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.

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)

Factors in Wind Rating

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. 

N1 Wind Classification

N1 wind classification is the lowest of all wind classifications, and it is the weakest classification of wind. Generally, winds of this classification are considered light breezes and have an average speed of up to 34 m/s.

Though N1 winds are the weakest, they can still cause minor damage to roofs and gutters, and they can also bring down small branches and other loose objects. In addition, N1 winds can cause light livestock to be affected and can lead to minor sand-blasting of surfaces such as buildings and cars. 

N2 Wind Classification

The N2 wind classification is the second category of the Beaufort scale, and is divided into two parts. This category is characterised by wind speeds of 40m/s In this range, the wind can be felt on the face, and can cause small branches to move. At the upper range of this category, loose items may be moved or blown away. It is often accompanied by short, scattered gusts of higher speeds.

N3 Wind Classification

The n3 wind classification is up to 50 m/s,  It is generally considered a strong wind, causing branches of trees to sway and minor damage to structures and crops. Large trees may be uprooted under these winds, and people should not walk outdoors in this wind condition.

 The n3 wind classification is often associated with thunderstorms, as they are capable of producing winds of this magnitude.

N4 Wind Classification

The n4 wind classification, which is the fourth highest rating on the Beaufort scale, is typically associated with strong winds. These winds ultimate speed is 61 m/s

They can also pose a physical threat to pedestrians, as the winds are strong enough to knock an individual off balance.

N5 Wind Classification

The n5 classification of wind ratings, which is considered a strong wind, can be described as having sustained winds up to 74 m/s. 

N6 Wind Classification

The N6 wind classification is the highest level of wind speed and is classified as a hurricane.  N6 ultimate wind speed is up to 86 m/s

They cause massive destruction and can uproot trees, destroy homes, and cause widespread flooding. Hurricanes typically form in warm ocean waters and can last for days or even weeks. In order to prepare for a hurricane, people need to evacuate the area and store essential items in safe places. 

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