Specialty Timbers New ZealandSearch technical reports
Understanding outdoor durability of specialty species - Assessments for 2024 and 2025
By Diahanna O’Callahan, Jackie van der Waals, Carolina Gous, Colleen Chittenden and Rosie Sargent, December 2025.
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Executive summary
The problem
A key barrier to the wider use of New Zealand's specialty species in outdoor environments is the limited durability data available to support market approval. Durability is a defining performance attribute for exterior wood products, shaping service life, structural reliability, and commercial viability. It may be inherent in naturally durable species or achieved through modification processes such as thermal treatment. This project addresses the data gap by undertaking accelerated outdoor trials to benchmark and compare durability under realistic exposure conditions. Although these trials are designed to hasten decay relative to typical service environments, multi-year assessments are still required to generate meaningful results. By building a robust evidence base, the project will expand the range of high-value products that can be manufactured from New Zealand-grown specialty species. This broader product offering will strengthen grower confidence, open new market opportunities, and encourage the planting of diverse forest species.
This project
Since 2016, outdoor durability trials of selected specialty species - both modified and unmodified - have been installed at the Whakarewarewa test site on the Rotorua campus of the Bioeconomy Science Institute (BSI). This report consolidates results from durability assessments conducted in 2024 and 2025 of trials set up by the Specialty Wood Products Partnership (SWP). Although samples were evaluated in 2024, no formal report was prepared at that time; therefore, this document integrates findings from both years to provide a comprehensive overview of product and species performance across multiple trial types.
Key results
- Thermal modification improves durability and stability: Cupressus and Douglas-fir heartwood consistently resisted decay better than unmodified controls and P. radiata, with reduced checking and greater dimensional stability (e.g., less cupping in weatherboards).
- Surface degradation remains a weakness: Despite improved decay resistance, thermally modified samples were more prone to mould and weathering, and Douglas-fir heartwood showed no improvement in surface performance.
- Treatment parameters are critical: Douglas-fir modified at 220ºC outperformed 230ºC treatments in accelerated trials, and sapwood benefits were limited compared with heartwood, underscoring the need for process optimisation.
- Untreated Eucalyptus fastigata laminated veneer lumber (LVL) surpasses the decay performance of untreated solid E. fastigata heartwood framing after four years of exposure, though it remains less durable than industry-approved species such as C. macrocarpa and larch.
Implications of results for the client
Early results indicate that thermal modification significantly improves decay resistance and dimensional stability in Cupressus and Douglas-fir, positioning them as viable alternatives to traditional durable species and chemical treatments. However, surface degradation and treatment optimisation remain challenges. While premium references (Kebony, Accoya) still outperform, thermal modification provides a promising pathway to expand specialty timber resilience and market opportunities with reduced reliance on chemical preservatives.
Further work
Many of the installed trials remain at an early stage in generating the data needed for market approval of these products. It is therefore recommended that all outdoor trials continue to be assessed for the foreseeable future. Framing trials, however, can be considered complete at this stage and could be disestablished.
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