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what is a wind rating



We’ve had many people say to us over the years that their site is very windy and they want to make sure the foundations are stronger than normal. Our answer is always that their site is assessed for the strength of the wind and the engineering is then designed to suit. But it’s an interesting observation, and it begs the question as to whether there’s an understanding out there as to how stormy weather and strong winds affect a building, and what builders do about it. So, if the question is; What is a wind rating?…well, let’s answer it.


On a very broad scale, Australia is segregated into regions of varying wind strength, ranging from the lower wind speeds in the southern part of Australia, to the cyclonic areas in the north, and various coastal areas. If you’re interested in the technical side of it, click here to see the map. But even then, this map doesn’t go into the nitty gritty detail about particular sites – it’s really just a good starting point for the region you’re in.


Of course, if you’re in a cyclonic region, then the requirements for your home construction is higher before you even assess your particular spot.


So identifying the region you’re in is the obvious starting point. Most homes built in Australia are in Region A – this covers all of Victoria, SA, NSW, ACT & Tasmania, and also most of the remaining states and territories. But if you live in the coastal areas of QLD, WA and NT, then it will be in other regions from B through to D.


The next step is where someone – often an engineer, architect or surveyor – visits the site and conducts an assessment. This site assessment takes three things into consideration, which we’ll talk about in more detail.


Just before we jump into the detail though, we want to mention that this site wind assessment – and even the regions – is covered by two Australian Standards, especially one of them called AS 4055 Wind Loads for Housing. This standard sets out the rationale behind how the site is to be assessed, and provides clear details on how the final rating of the site is to be determined. This means that – in theory – two different persons assessing the same site should arrive at the same result. Which is a good thing!


Ok, let’s look at the 3 areas.


1. The terrain

This looks at the general nature, or the ‘smoothness or roughness’ of the site and the area around it. Is it fairly open? Or lots or minor ups and downs? Or a combination of both? What about further away – up to 500 mtrs away? These are questions that are systematically answered in the assessment that then provide a category of what the terrain is under the Australian Standards provisions. An example could be that a site for a farm home out in the open country is TC2 (Terrain Category 2), which is “Open terrain including sea coast areas, airfields, grassed with few well scattered obstructions, such as isolated trees and uncut grass, having heights from 1.5m to 10m.”


2. Shielding

In a parallel way to the terrain, this looks at whether the new house is going to be shielded from wind – mainly from 2 key areas such as trees and other buildings. There might be a large forest nearby which has a major impact on the wind coming through, or it might be the opposite – it might be sitting up on a hill with no trees for miles around. In this last case, it would be classified as NS (No Shielding), which has a rationale of “Where there are no permanent obstructions or where there are less than 2.5 obstructions per hectare, such as the first two rows of houses abutting open parklands, water or airfields.”


3. Topography

Once again, similar to terrain, but looking at more detail of the slopes (or lack of). Sloping sites will generally come under higher wind ratings if they’re facing in a certain direction and they’re sitting at the top end of a slope. We all know the scenario of the wind roaring out of the sloping end of a valley! So the assessor determines the actual gradients of the slopes that are applicable and uses them to classify the topography rating. An example of the wind roaring out of the sloping end of a valley could be T3 “Top third of slopes between 1:7.5 and 1:5.”


So the results of the 4 areas (the region, and the 3 points above) then provide a single rating. This rating is split into either N for non-cyclonic and C for Cyclonic. The rating then starts at 1 and goes up for higher wind speeds. So for a home right in the suburbs, shielded, flat, region A, it would N1. And then where there’s maybe some exposure or slope, it would move up to N2, and then for farms, N3, and then for really windy sites N4 or even higher in unusual situations or different regions. And then the same thing applies in Cyclonic areas – it starts at C1 and moves up.


Without getting too technical, there’s also other data which comes out of the assessment, and these are related to the actual wind speed and also the impact of water in a storm. The engineers calculate this as pressure, and it’s expressed in pascals, or pa for short. This isn’t a physics lesson, but the outcome of the report basically tells the structural engineer some gory detail of the pressures that the home will exposed to. It mightn’t make much sense to you and me, but to an engineer it’s like a cook reading a recipe – it makes perfect sense.


So there you have it – a report that tells the story of how windy your exact site is.


But to come right back around to the original comment in this blog. What does the builder do with this report? Does it have an impact on the home itself? Absolutely!


We’ve got two comments here. One is that the National Construction Code, combined with the relevant Australian Standards, provide details on how a home is to be built to meet a certain wind rating. As an example, within the Code, there are various tables and diagrams relating to different parts of the home that have columns for N1, N2, etc.


So if a wind rating comes back as N2, then the builder looks up the Code and uses the N2 column. This is a very simplistic view of it, as many items in a home are the same for all wind ratings, and some things in the code only cover up to N3, which means that the Australian Standards have to be consulted, and of a course an engineer is sometimes engaged to calculate what’s needed.


The other comment is what’s affected on the home itself by higher winds. Interestingly enough, high winds have minimal impact on footing size.


We’re not saying there’s no impact, but if you’ve ever seen photos of old homes that have been ripped apart in cyclones, you generally see the footings are still there, still in the ground. It’s the ‘tie-down’ that’s the most important – the tying down of the roof to the roof frame, and the roof frame to the wall frame, and the wall frame to the floor, and the floor to the footings.


Wind applies pressure to the roof, walls and windows, and it’s these three areas that are mostly changed in building your home in a higher wind spot, not the footings or foundations. It might mean more screws in the roofing iron. Or the trusses being closer together. Or more brackets in the roof to wall frame connections. Or it might even mean smaller window pane sizes. You get the idea.


To put it simply, a builder doesn’t have the same set of details they work on for a windy site as a non-windy site. Every site is slightly different, and calls for an individual assessment, and then constructed to that particular outcome. It’s all for your peace of mind that your home isn’t going to blow away.


As always, ask for more information!

Disclaimer. This blog is our opinion only. The information provided in our blogs is accurate and true to the best of our knowledge, but there may be omissions, errors or mistakes. The information presented in our blogs is for informational purposes only and we are not professionals, so the content we provide shouldn’t be taken as legal advice. We strongly recommend consulting with a professional before taking any sort of action. We reserve the right to change how we manage our blog and we may change the focus or content at any time.

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