New rules for timber framing for housing
On 1 April 2007 the Compliance Document for B1 Structure was changed. The new version cites Amendment 2 of the Standard NZS 3604.
Builders need to know about this change because it affects the type of timber that can be used in housing framing.
Main changes
No. 1 Framing timber can still be used, but the required sizes, spacings and spans have changed.
It is now an option to use verified VSG or MSG timber.
There are three groups of timber grades:
- No. 1 Framing or MSG 6
- VSG 8 or MSG 8
- VSG 10 or MSG 10.
Both grades in each group are equivalent for the purposes of NZS 3604.
All timber sizes are now described by their actual size (eg, 90 x 45 mm), rather than the call size previously used (eg, 100 x 50 mm).
Wet timber is specifically mentioned for use in certain places, such as on decks.
Amendment 2 gives options for replacing larger member sizes with smaller ones (which are more commonly available). These options require either closer spacings or built-up members.
Follow the plans
This is the key advice for builders. You must follow the consented plans and specifications for a building project otherwise Code compliance may not be achieved, and the code compliance certificate (CCC) could be refused.
Plans and specifications must nominate the grade of timber, as well as show all other necessary information, such as size, span and spacings.
Verified timber must be marked with certain minimum information at 1500 mm centres, including the grade and the auditor. If the auditor's mark is not present, the timber is not verified - don't use it.
Switching products
Switching products within a grouping (eg, using VSG 8 in place of MSG 8) is acceptable in terms of NZS 3604, and therefore would comply with the Building Code, but any substitution should be checked with the owner/designer first.
A further word of caution. The building consent authority can refuse to issue a code compliance certificate if the finished work does not exactly match what was consented in the original plans. So, check with it first, before making any change.
Switching products between groupings (for example using MSG 8 in place of MSG 10) is not acceptable without first obtaining approval from the owner/designer and the building consent authority. A change like this would require an amendment to the building consent.
Planning ahead
Designers should check the availability of particular grades of timber before specifying them on the plans and specifications. Because certain grades are not always widely available, it's wise to place orders as early as possible.
If the plans and specifications specify different grades in different places, be sure to keep these grades separate on site. Mixing them up could be costly.
If the building consent was granted before April 2007 and was based on Amendment 1, there is no legal need to upgrade the design to Amendment 2.
More help
For more information on the grading of timber framing call the Department's contact centre on 0800 242 243 or this website.