Specialty Timbers New ZealandThe New Zealand Building Code
Both durability and structural performance of timber must comply with the Building Code. Some building elements are required to last a minimum of 50 years (those that are hard to replace, e.g structural timber), whereas those that are easy to replace, e.g. weatherboards or decking are required to last a minimum of 15 years.
Proving compliance is either via "Acceptable Solution" or "Verification Method". Acceptable Solutions are step-by-step solutions that must be accepted as compliant. Verification Methods are tests, calculations or evidence that prove compliance.
Acceptable Solutions
"Acceptable Solutions" are sometimes referred to as "cookbook" solutions because they prescribe a "recipe" for ensuring compliance, usually documented in Standards. Acceptable Solutions MUST be accepted by your Building Consent Authority (BCA) as complying with the Building Code.
The two key Acceptable Solution standards used for timber buildings are
- NZS 3602 Timber and wood-based products for use in building (for durability) and
- NZ 3604 Timber Framed Buildings (for structure).
If your timber species is not listed in the Acceptable Solution standard NZS 3602, you'll need to prove to your BCA that the timber will perform adequately using an Alternative Solution.
Alternative Solutions
An Alternative Solution is a building design, of all or part of a building, that demonstrates compliance with the Building Code. It can include a material, component or construction method that differs completely or partially from the acceptable solution clauses of the building code.
The BCA will requires sufficient evidence that demonstrates adequate performance before issuing a building consent. If a dispute arises over a BCA's decision on a building consent, a "determination" may help to resolve this. A determination is a binding decision made by the regulator, the Ministry of Business, Innovation and employment (MBIE) to solve disputes or questions about the rules that apply to buildings. MBIE can also initiate a determination where it believes it is necessary to achieve the aims of the Building Act.
Specialty Timbers NZ are publishing our own Alternative solutions for our members.
Verification Methods
Verification methods are the ways to prove that your timber will perform adequately in service.
For species or products not listed in NZS 3602, proving that your timber species is sufficiently durable for your application uses the Verification Method B2/VM1, and requires evidence that takes into account:
- exposure conditions that the timber component will be subject to in service; and
- how long it must last (15 or 50 years).
If you are going to use your timber species for structural applications, things get a wee bit complicated. This is because the Acceptable Solution, NZS 3604 Timber-framed buildings, applies only to radiata pine and Douglas fir. The solution is to engage a structural engineer to design your building, who'll use NZS 3603 Timber Structures Standard as their verification method. DO THIS BEFORE YOU DESIGN YOUR BUILDING. Specialty Timbers NZ are working on a number of initiatives to address this issue, which arbitrarily penalises structural specialty timbers.
For more detail on durability see Durability and the building code »
For more detail on structural timber see Structural Specialty Timbers and the Building Code »
Disclaimer: While every effort is made to ensure the accuracy of the information provided on this site, Specialty Timbers New Zealand inc. do not accept liability for any consequences arising from reliance on the information published. If readers have any doubts about acting on any articles they should seek confirming, professional advice.