asks how the surface looks: knots, wane, discoloration. In Europe softwood appearance grading follows EN 1611-1; the Nordic mills also use the Nordic Timber grading rules, which sort into the familiar A to D classes.
• Strength grading asks what the board can carry. Structural timber is assigned to a strength class under EN 338, with the grading itself governed by EN 14081-1. Grading can be visual or machine based, and machine grading has largely taken over at volume because it measures stiffness directly rather than inferring it from appearance.

The two systems are separate and a board can score well in one and poorly in the other, but they are not unrelated: knots drive both, and knot measurements alone explain roughly a third of the variation in bending strength in large European test series. What knots cannot do is grade finely. Knot criteria alone will not reliably reach above the C30 strength class, and the scatter within a grade stays wide, so a knotty board may carry load perfectly well and a clean board may be structurally mediocre: neither grade can be read off the other with confidence. This is why the same log can yield boards that go to completely different markets. See sawn timber products.

Yield and why it is the central number

The share of log volume that leaves as sawn timber is the number a sawmill lives or dies by. Conversion factors used in international forest products statistics put softwood sawnwood recovery at roughly half the log volume, but the spread is wide. Reported national factors run from about 1.5 to just over 2.0 cubic metres of roundwood per cubic metre of sawnwood, and much of that spread is not about mill efficiency at all: it reflects where in the process the output is measured, since the same softwood chain moves from about 1.6 rough green to about 2.1 once the timber is dried and planed. Log size is a real driver on top of that: the same source puts recovery at 0.41 cubic metres of sawnwood per cubic metre of logs at a 15 centimetre top diameter, against 0.63 at 60 centimetres, a factor of about 2.4 against about 1.6. A mill cutting small diameter logs is therefore working with a structurally worse yield than one cutting large logs, whatever its equipment.

The remainder is not waste. It is chips, sawdust, shavings and bark, and most of it is sold. A sawmill that improves its yield by a percentage point moves value from the fibre stream to the timber stream, and both streams have customers. See sawmill byproducts.