Although building with timber is part of our heritage, the way we use it has changed over the years. Keeping up with the different specifications is a must for builders.
Some people are of the opinion that, since our building consent process is now more rigorous and detailed, our site operatives need to know less. This is based on the view that all the details needed to construct should be contained in the consent documents.
But if we go down this path, we could end up producing builders who can follow a plan and interpret specifications but have no real experience of their own field. This would be foolish, as it is important to have as many pairs of eyes participating in and reviewing the construction process as possible.
Even when construction doesn’t require a building consent, it is still vital that the builder conforms to New Zealand Building Code requirements. So it is important that hands-on constructors understand their work, and a key part of this is the timber that is used.
Timber construction part of our heritage
Since pioneer settlement, one of the principal components of our housing stock has been timber. Generally, our builders have great skill with timber and are often justifiably proud of this innate part of their heritage.
Over the years, our timber use has changed as the supply sources changed. Our forefathers used native timbers exclusively. These were often naturally durable and eminently suited to our construction. Rimu, kauri, matai, totara, tawa, rewa rewa and many other local timbers were commonly used.
After World War II, Pinus radiata became the timber of choice as huge plantations planted during the depression years matured, thus creating a prolific and cheap source. Untreated ‘pine’ quickly became the normal framing timber. For longevity, it was later treated with either boron salts or copper-chrome-arsenic solutions.
Changing preservatives, changing strength
As researchers learnt more, standards for timber preservation were produced and controls put on which timber treatment could be used where. Over time, this has continued to be a moving target.
We learnt, for instance, that some preservatives weren’t good for us, so new treatments were developed. Future changes are assured as we try to find which preservatives are best for people and the planet while also protecting against rot and insect attack.
Another discovery was that, when timber was grown quickly and milled earlier than previously, the strength was lower. This meant changes to span tables and the need for the timber strength to be ‘verified’ (the grading independently checked) to show that it really was as strong as expected. The old ‘F-grade’ suite of grading has been discontinued as have the ‘engineering’ grades.
Keeping up at a glance
So, are builders keeping up with all the changes? Possibly, but here is a recap of the key points.
- NZS 3602:2003 Timber and wood-based products for use in building covers what you can use and where. It is a vital standard for all builders.
- NZS 3604:1999 Timber framed buildings is the gospel for residential construction details. To be current, it must include revision 2 with the coloured span tables.
- NZS 3604 now uses the actual dry sizes of timber being used (for example, 90 × 45).
- Timber can be machine stress graded (MSG) or visually graded (VSG) using NZS 3631:1988 New Zealand timber grading rules.
- Grading identification will show the grading organisation and the stress grade, indicate if seasoned (kiln dried) and include reference to AS/NZS 1748:2006 Timber – Mechanically stress-graded for structural purposes.
- Only timbers that have been ‘verified’ can be marked VSG or MSG. This must be done by an accredited auditor.
- Identifying colours are used for machine stress grading (see box on page 33).
NO. 1 FRAMING AND G8
- No. 1 framing timber has been downgraded in where it can be used, because it is not a ‘verified’ timber. It is now rated only as an equivalent to MSG 6, the lowest strength timber covered by NZS 3604.
- No. 1 wet framing can replace No. 1 dry framing if it is similarly propped and dried before loading and is kept dry in service.
- NZS 3604 includes a wet timber, graded and identified as G8, for use where the moisture content might be higher than 25%. It has the same spanning capabilities as MSG 8 and VSG 8.
- G8 can replace MSG 8 if it is propped and dried before loading and is kept dry in service.
SPANS, NOTCHING AND FIXING
- NZS 3604 has coloured span tables for No. 1 framing, MSG 6, VSG 8, MSG 8, G8, VSG 10 and MSG 10.
- Notching and drilling of structural timber members is to be strictly in accordance with NZS 3604, not just to suit the pipe sizes involved.
- Fixings used must be strictly in accordance with Section 4 of NZS 3604.
- Hazard class marking on framing timber will record the plant (by number) and the chemical preservative used (by code) as well as showing the hazard class.
- There are Material Safety Data Sheets (MSDS) available for most treated timbers. For your health’s sake, follow them. Use gloves, masks and goggles when handling treated timber, wash hands before eating or smoking and always wash clothing separately.
- Treated timber sawdust and off-cuts must not be burned or buried, but disposed of in authorised landfills.
Check NZS 3602 for correct timber
Many attempts have been made to produce ‘ready reckoners’ to show where the different hazard class requirements apply and where the various species of timbers can be incorporated in our houses, but there are so many exceptions that only careful use of NZS 3602 (looking at both the tables and related text) will give correct and complete answers. There are many instances where untreated pine, larch, Douglas fir and macrocarpa (Cypress species) can be used in general house construction (see Table 1), but getting the use wrong may result in premature failure if the timber gets wet in service.
It is important not to guess at the content of the New Zealand standard. Always check. Accuracy is paramount.
Articles are correct at the time of publication but may have since become outdated.