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NZ Wood talks to a building consent authority about using timber where the work is not automatically covered by the Building Code and is considered an alternative solution.
In the first of a series of articles, Wellington City Council’s Manager of Major Projects and CBD Building Consents and Licensing Services, Robert Tierney, offers advice and explains the process of gaining consent for alternative solution proposals.
What is an alternative solution?
There are two ways of demonstrating compliance with the building code, which is a performance-based requirement for building work.
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An acceptable solution comes from a suite of documents issued by the Department of Building and Housing (DBH) called compliance documents. If you follow the prescribed method in those documents it is deemed to comply. If you build with those details the building consent officer or council or local authority will accept it because they know it’ has been issued by the DBH and it is deemed to comply.
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Everything else is an alternative solution. An example I often give is that if you to build a 20-story building out of recycled coke bottles, then that’s fine but then the onus is on you to demonstrate how you are going to make it stand up, how you are going to stop it from leaking and how it is going to perform adequately for the purpose required and for people to live in it.
Window framing is one example of where this situation occurs and alternative solution consent is required. Why is that?
A compliance document is available for window framing as a result of the issues around weather tightness. The compliance document is called E2 AS1 and it shows acceptable solutions for doing cladding with particular materials, which could be timber weatherboards, board and batten as well as profiled metal in stucco systems. This document, however, only has generic details for window frames. It doesn’t provide specific details for different types of frames, for example metal, wooden, plastic or whatever. So theoretically an alternative solution is required for window frames because these details are not mentioned in the compliance document.
So how do you go about the consent process?
There is, however, another way of viewing consents for timber window framing and this is to refer to its in-service history. People have been using timber windows and doors for centuries so it has an in-service history. But this way of getting compliance is not very well-utilised.
How do we know that in-service history works?
The simple answer is look around you, there are hundreds and thousands of buildings around Wellington that were built that way. It works; it’s proven itself so its in-service history is a very valuable means of demonstrating compliance with the Building Code.
How do you use in-service history when making a building consent application?
Basically an architect or designer can reason that what they are proposing in a drawing will work. They can argue that what they are proposing has been proven in its in-service history. A lot of timber windows very much fall into this consent category and it should be acceptable. People still need to detail their applications correctly for insulation and flashings and things like that. Theoretically there isn’t a problem with such consents for timber window frames that are made correctly in accordance with the standard profiles for their use, and as long as they are installed correctly with the right flashings. Obviously the choice of materials that is used is important and that’s something that comes down to durability. There is a particular standard for this, the NZS3602 compliance document.
What happens with a compliance document?
The compliance document will say thou shalt do it in this particular prescribed method. And what they will often do is reference a joint New Zealand Australia standard. If you follow that standard it is deemed to comply. So this is the case in terms of timber durability and the choice of the right materials for a particular job, obviously you can’t use balsa wood for a timber door or frame. That standard clearly references what timbers are acceptable for particular durability uses. And that’s all you really need to consider.
Tags: Architecture, Building, Building Consents, Design
