Where to with timber treatment?

This Issue This is a part of the Building materials feature

By - , Build 107

Findings from a recent survey give a clear mandate for improving the way we assess timber treatments.

Figure 5: Should there be maximum recommended exposure periods for timber framing left open to the weather during off-site fabrication, delivery and the construction period?
Figure 1: Are the differences between H1.1, H1.2, H3.1 and H3.2 clear to you?
Figure 2: Do you require more or less choice in timber treatments for framing?
Figure 3: How do you know that the LOSP treatment has flashed off to a safe level?
Figure 4: If a boron-based treatment was available for the H3.1 treatment class for timber framing, would you use it?
Figure 6: How practical is it to require builders to use site-applied treatments to cut timber?

Timber treatment choices used to be simple – either boron or CCA (copper chrome arsenic). In recent years, chemical companies have introduced an increasing range of treatment choices to the New Zealand market. With at least four major international companies competing here, this looks destined to continue. It is important for the New Zealand building and regulatory sectors to communicate their ‘customer needs’ to treated timber suppliers, so the regulatory processes (Standards, codes of practice and guidelines) for treated timber will continue to meet sector and consumer needs.

Figure 1: Are the differences between H1.1, H1.2, H3.1 and H3.2 clear to you?
Figure 2: Do you require more or less choice in timber treatments for framing?

Last November, the Department of Building and Housing undertook industry discussions with key people among timber suppliers, merchants, processors and builders. Findings from these discussions were widely circulated for comment throughout the building industry. Some of the survey questions, and their summarised results, are shown in Figures 1–6.

Simplicity and clarity sought

There were two key drivers to this consultation. There was a view that selections of framing timber were too complex. Changes were required to achieve simplicity of choice and clarity of purpose. There was also clear indication that LOSP (light organic solvent preservation) treatments were increasingly ‘out of favour’ with industry users. Boron-based alternatives are available for H1.2 category timber, but LOSP treatments are needed to meet the higher risk framing use of H3.1 because there is presently no approved boron alternative in this category. The Department needed to know whether the anecdotal comments were representative of industry’s views.

At the same time, the Department was aware that timber treatments have traditionally been assessed for effectiveness of fungal, mould and insect resistance only. This view had developed from NZS 3640 Preservation of round and sawn timber being traditionally for treatment plant use, and not intended for capturing the broader needs of timber users.

Figure 3: How do you know that the LOSP treatment has flashed off to a safe level?
Figure 4: If a boron-based treatment was available for the H3.1 treatment class for timber framing, would you use it?
Figure 5: Should there be maximum recommended exposure periods for timber framing left open to the weather during off-site fabrication, delivery and the construction period?
Figure 6: How practical is it to require builders to use site-applied treatments to cut timber?

If it is to meet its potential as a true industry Standard, NZS 3640 needs to include certain secondary performance criteria, such as:

  • clearly agreed exposure times for treated timber framing prior to close-in
  • treatments that allow for normal carpentry practices of cutting, ripping, notching etc, without requiring further on-site treatments
  • treatments that can be easily identified on site, do not affect timber strength and allow for accurate site measurement of moisture content
  • treatments that are compatible with other building materials
  • safe environmental practices for treated timber throughout production, construction, in-service performance and disposal.

Always room for improvement

The results from this survey appear to give a clear mandate for improvements in some of the ways we assess timber treatments. There is no indication that the timber treatments commonly used to meet NZS 3602 Timber and wood based products for use in building are inadequate, but there is always room for improvement.

The survey aimed at drawing out the wider needs of the building sector for treated timber. The information gathered will be used for the development and coordination of industry guidance and, in particular, NZS 3602 and NZS 3640.

For more

The full survey is available for download from www.dbh.govt.nz. The survey report explores the responses to all 14 questions, together with numerous associated comments.

Download the PDF

More articles about these topics

Articles are correct at the time of publication but may have since become outdated.

Figure 5: Should there be maximum recommended exposure periods for timber framing left open to the weather during off-site fabrication, delivery and the construction period?
Figure 1: Are the differences between H1.1, H1.2, H3.1 and H3.2 clear to you?
Figure 2: Do you require more or less choice in timber treatments for framing?
Figure 3: How do you know that the LOSP treatment has flashed off to a safe level?
Figure 4: If a boron-based treatment was available for the H3.1 treatment class for timber framing, would you use it?
Figure 6: How practical is it to require builders to use site-applied treatments to cut timber?

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