The Hit and Miss Approach
Traditional timber claddings use standard timbers, profiled to “industry standard profiles” and coated according to a given specification with a third party coating product. Every time a change to one component of this sum of parts is made, there is the potential for problems.
Typical examples of failure include inappropriate code compliant timbers combined with a certain profile, or more commonly the incorrect coating being selected. These problems manifest themselves in different ways, and are highly expensive to remediate after they are found.
“In many instances product failure becomes a finger pointing exercise between the timber manufacturer, the coatings company and the specifier.”
There is little prescriptive specification in Australia for timber cladding, both the NCC and BCA are both ‘performance based’ codes – beyond performance criteria, there are no specific requirements regarding the thickness or profile of timber cladding.
Some standards, for example AS 2796 (Milled Hardwood Products) make reference to a minimum effective board overlap of 13mm for products with a moisture content up to 18%. Unfortunately not all timbers are suitable for all timber profiles.
As an example H3 Radiata Pine could be specified as a stain finish cladding, While durable this product has woeful stability is liable to cup and warp within months of installation. In other cases a Class 1 durability hardwood may be specified to provide durability in a highly exposed situation, however this may exhibit significant movement in service.
Appropriate timbers need to matched with appropriate finishes and profiles. In New Zealand there is a weathertightness mandate that requires manufacturers to pass an E2/VM1 performance test. E2/VM1 is method for proving the weather-tightness of wall claddings on low-rise buildings. Both design of the profile and installation need to be designed to allow movement of timber – profiles should be matched to the timber’s respective stability.
The test was derived from AS/NZS 4284:2008 Testing of building facades, which, in turn, was derived from work by CSIRO in Australia during the 1970s. Little thought or testing has been put into the traditional profiles used in Australia today – this creates a quantum of risk for specifiers and builders.
There is a huge amount of variance in the types of fasteners used for timber cladding, from nail guns to annual grooved nails to many different screw types. Many contractors are tempted to use the fastest solution without considering longer term problems.
Fasteners represent the weak point in timber cladding. Moisture is most likely to enter weatherboards through fastener penetrations. Abodo’s hidden fix system ensures no fasteners penetrate through the exposed face, minimising the chance of installation issues and moisture ingress. We have tested our cladding systems with specific fastener type, material specification and exact fixing point on the weatherboard. It is important that builders refer to our installation instructions to get it right to ensure the long term performance.
It is virtually impossible for coatings manufacturers to test their coatings with every timber substrate, there is simply too many options. The best they can do is identify commonly used timbers. For that reason it is common to see short term failure of coatings, particularly semi transparent coatings on timber cladding.
The use of darker colours is a minefield for specifiers – darker colours attract heat, which can manifest itself in increases substrate movement and resin bleed (for softwoods). Increased substrate movement means coats prematurely fail, in some cases flaking off.
Little attention is paid to the role of surface texturing prior to coating. Texturing a surface increases both the loading, penetration and adhesion of coatings – simply put coatings applied to a textured surface will out perform a smooth face every time.
The System Approach
Modern day facades includes screening systems and other complementary timber accessories.
A system is needed to allow designers to have the freedom to specify a range or timber elements with the same look, maintenance cycles and weathering characteristics. This should incorporate an installation guide covering off key installation details, such as corner detailing and end grain protection.