Understanding yield and recovery

In most sawmills logs are the largest single cost, so the share of each log that becomes saleable boards, or the value those boards earn, decides the margin. This lesson explains how yield is defined and what moves it. The warning of the source article applies to every number here: there is no single typical recovery for spruce or pine.

Definitions

Volume yield is sawn volume divided by log volume:

volume yield (%) = sawn volume (m³) / log volume (m³) x 100

Three choices change the number without any change in mill performance:

• the state of the lumber (rough green, kiln-dried rough or planed dry)
• the log measurement (over or under bark, mid-diameter formula or scanner)
• what counts as a product (main product only, or everything sold)

The conversion factor is the reciprocal: cubic metres of roundwood per cubic metre of sawnwood. The FAO, ITTO and UNECE publication (2020) lists 1.59 for coniferous sawnwood in Germany (Europe) and 2.04 for the USA (North America). Inverting gives 1 / 1.59 = about 63% and 1 / 2.04 = about 49%. The basis of each national factor was not verified, so the gap is not a performance ranking.

Value yield asks how much money a cubic metre of logs produces: revenue from lumber plus by-products, divided by log volume. It can rise while volume yield falls, for example when logs are rotated to put knots outside the best boards.

North American mills also use lumber recovery factor (LRF), board feet per cubic foot of log, and cubic lumber recovery (CLR), green lumber volume over log volume. A CLR of 0.50 means 50%.

Factors that move recovery

The US review by Steele (1984) lists log diameter, length, taper and quality; kerf; sawing variation; product mix; operator decisions; equipment condition; and sawing method.

Small logs

In a Swedish simulation (Fredriksson et al. 2015, Europe), reference cant sawing gave 43.7% planed-board yield for all logs but only 35.8% for logs under 185 mm top diameter, about eight points lower.

Taper

Boards must fit inside the narrow end of the log. For a glued-laminated lamella of 43 x 146 x 4,000 mm sawn from Austrian conifer logs (Blinzer and Treml 2022):

The same sawn volume needs more and more log: 0.075 / 0.152 = 49%, 0.075 / 0.212 = 35%.

Sweep

In a US study of small hardwood logs (Hamner et al. 2006) curve sawing raised yield from 58% to 62% overall; for logs with sweep deduction above 30% the figures were 48% (straight) and 60% (curve).

Kerf, sawing variation and oversizing

Kerf savings are real but discontinuous: they show up when they tip a log into a better pattern (Steele). Sawing variation forces mills to saw oversize so that planing can reach final size. An Alaskan study (Kilborn 2002, 23 studies at 22 mills, 1,666 logs) found variation above 0.030 inch (about 0.76 mm) in 17 studies, oversizing in 18, and estimated roughly 2.5% more recovery for each 0.030 inch removed. Effects on a modern high-speed European line may differ.

Product mix and rotation

Allowing one more product (1 x 4 boards) raised yield by 9.7% on one 7.1 inch log (Steele). Best rotation in an X-ray simulation on Douglas-fir logs (southern Germany) raised product value by 4 to 20%, with volume nearly the same.

Losses before the mill

In eight silver fir stems in Romania (Brandstetter et al. 2020), crosscutting, grading and debarking lost on average 11%, 7% and 6% of stem volume, leaving 76% as debarked sawlogs.

Checklist: measuring your own yield

• Fix definitions: over or under bark, lumber state.
• Choose a sample: straight, sound logs isolate the machine; all logs over a period show overall performance.
• Report yield by log size class, product mix and by-product share.
• Report value as well as volume, with price basis and date.

Key points

• Yield depends on definition: lumber state, log measurement and product count must match before numbers are compared.
• Conversion factor is the inverse of yield: 1.59 corresponds to about 63%, 2.04 to about 49% (different bases, not a ranking).
• Small logs, taper and sweep all lower recovery; the taper table shows 49% falling to 35%.
• Reducing sawing variation and oversizing is the best-supported first improvement (Kilborn: about 2.5% per 0.030 inch).
• Volume yield and value yield can move in different directions.