How the wind rating for your home is calculated
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27 August 2018
Your Paal home will have a wind rating that’s determined by the consideration of four factors. These are the Region, the Terrain Category, a Shielding Factor and the Topography of the land.
1: WIND REGION
This refers to the part of Australia where your home is or will be located.
Region A – Normal refers to the southern half of the continent, with the exception of 100 km wide coastal strips on the east and west coasts, north of Coffs Harbour and Green Head respectively.
Region B – Intermediate refers to this 100 km wide coastal strip, which also extends around the northern half of Australia, except where it’s superseded by a 50 km wide cyclonic coastal strip.
Region C – Tropical Cyclones refers to this 50 km wide cyclonic coastal strip extending around the northern half of Australia, except where it’s superseded by an area of severe tropical cyclones on the west coast of Australia.
Region D – Severe Tropical Cyclones refers to this coastal strip on the west coast, roughly between Carnarvon and north-east of Port Hedland.
The main structural threat of cyclonic winds is the aerofoil factor where the roof is sucked upwards by air blowing across the convex roof line. Stronger tie-downs for the roof are combined with extra wall bracing, with sheet bracing sometimes added in critical parts of the frame.
2. TERRAIN CATEGORY (TC)
This describes the surface roughness of the area up to 500 metres from your home site.
TC 1 Very exposed open terrain with few or no obstructions. Enclosed water surfaces.
TC 1.5 Open water surfaces including near-shore water, large unenclosed bays, lakes and enclosed bays. TC 2 Open terrain such as farmland with isolated trees. Grassland with uncut grass. TC 2.5 Terrain with a few trees or isolated obstructions. This terrain is typical of outer urban areas with scattered houses, or rural developments with few buildings per hectare. TC 3 Terrain with many closely-spaced obstructions, such as suburban housing or light industrial estates.
3. SHIELDING FACTOR
FULL SHIELDING – FS. This occurs where at least two rows of houses or similar-sized permanent obstructions surround the house being considered. Full shielding is typical of suburban developments greater than 10 houses per hectare. In Regions A and B of Australia, heavily wooded areas within 100 metres of the site provide full shielding. PARTIAL SHIELDING – PS. This requires at least 2.5 houses, trees or sheds per hectare. It’s typical of acreage-type suburban developments or wooded parkland. The second row of houses are classified as partially shielded. NO SHIELDING – NS. This applies where there are no permanent obstructions or less than 2.5 obstructions, such as houses, per hectare.
4. TOPOGRAPHY EFFECT
The topographic classification is determined by the effect the wind has on the dwelling, due to its position on the hill. A minimal slope such as the bottom of the hill or a less than 1-in-20 gradient would be classed as T0.
A cliff is defined as a slope of greater than 1-in-3 and has the maximum topographic rating of T5 at the top. The maximum slope is measured at the steepest part of the hill, regardless of where the dwelling is positioned. Over the top of the hill, the wind pressures drop down.
5. WIND CLASSIFICATION
This wind classification system is a combination of wind region, terrain category, shielding and topography. The Australian Standard AS4055-2012 Wind Loads for Housing sets out 10 wind classes N1-N6 (non-cyclonic regions) and C1-C4 (cyclonic regions)
Paal designs its homes in non-cyclonic zones to wind classification N3. “This is even for regions that might require N1 or N2, so there’s a considerable safety margin,” said Paal product development manager Philip Weymouth.
Paal also builds to the C2 rating for cyclonic zones.
“Because we design strong, the customer doesn’t have to worry, since we always meet or exceed the Australian wind load standards. Essentially, we build in a security factor.”
ASSESSING YOUR SITUATION
“Mostly we can make an informed decision on all these factors by looking at the house plans, but if necessary Paal will refer a home builder to a local engineer for assessment of the block,” Philip said.
The structural integrity of Paal’s homes is a result of extensive in-house testing of the frames and roof trusses by engineers at Paal’s factory in Emu Plains. This work has been followed by further strength testing of the frame and roof components by the reputable CSIRO.
“Paal homes have long been renowned for being easy to erect, accurately built, and durable. The wind strength factor isn’t one that’s considered by customers every day, but Paal scores there as well,” Philip said.