When broaching the subject of metal fasteners and preservative treated wood it’s very much a matter of education, education, education as one former UK prime minister famously coined.
A wealth of information is now available from industry groups worldwide on this important topic, as well as from the preservative treatment and fixings manufacturers themselves.
Much of the newer work stems from legislation restricting the use of CCA treatment (including a ban on residential use) due to its arsenic content.
In the UK these restrictions took effect in 2006, while in the US, CCA manufacturers stopped production for residential uses in 2003.
Global timber connector manufacturer Simpson Strong-Tie has done extensive testing on connectors and fasteners with preservative-treated wood, much of the research in the US.
Much of its testing followed the time when newer non-CCA water-based treatments, such as alkaline copper quaternary (ACQ), copper azole or other copper-containing solutions or boron compounds came to the fore.
It first became aware of a potential corrosion issue after receiving reports of increased connector and fastener corrosion when installed with some of the non-CCA treated woods. It explored the phenomena and subsequently expanded its corrosion warning in its product literature to include specific language related to treated wood. It has tested more than 1,800 samples of various combinations of steels, coatings, preservative treatments, wood species, and over 3,000 fasteners to try to better understand the issues.
Using the American Wood-Preservers Association test standard E12-94 for many of these tests, its results indicate there were some significant differences between the corrosion rates of CCA-C and some of the “alternative” water-based products.
But it did note that many variables can affect the test results, including wood characteristics, plus the number of chemical additives added to the treatments. Its conclusion was the connector industry’s standard hot-dip galvanised specification of G60 needed to be increased.
In recent years, a new generation of exterior grade fixings has been born and specifically marketed as safe to use with the newer preservative-treated timber.
Simpson Strong-Tie offers many products such as ZMAX, hot-dip galvanized (HDG), mechanically galvanized and stainless steel for use with the treated woods of today. ZMAX is a continuous hot-dip galvanized steel with a G185 specification, designed to provide more than three times the zinc of the industry standard G60 galvanized coating.) UK manufacturer Timco has the C2 exterior screw – designed to provide additional protection against corrosion in exterior applications. It is plated with a multi-layer protection system that consists of an organic top coat barrier layer and a zinc base layer.
Other manufacturers FastenMaster and Tite-Fix also have products specifically for pressure-treated wood, the former “tested and approved for use in ACQ”, while the latter has a NetCoat green finish on some products. Preservatives sector contacts stressed to TTJ that a lot of work was done by the industry in the early days of the CCA alternative treatments.
“The corrosion question has eased a lot,” said one preservative manufacturer. “The industry has changed a lot to the formulations. Additives are added to the formulation to help prevent corrosion to fixings. From our point of view it is not so much about the preservative treatment that is a problem but the application.
“But users should ensure they following the manufacturer’s recommendations,” he said.
“All the advice is there and it’s about common sense. The treated timber will last a long time if you use the appropriate fixing.” The Wood Protection Association gives guidance in its WPA Manual: Wood Preservation Specification & Practice 5.1.4 Metal fasteners and fittings, while Eurocode 5 (BS EN 1995-1-1) gives minimum specifications for material protection against corrosion for fasteners and fixings used in internal building, low hazard situations (Use Classes 1 and 2).
