Can macrocarpa be used for weatherboards and does it need to be treated?
Under the NZ Building Code, cladding should comply with the minimum durability requirement of 15 years when maintained as specified. The serviceable life can be much longer and can exceed 50 years if coatings are properly maintained.
Under NZ Standards 3602 table 2 ref 2A.1 untreated macrocarpa “dressing heart” grade weatherboards are deemed to provide a 15 year durability performance and are therefore deemed to comply with the building code for weatherboards. As an example C. macrocarpa is ideal for weather boarding and framing but you need heartwood and look for clear boards if you can as it tends to check round green knots when exposed to sun while encased knots drop out. With an oil based stain clear weatherboards can prove to be very stable. C. macrocarpa is only class 3 durability so any areas that remain wet are susceptible to rot.
Note that Leyland cypresses, a hybrid of C. macrocarpa, should be even better for cladding than macrocarpa.
See Macrocarpa on the NZ Wood website
See Building Legislation and regulations on the website