Farm Forestry New ZealandThe decay resistance of some wood species used as framing - Final report on the condition of samples
By Ian Simpson and Tripti Singh, July 2023.
Download SWP-T096 (pdf)
Executive summary
Sets of framing samples were exposed in high humidity conditions of 85-90% relative humidity and 25 – 27°C. The species included in this test were:
- Untreated Cupressus lusitanica.
- Untreated Douglas fir.
- Untreated Eucalyptus nitens (installation was delayed by approximately six months).
- Untreated Eucalyptus regnans.
- H1.2 treated radiata pine.
- Untreated radiata pine.
All the samples were periodically sprayed with water at approximately two weekly intervals to maintain the wood moisture content at a level suitable for decay to progress. Before exposure in the high humidity condition, samples were also soaked in water for two hours.
The method of testing followed the procedure described in Australasian protocols, in this case for the Hazard class H1.2. This test method simulates the common framing joint in house framing between studs and plates, where in a leaky building, moisture may become trapped and provide suitable conditions for fungi to establish.
After twenty-four month’s exposure (in high humidity condition and including regular spraying with water on the test samples);
- Active mycelium had developed on all of the samples of Cupressus lusitanica, Douglas fir and Eucalyptus regnans.
Decay had developed in all of the Cupressus lusitanica, Douglas fir and Eucalyptus regnans samples, and samples in each group had failed due to decay.
After eighteen month’s exposure (in high humidity condition and including regular spraying with water on the test samples);
- Decay had developed in all of the Eucalyptus nitens samples, and sixteen of the twenty samples had failed due to decay.
- Decay had developed in all of the untreated radiata pine samples, and six of the ten samples had failed due to decay.
This study shows that untreated Eucalyptus nitens, E. regnans and Douglas-fir are susceptible to decay and unlikely to meet a 50-year durability requirement for house framing where there is risk of prolonged exposure to moist conditions and high humidity. Further testing of these species including H1.2 boron treated timber is recommended.
Other durability testing, including stake test conducted at Scion showed Cupressus lusitanica and Macrocarpa (Cupressus macrocarpa) heartwood to be moderately durable and classified as class 3 (towards the upper end) of the Australasian natural durability classification system. (Page and Singh; 2014). The Cupressus lusitanica framing in this study, including heartwood, showed some susceptibility to decay in prolonged damp and humid conditions. It is recommended that further testing of Cupressus lusitanica and C. macrocarpa, including H1.2 treated timber, be conducted to determine whether C. lusitanica is less durable than C. macrocarpa and hence less suitable for use as framing in situations where wetting is likely.
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