New Timber Treatments & Corrosion of Plasterboard fasteners

Article by Kevin Sceats
Winstone Wallboards Ltd.

The GIB® helpline has received the odd call recently asking about potential corrosion of drywall or plasterboard fasteners when applied to H3.2 treated timber framing. We suspect this may be an outcome of BRANZ publicising some of its research into fastener corrosion when used in exterior situations particularly in our coastal zones. This research has resulted in (many cases) mandatory use of stainless steel fasteners and connectors in these highly corrosive zones.

For interior lining fasteners however it’s useful to firstly refer to NZS 3602 Table 1 which states ‘To achieve 50 year durability performance – (framing) Members not exposed to weather or ground atmosphere and in dry conditions.

1E.4 Timber framing in exterior walls – Kiln dried and gauged radiate pine – In-service moisture must be 18% or less.’

So in other words the timber framing onto which our GIB® lining systems may be applied, MUST remain dry in service.

We’re not sure of any rational for using H3.2 bottom plates (or for that matter other framing in external envelope walls) given our understanding that ‘H3.1 is (only) required for cavity battens and H3.2 is required for enclosed cantilevered decks or framing exposed to the weather and above ground.’ I just can’t imagine why a bottom plate would have a high enough in-service Moisture Content to justify use of H3.2, unless the detailing doesn’t comply with E2 AS?

As far as I’m aware there has never been any issue raised regarding galvanic type corrosion with either our GIB® Grabber® screws, GIB®Nails or galvanised components such as GIB HandiBrac® which could result in durability performance being compromised.

So the bottom line is that specifying H3.2 CCA treated timber framing does not present problems with GIB® Systems when its use is confined to ‘dry interior conditions’ as outlined in NZS 3602 Table 1.

However a further and recent confusion has arisen relating to potential use of new generation Copper treatments which also meet the H3.2 requirements which are called either Micronised Copper Quaternary (CuQ) or Copper Azole (CuAz) treated timber framing, and which inherently have much high levels of copper than the traditional CCA treatment that we are all familiar with. These two newer Copper based timber treatments are not widely used in New Zealand as (unlike the USA) CCA is still the most widely accepted and used treatment to achieve H3.2, H4 and H5. However information from a major Forest Products industry group in the USA, ‘When CuQ preserved wood is used for interior applications with continuous dry conditions, where the wood in service will remain below 19% equilibrium moisture content, the performance of fasteners, hardware and other metal products in contact with the treated wood should be similar to that experienced with untreated timber.’

There is little doubt that CuAz and Copper Quaternary treated ‘exterior’ timber framing in situations exposed to the
exterior environment are quite corrosively aggressive with Hot Dip Galvanised (HDG) non-stainless steel fasteners.
We have enquired with BRANZ to validate these views and initial discussions appear to confirm the views expressed above.

So keep an eye out for treated timber framing with the markings as follows;

  • A preservative code number of 89 which is Micronised copper quaternary
  • A preservative code number of 88 which is Micronised copper azole

We’ve made some enquiries with the timber industry and initial feedback is that these 2 treatments are not widely used. We’ll get back to you after hearing back from BRANZ.

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