Official website of the New Zealand Farm Forestry Association

Dimensional Stability of Specialty Species

By Rosie Sargent, Gavin Durbin, May 2019.

Download SWP-T074 (pdf)

Executive summary

Dimensional changes caused by changes in wood moisture content (dimensional stability) can have a large impact on how the wood performs in service. Poor dimensional stability can lead to cracking, poor paint adhesion and problems with clearances in moving parts like doors and windows. To better understand this the dimensional stability of 16 different wood types were compared (8 species, plus variations such as modified wood, or different tree ages).

Two tests were used, one using short term contact with liquid water, and one exposing the wood to different air humidities. The test using humid air is not complete, and only interim results are given here.

For the short term water soaking (swellometer) test, there was a wide range of different behaviours. The hardwood samples tended to swell the most, but did so quite slowly. The softwood samples swelled less, but did so quite quickly, so after 30 minutes soaking, they had similar, or higher, levels of swelling than the hardwoods. Radiata pine had the third-to-highest level of swelling, behind E. globoidea and E. fastigata. It also had the highest level of swelling after 30 minutes.

Thermal modification reduced both the degree of swelling and the rate of swelling for all the species included in this testing (C. lusitanica, E. nitens, radiata pine).

Understanding differences in dimensional stability between different species makes it easier to understand how each species would behave in service and makes it easier to specify timbers that will work well in a particular application.


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