This collection of details is based on the traditional New Zealand rusticated and bevel-back wall claddings over timber framing, either direct-fixed for lower weathertightness risk applications or installed over a drained and vented cavity. These claddings rely on traditional skills and careful detailing for their successful application. Flashing systems are an essential element of any cladding system.
This extensive range of details, generally not given in E2/AS1, covers the junctions and openings in horizontal timber weatherboard claddings that are likely to be encountered on buildings in New Zealand. The details may also serve as a guide for checking details that are adapted, or differ, from those illustrated in this collection, such as those with metal or other supporting framing, to assess them for the levels of weathertightness achieved.
The installation of an aluminium window in bevel-back weatherboards incorporating a cavity has been tested by BRANZ to Verification Method E2/VM1. The test has demonstrated that these particular details will meet the performance requirements of clause E2 External moisture of the Building Code for all exposures and wind zones.
This collection provides construction details for junctions and openings in horizontal timber weatherboards on timber framing to provide a means of supporting compliance with Building Code clause E2.3.2.
The details are limited to buildings that fall within the parameters of E2/AS1, that is buildings that are:
BRANZ Good practice guide - timber cladding and BRANZ Selecting timber detail the selection and fixing requirements for horizontal weatherboard systems, including framing, building wrap, cavity construction, fixings, profiles and application of decorative coatings.
E2/AS1 risk matrix
The first step in the design of any cladding system and the associated opening and junction details is to refer to Section 3 of the E2/AS1 risk matrix. This will establish if the timber cladding can be fixed directly to the timber frame or is required to be fixed over a cavity. Two details are often provided for a particular junction - one with and one without a cavity.
The wind zones are as defined and determined for a particular site in NZS 3604 Section 5. The wind zones are established from wind regions, based on data supplied by the New Zealand Meteorological Service and included in NZS 4203:1992 General structural design and design loadings for buildings. The regions are based on wind speeds that have a 5% probability of being exceeded in 50 years. The wind zones are calculated for a particular site by considering the modified wind speed as outlined in NZS 4203 for a building with an eaves height of 8 m above the ground.
Wind zones are identified as:
Flashings must be sufficiently rigid to maintain the shape specified when fixed and in continuous use. Materials must be compatible with other contacting elements, particularly contact with timber treated with copper-based preservatives. Back flashings in this collection differ from those shown in E2/AS1 details in that they provide 65 mm rather than 50 mm cladding cover, thus giving an increased margin of safety.
Wall underlays and backing
All external timber framing containing insulation within the framing cavity must be wrapped with a wall underlay (rigid or flexible). This is irrespective of whether a cavity is applied over the framing or not. No wall underlay or wall wrap is required immediately behind the timber cladding if it is fixed over a cavity.
Air seals to joinery items
All window details incorporate an air seal between the framing and the window reveal or frame. Air seals are formed by inserting a compressible polyethylene foam (PEF) backing rod and filling the gap with a low expansion foam or sealant to give an airtight seal.
The details in this collection use a number of materials in contact or close proximity. Users must therefore ensure that framing, battens, flashings, claddings and fixings are compatible with each other to meet Building Code clause B2 Durability requirements and be suitable for the environment in which the completed building will be located. An example is isolating timber containing copper-based treatments from metal (zinc) components.
Aluminium window details in this collection show a wider flange to maximise the cover of the flange to the cladding materials and to reduce the risk of the cover being compromised during construction. E2/AS1 details for aluminium windows require a 10 mm minimum cover at jamb and sill flanges (although the sill cover may be reduced to 8 mm in some instances).
Roof underlay is noted on drawings as being laid over wire netting. Self-supporting roof underlay may be used (in accordance with the specific manufacturer's recommendations) as an alternative. At barge and similar details, folding the underlay up and onto the top of the barge board or similar is recommended.