Where the log goes: residues and losses before the saw

Three output streams

A sawmill converts a round, tapered, imperfect cylinder into rectangular pieces plus residues. Three streams leave the saw line:

1. Primary product: boards, planks, scantlings and cants.
2. Solid residues: slabs, edgings and trim ends, usually chipped and sold to pulp, panel or energy markets.
3. Sawdust and bark: sawdust from the saw kerf, bark from debarking.

A generic mass balance (FAO example)

An FAO worked example (T0269E, Appendix VI) takes net log input as 100 percent: bark about 12 percent of gross input, slabs, edgings and trimmings 34, sawdust 12, and air-dried lumber 52 after 2 percent shrinkage. Finished kiln-dried lumber is 40 after further shrinkage and 8 percent planer shavings, sawdust and trim ends. The source does not state year, mill type or region, so this is a textbook illustration, not a benchmark.

Check that it adds up: 34 + 12 + 52 = 98, plus 2 for shrinkage = 100.

A European simulation: Norway spruce in Finland

Kantola (2008) simulated sawing of Norway spruce from southern Finland (Europe) for three thinning regimes, using a stem-and-sawing simulator following Finnish industrial practice. The ratios below are the portal's calculations from the published volumes per hectare:

The three streams add up to roughly 96 percent of log volume; the source does not explain the remaining 4 percent. The order of magnitude is the lesson: in this simulated case a little over half the log leaves as sawn timber, a little over a quarter as chips and about one-seventh as sawdust. It is one stand and a simulator, not mill measurement.

Losses before the log reaches the mill

Recovery is usually calculated from the log yard gate, but the standing tree yields less than the log pile.

• Romania (Europe), silver fir: Brandstetter and colleagues (2020) followed eight stems. Crosscutting lost on average 11 percent of stem volume, sawlog grading 7 percent and debarking 6 percent, leaving 76 percent as debarked sawlogs (range 67 to 85 percent). The manual felling cut, which removed the largest-diameter butt section, caused most of the crosscutting loss. Small sample, no sawing.
• Estonia (Europe; country inferred from author affiliation): Padari and Kangur (2023) compared harvester data. Damaged or deformed stems showed sawlog volume reductions of 7 to 28 percent for Scots pine and 5 to 30 percent for Norway spruce, depending on site and age. These are shifts in assortment, not sawmill recovery.

By-products are another mill's raw material

In Germany (Europe), the Thuenen Institute reports for 2021 that sawmill by-products supplied about 5.9 million solid m3 to the wood-based panel industry (37.1 percent of its fibre wood use) and about 3.98 million solid m3 to pulp producers. Changing the sawing pattern therefore changes fibre supply to other industries. These flows cannot be converted into a sawmill yield because sawlog input and sawn output are not given.

Worked example

A stem has 20 m3 of volume. Using the Romanian fir averages: crosscutting lost 11 percent, grading 7 percent, debarking 6 percent.

• Remaining: 100 - 11 - 7 - 6 = 76 percent.
• Debarked sawlog volume: 20 x 0.76 = 15.2 m3.

If the mill then recovers 53 percent as sawn timber (Finnish simulation level), it obtains 15.2 x 0.53 = 8.06 m3 from this stem, which is only 40 percent of the stem volume.

Checklist: what is the yield baseline?

• Is yield counted from standing tree, roadside, yard gate or saw infeed?
• Does the balance close (outputs plus losses = input)? Note any unexplained share.
• Where do chips, sawdust and bark go, and at what value?

Key points

• Output streams: primary product, solid residues (slabs, edgings, trim ends), sawdust and bark.
• The generic FAO balance and the Finnish simulation are illustrations, not mill benchmarks.
• In the Finnish simulation about 53 to 54 percent of log volume became sawn timber, with about 4 percent unexplained.
• Pre-mill losses (11 + 7 + 6 percent in the fir study) can be as large as machine effects; agree the measurement basis before benchmarking.
• Sawmill by-products are significant raw material for panel and pulp industries.