CHANGES TO H3.1 FOR SOLID TIMBER FRAMING   

MBIE have just released a notice that H3.1 is no longer an Acceptable Solution for solid timber framing.  NZBC B2/AS1, clause 3.2.3 makes it clear that it is not to be used for structural purposes.  Also it limits H3.1 to non-structural timber such as; cavity battens, jamb battens, cladding, lining and trim.  The term "solid" timber may be confusing as it is not defined and is not a common term, we assume it is similar to the term "sawn timber (whether or not dressed or gauged)" from NZS 3602.

Masterspec timber sections are generally still correct, specifying structural and framing timber as H1.2 or H3.2, and other timber, cavity battens, jamb battens, cladding, and trim timber as H3.1 or better depending on circumstances.  We found only one area where we needed to update the treatment.

Normal non-structural timber cavity battens, for drained cavities, are correctly specified as H3.1.  However, structural cavity battens, an Alternative Solution covered by the BRANZ Bulletin BU 475 Structurally Fixed Cavity Battens, are specified as H3.1 in the bulletin.  With this new MBIE notice it would seem that structural battens now need to be H3.2, as they are structural and outside of the wall underlay.

Masterspec has updated the timber sections and some of the timber cladding sections to include;

  • Non-structural timber cavity battens treatment remaining as H3.1.
  • Structural timber cavity battens treatment to be H3.2
As usual all changed sections will be flagged in the NextGen2 system the next time you open your specification or Office Master.

The following is an excerpt from the MBIE release.

Changes to H3.1 for solid timber framing

Notice

This is an important notice for framing specifiers.

H3.1 treatment class is no longer an Acceptable Solution for solid timber framing.

Background

In 2003, the review of NZS 3640 (Chemical preservation of Round and Sawn Timber) split H3 into H3.1 and H3.2. H3.1 was developed as a dedicated “cladding treatment” class, and H3.2 continued for external structural timber not in ground contact.

Partly in response to the leaky building crises, H3.1 was adopted in 2003 for some enclosed framing use. However, in 2011 this was discontinued under B2/AS1 when all framing was cited as H1.2 (except for H3.2 cantilevered deck joists)

The discontinuance of H3.1 as a framing treatment allows it to revert to its original purpose as a cladding treatment class. B2/AS1 reinforces this in its 2014 amendment which states, under 3.2.3:

...Timber used for structural purposes is required to be durable in-service for the life of the building, being not less than 50 years unless.... This is applicable to H1.2, H3.2, H4, H5, and H6....Timber used for non-structural purposes, such as H1.1 and H3.1 is required to be durable in-service for a minimum of 5 years and 15 years respectively.

H3.1 is no longer an Acceptable Solution for solid structural framing.
H1.2 or H3.2 is required for enclosed framing under B2/AS1.
  

Comment

NZS 3640:2012, cited in B2/AS1, has an expanded range of H1.2 treatments comprising boron plus most of the azole based treatments from H3.1. These H1.2 azoles are only available as water-based, not solvent (LOSP), and have a slightly higher concentration than for H3.1.

Those wishing to continue using azoles for framing can now do so under H1.2 without recourse to H3.1 and without involving solvents.

H3.1 continues as a dedicated cladding class, and retains the azole-based treatments (and others) as both solvent (LOSP) and water-based options. Reinstating H3.1 as a cladding class allows for possible development of dedicated cladding treatments in the future.

Treatment options in B2/AS1 for Framing and cladding (Solid timber)
Taken from NZS 3640:2012
Class Use Treatment Carrier
H1.2 Framing class Boron Water
 Azoles - propiconazole and tebuconazole Water
H3.1 Cladding class  CCA Water
 Copper Azole Water
 Boron Water
 Azoles - propiconazole and tebuconazole Water
 Azoles - propiconazole and tebuconazole Solvent
 CuN Solvent
 TBTN, TBTO Solvent

 



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