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

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.

In a Scion Research accelerated decay trialwhere timber was regularly wetted, kept wet and well above fibre saturation as well as inoculated with decay fungi untreated Douglas-fir maintained strength equivalent to H1.2 treated pine for over 70 weeks and showed significantly better resistance to decay than untreated pine.

It is completely unrealistic to expect a frame to be continuously wet for a month let alone more than a year. With proper building practice that incorporates the 4 D’s principles of moisture management a weathertight building solution is possible.

The 4 Ds Hierarchy of water management

  • D1- Deflection
  • D2 -Drainage
  • D3- Drying
  • D4- Durability

Using this hierarchy it is reasonable to expect that moisture will not reach the timber frame. An analysis of the Weathertight Homes Resolution Service database shows just how effective eaves can be at reducing the risk of frames becoming wet.

Douglas-fir is suitable to use in many building elements without treatment but treated H1.2 is also available. This allows Douglas-fir to be used in higher risk building elements as well.