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FAQ: Can modular homes be cyclone rated?

So, with a cyclone brewing of the SE QLD coast for a change, its got us thinking about cyclone ratings for modular homes. (P.S. lets hope we get some rain though!)

So, a common question we frequently get asked is “can your homes be cyclone rated?”. And the answer is, yes, quite easily.

However, there is quite a bit more to it than just simply ‘cyclone rating’ a home. So, read on to find out more about what this is and what difference it makes.

Cyclonic Wind Ratings

Ok, so you will hear the term used all the time – cyclone rating. What does it mean? So, it is the wind speed that a home is designed to withstand for a certain period.

So, the Australian Standards for wind design break the categories up as per the below table. The wind class’s starting with the letter N are for Non-cyclonic areas while the wind classes beginning with a ‘C’ are for cyclonic regions.

Wind ClassCommon
Notation
Serviceability
(m/s)
Ultimate
(m/s)
N1 W28 26 34
N2 W33 26 40
N3 W41 32 50
N4 W50 39 61
N5 W60 47 74
N6 W70 55 86
C1 W41C 32 50
C2 W50C 39 61
C3 W60C 47 74
C4 W70C 55 86

Lastly, most homes in South East QLD are designed to suit an N3 wind rating. So, this means your home should be able to easily withstand a wind gust of at least 32 m/s (115km/h) and ultimately withstand a wind gust of 50 m/s (or 180km/h).

Also by way of comparison, a home in a C4 region must be designed to suit an ultimate wind gust speed of over 300km/h!

How do I know if I’m in a cyclonic region?

Now to find out if your new home is going to be in a cyclonic region there is a helpful map published in the National Construction Code. If you are any further north than 25° south and within 50km of the nearest adjacent coastline, then you are most likely in a cyclone area.

This map highlights the major wind classification regions of Australia. Moreover, this is one of the parameters that a structural engineer must take into consideration when designing your new home.

What else affects the wind class of my new modular home?

So, deciding on the appropriate wind class for your new home is not as simple as it may seem. And ultimately the wind class for the house must be decided on by a qualified structural engineer.

However, what we can say, is that the following three points are the primary considerations that go into deciding the appropriate rating for your new prefab home.

  • The wind region your home is located in as shown on the map above.
  • The terrain category determines if your home is located in a built-up area with lots of shielding or in a flat, open space right on the beach front with no shielding. The category will take into account what the terrain is like within 500m of your home.
  • The topographic class takes into account whether the location of your home is on a slope, how steep the hill is and whether you’re on the windward side of the mountain.

So, what do we have to do to make my home cyclone rated?

Once the structural engineer has analysed your site, your design and the relevant standards they will decide on the required wind rating.

The beauty of a modular home is that, because it is already designed to withstand high wind loads and movement for transport there aren’t a lot of structural changes needed to acheive a cylonic rating!

Windows

Firstly, upgrading your windows is quite often the most expensive part of achieving the correct wind rating. Because, not only does the window have to withstand the high wind pressure, it also prevents water penetrating through to the inside of the home. Typically though, for a C1 or C2 rating, this isn’t too difficult as the actual wind rating isn’t that much higher than the standard N3 rating used.

Check out this great video by AWS on cyclonic windows.

Roofing

Another significant change often required to achieve a cyclone rating is to upgrade the standard roof sheeting. So, this involves using slightly thicker roof sheeting and installing additional screws. Moreover, all screws are required to have cyclone washers.

Internal Wall Tie Downs

And lastly, depending on the required wind rating, additional strapping and rods may be necessary to achieve the correct tie down parameters. Because of the inherent structural strength of a relocatable home, it is not too expensive and is relatively simple to produce.

Custom Design

Another important point to consider in all this is that your home is unique. And because your site is single, and quite often the topography and terrain are unique to your site. So, the wind rating for your home will always differ.

That is why it so essential to employ the services of a qualified structural engineer. Because a qualified structural engineer will consider all the relevant standards and codes, make the required calculations and provide us, the builder, with the correct wind classification to ensure that your home is built right!

Find out more about custom designing here.

Final Word

As a final note, we would like to point out that Westbuilt has built many homes in cyclonic areas throughout the last 25 years. So, we’re proud to say that our client’s homes have ridden out some of the most extreme cyclones to hit the QLD coast including Cyclone Yasi and Cyclone Debbie. Moreover, all with no issues or failures!

Find out more about our Custom Design Service here.

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