Wind Rating Classification

What is Wind Classification?

Wind classification simply refers to the gust wind speed that can affect a construction site. Different regions experience different wind speeds, which are also affected by the surroundings of your site. 

Why is it Important to Know My Area’s Wind Rating Classification?

Engineers and builders take note of wind classification so that they can ensure a structure’s durability to withstand the wind it will experience. In case of strong winds, they make sure that what they built will hold up. 

Wind classification of your construction site is actually quite an important consideration to note and is outlined in the Australian Standards. In particular, it has everything to do with bracings and tie-down fixings, both of which will affect how your design turns out.

Where you are building a residential property depending on the wind classification it can alter the cost of building a new home.

Wind Classifications

Wind can be classified as cyclonic or non-cyclonic, given by the letters N and C and paired with a number. They are also given common notations, given by W also paired with a number. Increasing numbers indicate increasing speed.*serviceability limit refers to the conditions in which a structure can be used, while the ultimate limit refers to the threshold at which the structure can no longer be guaranteed for safe use

Wind ClassNotationServiceable Wind Speed (m/s)Ultimate Wind Speed (m/s)
N1W282634
N2W332640
N3W413250
N4W503961
N5W604774
N6W705586
C1W41C3250
C2W50C3961
C3W60C4774
C4W70C5586

Factors in Wind Classification

A builder, building broker or engineer will account for four factors in classifying the wind of the construction site.

Wind Region

Wind region refers to categorization of different areas of Australia based on the wind that each area experiences on average. 

  • Region A: Normal
  • Region B: Intermediate
  • Region C: Tropical Cyclones
  • Region D: Severe Tropical Cyclones

Terrain Category

Terrain category refers to the area around your site. The surrounding objects may act as blocks that may reduce wind speed. 

  • Category 1 (TC1): Generally open and plain terrain
  • Category 1.5 (TC1.5): Generally open and plain terrain with bodies of water
  • Category 2 (TC2): Open terrain with few scattered trees or structures, etc. generally between 1.5m and 5m high (isolated trees, uncut grass, etc.), around 2 structures or trees, etc. per hectare
  • Category 2.5 (TC2.5): Open terrain with a few trees and isolated structures (ex. outskirts of suburban areas), with around 10 buildings per hectare
  • Category 3 (TC3): Numerous (at least 10 per hectare) close structures and/or trees, etc. (ex. suburban neighborhoods, light urban areas)

Shielding Factor

Shielding factor is about the obstructions nearby your structure, such as houses or trees. It is more about the immediate vicinity.

  • Full Shielding (FS): 10 houses per hectare or at least two rows of houses upwind surrounding the structure
  • Partial Shielding (PS): Around 2.5 other houses per hectare, houses surrounded by one row, next to open water or parks
  • No Shielding (NS): Less than 2.5 structures per hectare, no row of houses surrounding the structure

Topography Effect

Topography refers to the lay of the land, such as plains, hills or slopes. This varies with the slope and height of the hill, ranging from T0 at the lowest areas to T5 at the highest areas.

The general trend is that as each of the factors increase, the wind speed increases as well, meaning it would fall in a higher category from N1 to N6 or C1 to C4. It is essential for your builder or engineer to take the wind classification of your area into account so that you won’t ever have to worry about gust damage. With care and awareness, your home will be safe, sturdy, and steadfast. 

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