Structurally fixing cavity battens for horizontal timber weatherboards 

Using longer and thicker nails to fix horizontal timber weatherboards to structural framing through cavity battens can damage the weatherboards. However, there’s a way around this 

The drained and ventilated cavity behind horizontal timber weatherboards is formed by nominal 20mm thick x 45mm timber cavity battens fixed vertically to the outer face of the exterior wall framing. The battens are fixed to the framing – primarily studs – to act as packers that form a cavity that keeps the back of the weatherboards clear of the wall underlay or rigid air barrier (RAB), which is fixed to the face of the framing. 

The relevant Acceptable Solution 

Under E2/AS1, the Acceptable Solution for Building Code clause E2 External moisture, the battens are non-structural and batten fixings are required simply to hold the battens in place until the timber weatherboard cladding is installed.  

The required fixings for the weatherboards are increased in length by 20mm (minimum) to ensure that the fixings penetrate through the batten and into the structural wall framing to the required depth. These fixings ultimately secure the battens in place. 

E2/AS1 also incorporates Table 24 Fixing selection for wall claddings, which covers fixing length, diameter and type as well as the minimum structural framing penetration and fixing patterns for a range of cladding systems, including timber weatherboards. Fixings for both direct-fixed and cavity-fixed claddings are included in the table. 

The downside of increasing the weatherboard fixing length to allow for the cavity batten thickness is that the shank diameter of the fixing also increases. This increase in length and diameter can then have a detrimental effect on the weatherboard, often resulting in cracking or damage to the face of the board. It is also more difficult to accurately install longer weatherboard fixings. 

Solving the problem 

One way of resolving this potential problem and maintaining the standard length/diameter for weatherboard fixings is to structurally fix the timber cavity battens to the wall framing. In effect, this laminates the batten to the framing, forming a nominal 20mm deeper framing member. For example, a 20mm timber batten structurally fixed to a 90x45mm timber stud forms a 110x45mm stud. This is not covered in the Acceptable Solution and will need to be submitted for consent as an alternative method.  

To support the use of structural cavity battens, BRANZ testing has identified the type of timber batten required along with the fixing requirements. Using the correct fixing type and size ensures that there is sufficient structural resistance to prevent the batten fixings pulling out of the frame. 

Fixings for different timber weatherboard profiles have also been identified. Timber structural cavity battens must be: 

• 18-20mm in thickness. 

• Installed vertically to exterior wall framing (studs) where required to support the weatherboard cladding. 

• Kiln-dried, SG6 minimum, H3.1 minimum treated. 

• 40mm minimum width – fixed on the centreline of the stud. 

The battens must be structurally fixed to the framing with either: 

• Hand-driven 60×2.8mm jolthead hot-dip galvanised nails. 

• Power-driven 60×2.87mm D-head hot-dip galvanised nails. 

• Power-driven 64×2.8mm D-head stainless steel annular-ringed nails. 

Structural batten fixings must be at 300mm centres maximum up to the very high wind zone and 250mm centres maximum for the extra high wind zone. Fixings should be staggered 12mm either side of the batten centreline. 

Timber weatherboard fixings used with structurally fixed battens based on a flexible wall underlay must be: 

• 75×3.15mm jolthead hot-dip galvanised (or stainless steel equivalent) for bevel-back and rebated bevel-back weatherboards. 

• 60×2.8mm jolthead hot-dip galvanised (or stainless steel equivalent) for rusticated weatherboards. 

Where an RAB is used, the weatherboard fixings must be increased in length by the thickness of the RAB. Where proprietary horizontal timber weatherboards are specified, follow the manufacturer’s instructions for the fixings and installation. 

In all solutions other than those that have a current BRANZ Appraisal certificate or where the manufacturer has undertaken to have the cladding system specifically designed to meet the New Zealand Building Code, it is recommended that solutions for fixing cladding systems outside E2/AS1 should be covered by specific engineering design to suit each project.