Under the old code, engineers generally certified Macrocarpa beams as the same strength as pine.
How does Amendment 4 to NZS 3603 and flow-on effects to NZS 3604 affect alternative structural timbers such as Macrocarpa?
Much of the structural Macrocarpa supplied is large piece size beams which cannot be machine stress graded. Suggestions are to let all structural Macrocarpa fall into the existing no.1 framing visual grade that the above amendment allows, although it does downgrade its engineering properties. There is ever increasing feedback of the growing concerns both around the world and in NZ to chemically treated timber being in public amenity areas and even in domestic housing in general. Many people, especially new citizens from Europe and America, are insisting on an alternative to chemically treated timbers in their homes. They see Macrocarpa as a logical choice.
Macrocarpa is renowned for its natural durability.
Where Macrocarpa has been recorded as lasting many years even in in-ground situations. It must be recognized that like all naturally durable timbers there is large variability in durability. Under Ensis field trials 50 x 50 stakes were shown to deteriorate after 10-15 years in the ground. Larger sections such as fence posts will last longer.
Therefore this gives heart Macrocarpa a durability rating similar to H3.1 treated radiata pine according to the NZ building code. Macrocarpa has been shown to be very durable where used for internal structures, and weatherboard/fascias externally. Where used externally structurally, Macrocarpa should be able to be easily examined and replaced if necessary (Building Code 15 year life).
MacDirect sells considerable amounts of decking from heart grade. This is one of the areas where there is considerable variation in exposure stress. It can be expected that some boards could have to be replaced within 8 years although we have had very positive feedback regarding the durability of decking sold. We recommend that ‘grooves’ are placed down to facilitate drying between joist and deck. In the past some have used Macrocarpa deck joists; this is not recommended and is not allowed under the building code. MacDirect sells many pergolas both kitset and cut to order, for private homes and public park areas. Some of these are in-ground posts and are at owners discretion.