Douglas-fir is a refractory species. This means that it resists wetting and has a natural ability to withstand decay for longer.

The risk of wood becoming wet and suffering swelling and shrinkage movement during building is therefore much less resulting in higher stability. Movement risks are especially high for multi-storey buildings where building closure is slower.

This vital difference is confirmed by research conducted by Forest Research. The figure below shows the change in the moisture content for Douglas-fir compared with Radiata Pine when exposed to rain. 27% moisture content is the critical point at which decay is initiated. Results show that Douglas-fir did not exceed this at any point during the trial. This difference is still evident even within an enclosed wall.

For stability against the harsh New Zealand environment, Douglas-fir stands out as a top performer. Builders and home owners can rest assured that their homes are safer against humidity and moisture.

Douglas-fir does not have issues with 'longitudinal shrinkage near the pith, spiral grain and compression wood' so the shape of Douglas-fir is retained when dry. This ensures a cleaner finish for interior linings. The steady nature and strong character of Douglas-fir helps prevent movement in building structure.

Douglas-fir Stability Comparison with radiata pine

Works cited: FRI Bulletin 168: New Zealand Radiata Pine and Douglas-fir Suitability for Processing.