Untreated macrocarpa, Lawson Cypress, Lusitanian Cypress or Larch are possible options.
To achieve an “acceptable solution” within an enclosed frame H3.1 treated radiata needs to be substituted with either:
- Structural grade untreated larch, or
- Untreated heartwood from the cypress species (or H1.2 treated sapwood). Macrocarpa is the most likely source, but Lawson cypress may be available although unlikely to be in large supply. (The heartwood is easy to identify, and available, if you find a supplier of the species.)
For both of these, because they are specialist species, they could be hard to source for structural purposes ie. MSG or VSG 8 or better. Very few producers will go through the “verification” steps for small scale volumes.
If MSG 6 is sufficient then visually graded “number one frame” will be an acceptable alternative. (Unverified “number one frame” is deemed MSG 6 and it can be used in place of MSG 8 but will require larger dimensions to give equivalent strength)
That leaves “alternative solutions”:
- Research would support H1.2 boron treatment as being suitable for enclosed frames in any situation and some Building Consent Authorities (BCAs) will approve H1.2 Douglas-fir in place of H3.1 Radiata.
If it is an outdoors or exposed structural application eg. posts where 50-year durability is mandated, then the building standard NZS 3602 does not designate any commonly available untreated wood as an “acceptable solution”.
NZS 3602 allows untreated heartwood from redwood, western red cedar, cypress species and New Zealand beech for outdoor use in stairs, decking, handrails where a 15-year durability is required.
(Note: because of the risk of leaching, H1.2 boron should not be used in exposed areas unless it is primed and three-coat painted.)
For more information on treated timber and building legislation visit the Treatment and Durability section of the website.
Look at the table of alternatives to pine and their structural properties.