Code of Practice Online
The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
The wind load imposed on a roof structure is taken to apply perpendicularly to the roof cladding over a nominated area. The design wind load is affected by the pitch of the roof and is modified using factors called pressure coefficients. Wind design load is measured in kilopascal (kPa); 1 kPa equals 1 kN/m².
AS/NZS 1170.2 and NZS 3604 contain the basic wind speed regions for New Zealand and the modifying factors that govern the design wind load. The predominant wind speed for New Zealand is 45 m/s. The exceptions are either side of Cook Strait and areas in the lee of mountainous areas.
3.7A Sheltered Building
Scattered obstructions of a similar height or lower within 500 m from a building will considerably lessen wind speeds and lower design wind pressures.
3.7B Unsheltered Building
Structures in open land such as flat pasture and playing fields, or by water, will be subjected to higher design wind pressures.
3.7C Topographical Influences
Terrain also has a big effect. Structures near the crest of a rise or on flat land near a steep face will have increased design wind pressure.
Wind Design Load is affected by building design factors such as building height, shape, proportions, orientation, and roof pitch. Permeability can also be a big factor. Buildings with large openings on one side but completely closed on the other three sides will suffer high internal wind pressures. These internal pressures must be added to the suction load on the outside of the roof when calculating wind design load.
Local territorial authorities are usually able to give wind speed figures for a specific address in their area. All other factors, including topographical influences, internal, and local pressure factors must be considered by a suitably qualified professional to calculate the design wind load on a structure.
