Natural durability is a key property for untreated timber used outdoors and exposed to the weather. The natural durability of fast- and slow-grown Douglas fir heartwood was compared in an above-ground L-joint field test near Brisbane, Queensland, Australia. The L-joint test provides conditions that are favourable to decay. Fast-grown (regrowth) Douglas fir had ≤ 10 growth rings per 50 mm, while slow-grown Douglas fir had ≥ 20 growth rings per 50 mm. Juvenile wood was excluded. Douglas fir heartwood's natural durability is due to the presence of polyphenolic extractives (including taxifolin) and some water repellency. Achieving good performance of Douglas fir requires that quality workmanship, fixings, finishing and maintenance are sustained in use. Failure to maintain this quality may lead to rapid failure. In this study, slow-grown Douglas fir was significantly more durable than fast-grown Douglas fir. Douglas fir is classified as a Durability Class 4 species in the Australian Standard for the natural durability of timber AS 5604:2022 (probable outside above-ground life expectancy 0 to 7 years). Fast-grown Douglas fir performed consistently with this classification, while slow-grown Douglas fir performed consistently with expectations for Durability Class 3 (probable outside above-ground life expectancy 7 to 15 years). However, differentiating fast- from slow-grown Douglas fir in the marketplace may be impractical.

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