8/9/2019 Ac 2238 Structurally Fixed Cavity Battens

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Document type:Practice NoteTitle:Structurally fixed cavity battensDocument number:AC2238Version: 2

1. Background

There have been a number of enquires surrounding the structural fixing of timber cavity battens.E2/AS1 paragraph 9.1.8.1 gives the limitations of the construction of a drained and vented cavity byrequiring the cavity battens to be fixed by the cladding fixings to the wall frame and the claddings to befixed through the cavity battens into the wall frame.

E2/AS1 table 24 gives the acceptable fixing methods for different profiles of timber weatherboards overa cavity to enable sufficient fixing penetration to secure the cladding and batten into the framing. As analternative solution, BRANZ Limited have investigated structurally fixing the battens to the studs, whichenables normal nail sizes to be used to fix the cladding.

If the cavity battens are structurally fixed to the structural frame, the lengths of the fixings do not have toincrease for weatherboards installed over a cavity. The structurally fixed battens become part of theframe or stud.

Under ES/AS1 the fixings for weatherboards over a cavity need to be at least 20 mm longer than fordirect-fixed to give sufficient framing penetration. In general, as the length of the fixing is increased,there is an increase in the shank diameter. Larger fixings (longer than 75 mm) cause splitting of theweatherboards. Structurally fixing the cavity battens to the frame can resolve this problem, as it willallow standard length fixings to be used.

Battens must be:

20 mm maximum thickness to give a 20mm maximum width cavity

40 mm minimum width

Kiln dried (no requirement but this is industry norm) H3.1 treated

positioned mid-width to the stud and over the wall underlay

To achieve structural fixing of the batten onto the structural frame, the fixing must be either:

60 x 2.8 mm jolt head hot-dip galvanised nails

64 x 2.8 mm power-driven stainless steel annular grooved nails, or

power-driven galvanised nails, as per BRANZ Appraisal Number 546 (2007 and amended May2011) Paslode nails

The batten fixing positions must be:

at a maximum of 300 mm centres vertically

staggered 12 mm either side of the centreline of the batten

The weatherboard fixing must still be long enough to achieve 20mm minimum penetration into theframing studs, as well as fixing through the batten and weatherboard. Therefore, jolt-head hot-dipgalvanised nails should be used, that are:

75 x 3.15 mm for bevel-back weatherboards

75 x 3.15 mm for rebated bevel-back weatherboards

60 x 2.8 mm for rusticated weatherboards.

fixing positions in the weatherboard must be mid width of the batten and stud

2. References

New Zealand Building Code clause E2 External MoistureNZS3602:2003

Page 1 of 1 November 2014 AC2238 (v.2)