New Home Wind Ratings

What are wind ratings and why do they matter?

Every home we build is assigned a wind rating by our structural engineer. This rating is used by the engineer and our building designers, builders, and suppliers to determine the structural properties required by footings, framing, roofing, windows, and other building elements, to withstand likely wind-generated forces.

A higher wind rating generally means a dwelling is likely to experience stronger wind gusts. As a result, the dwelling will need additional bracing and tie downs, stronger windows, additional roofing fasteners, and other structural upgrades.

Wind Ratings

Wind ratings are determined using Australian Standard AS/NZS1170.2:2011. A concise version of the standard, AS4055-2012, may be used to determine wind ratings for standard residential buildings that meet set criteria. The different wind ratings correspond with expected maximum wind gust speeds (and other factors, mentioned below).

We expect that most homes we build on the Coffs Coast will be assigned a wind rating of N2. Using the table above, this means that a home built to the N2 standard is designed to withstand wind gusts of up to 86km/h, with occasional gusts up to 144km/h. According to the Bureau of Meteorology, the strongest wind gust ever recorded at the Coffs Harbour weather monitoring site near the airport was 124km/h. Keep in mind there may have been stronger gusts in other locations on the Coffs Coast.

The N3 wind rating is not unusual for homes in elevated or isolated locations. Occasionally we are required to build to the N4 standard, but this is generally limited to homes in exposed coastal locations.

Calculating Wind Ratings

To calculate the wind rating that applies to a specific structure, our engineer must consider four variables:

• The Region

  Australia is divided into four regions for wind rating purposes – Region A, B, C, and D. Most of Australia, including the Coffs Coast, falls into Region A. Region B in an intermediate zone. Region C covers coastal regions around northern Australia where tropical cyclones are likely. Region D covers a portion of the north-west coast of Western Australia where severe tropical cyclones are likely*.
  

• Terrain category
  

  The wind speed at a structure is affected by the terrain it flows over as it approaches the structure. For example, wind flowing over an unbroken, flat field behaves differently to wind flowing through a forest.
  

• Shielding factor
  

  The presence of large trees or other houses nearby may shield a structure from the worst wind gusts.
  

• Topographic classification

  A house on top of a hill is likely to experience stronger wind gusts than a house at the bottom of a hill. Engineers evaluate the gradient of a hill and where a house is placed on the hill.

Engineers also look at the geometry of the proposed structure to determine how winds may interact with the building.

*Note that the line between Zone A and Zone B falls near Corindi Beach, at 30 degrees latitude south. Any home built north of 30 degrees latitude will be constructed to a minimum N3 wind rating standard.

Costs

Our standard pricing assumes a wind rating of N2. A higher wind rating will result in some additional costs – usually in the range of $2,000 to $5,000 depending on the wind rating and size of the dwelling.

The extra costs cover additional tie downs (metal rods connecting the framing to the foundations), stronger windows, stronger framing and trusses, and other upgrades needed to meet the requirements of the National Construction Code.

Windows are a very important consideration, as glazing is expected to withstand significant stresses. Corners, in particular, experience significant pressure changes, so windows within 1200mm of a corner are built to the next highest wind rating standard. For example, general windows in an N2 rated dwelling are built to withstand serviceable loads of 400 Pascals, while corner windows are designed to withstand 600 Pascals, which is the same load rating as general windows in an N3 dwelling.

Got questions?

Wind ratings, Bushfire Attack Level, BASIX, NatHERS, Construction Codes, complicated terminology – at first glance, building a house might seem confusing! We’re here to make the process enjoyable, from start to finish. We take care of hard stuff, and explain the rest in simple terms, so you can enjoy the experience of planning your new home. Contact us today to get started!