Designing Cladding for High Wind Zones
Designing cladding correctly for “High Wind Zones” is critical to the design of a weathertight façade.
The Standard NZS3604 - Timber Framed Buildings table 5.4 denotes wind zones L (Low), M (Medium), H (High), VH (Very High), EH (Extra High).
Maximum ultimate limit state wind speeds are:
- Low wind zone = wind speed of 32 m/s
- Medium wind zone = wind speed of 37 m/s
- High wind zone = wind speed of 44 m/s
- Very high wind zone = wind speed of 50 m/s
- Extra high wind zone = wind speed of 55 m/s.
Specific design is required for wind speeds greater than 55 m/s.
Wind zone influences the risk level of the building envelope under Clause E2 of the NZBC, so it is critical to detail cladding systems for appropriate wind zones.
Some critical factors for Extra High Wind Zones include:
The detailing for Extra High Wind Zones under E2/AS1 requires a rigid underlay and cavity cladding.
Rigid underlays (also known as rigid air barriers) can take the form of 7mm plywood or 6mm fibre cement covered with flexible underlay.
There are also other proprietary systems available such as OSB (OSbrace), pre-coated plywood (Ecoply Barrier). Rigid underlays protect linings from undue air pressure loadings allowing for better weathertightness and allow the building to be closed in earlier.
Flashings in Extra High Wind Zones shall have hooks and hems, and flashing upstand dimensions increased by 25 mm beyond those shown in Table 7 or elsewhere in the document. Window heads must have sealant installed between underside of head flashing top edge of window head flange.