Can Lawson cypress, lusitanica or macrocarpa be used for structural framing where H3.1 treatment is specified?

Locally grown Lawson cypress, lusitanica and macrocarpa has heartwood that is in Australasian durability Class 3  – i.e., in testing of ground contact 50 x 50 mm stakes it has been shown to have an average life of 5-15 years. The average life is towards the upper end of this range.

Note that the sapwood of all species is non-durable.

For macrocarpa and most lusitanica there is sufficient colour variation between sapwood and heartwood that they can be easily segregated but in the case of Lawson cypress, the pale colour of heartwood makes it difficult to differentiate between heartwood and sapwood in finished products. Pieces containing sapwood should be segregated out immediately after sawing if timber is to be used in external situations.

Away from ground contact but fully exposed to the weather, the average life of 50mm thick heartwood of these species is 15-25 years. This is similar to the durability that could be expected from radiata pine treated to the H3.1 specification with light organic solvent preservatives (LOSP).

In situations where it is protected from the weather a service life exceeding  50 years could be expected.

These heartwoods are resistant to preservative penetration and there is little to be gained by putting them through a preservative treatment process. The sapwood is not durable and variable penetration is likely when conventional vacuum-pressure and light organic solvent preservative (LOSP) processes are used for H3.2 and 3.1 specification treatments. It can be treated using boron preservatives to the H 1.2 specification.

The NZS 3602:2003 requirement for some specific applications (such as a “flat” roof is H 3.1) treated pine. Providing that the Lawson cypress, lusitanica or macrocarpa is heartwood, it should be a satisfactory substitute for H3.1 treated radiata pine.

However, if the timber contains significant amounts of sapwood it will not be equivalent to H 3.1 treated pine. While the sapwood could be treated to meet the H1.2 specification with boron it would not be an acceptable substitute for applications where H3.1 treated radiata pine is specified.