Wind Loads & Shed Design
Wind is the single most critical design action for sheds in Australia. Here's how it works, why it matters, and what your engineer is calculating.
Why Wind Is Everything for Sheds
Unlike houses (heavy, enclosed, compartmented), sheds are lightweight structures with large surface areas and often large openings. This makes them uniquely vulnerable to wind forces. In fact, wind is the governing design action for virtually every structural component in a shed:
- Roof — wind creates massive uplift (suction) that tries to lift the roof off the columns
- Columns — wind pushes the columns sideways, creating bending moments at the base
- Connections — every bolt, bracket, and weld must transfer the wind forces from roof to column to footing
- Bracing — the bracing system prevents the shed from racking (parallelogramming sideways) under wind load
- Footings — footings must resist both sliding (horizontal wind force) and uplift (vertical suction on the roof pulling the columns out of the ground)
Australian Wind Regions
AS/NZS 1170.2 divides Australia into wind regions based on the statistical wind speeds expected at each location:
Region A (Non-Cyclonic)
- Most of southern Australia
- Sub-regions A1 to A7
- Ultimate design wind speed: 45–50 m/s typical
- Covers VIC, SA, TAS, southern NSW, inland QLD
Region B (Intermediate)
- Transitional zones
- Ultimate design wind speed: ~57 m/s
- Parts of WA coast, some inland areas
- Higher than Region A but non-cyclonic
Region C (Cyclonic)
- Tropical coastlines
- Ultimate design wind speed: ~69 m/s
- North QLD coast, NT coast, WA coast (Broome to Carnarvon)
- Cyclonic connection requirements apply
Region D (Severe Cyclonic)
- Extreme cyclone zones
- Ultimate design wind speed: ~88 m/s
- Limited coastal areas of far north
- Highest design requirements
Terrain Categories
Terrain category describes the ground roughness surrounding the shed. Wind speed at any given height depends on how much the terrain slows it down:
- TC1 — Open ocean, large smooth lakes, flat desert with no vegetation. Very exposed.
- TC2 — Open terrain with few obstructions. Typical for rural farmland. Most common for agricultural sheds.
- TC2.5 — Scattered obstructions 3–5m high. Low-density suburban fringes, small towns.
- TC3 — Suburban, industrial, well-wooded areas. Obstructions 3–10m high.
- TC4 — Dense urban with large buildings. Rarely applicable to shed sites.
A shed at TC2 (open farmland) sees significantly higher wind speeds at roof height than the same shed at TC3 (suburban). The engineer assesses your specific site terrain using satellite imagery, topographic data, and local knowledge.
Internal Pressure — The Hidden Force
This is the concept most people don't understand, and it's often the difference between a shed surviving a storm and the roof lifting off.
When wind hits a shed with open doors or large openings on the windward face, air rushes inside. This creates positive internal pressure that pushes outward on all internal surfaces — including the underside of the roof.
At the same time, wind flowing over the roof creates external suction (negative pressure) on the roof's outer surface. The combined effect: the roof is being pushed up from inside and pulled up from outside simultaneously. This is the maximum uplift condition.
For a shed with dominant openings (large roller doors on one face), the internal pressure coefficient can be as high as +0.7. Combined with external suction coefficients of -0.9 or worse, the net uplift coefficient on the roof can exceed -1.6. This is enormous.
This is why roller door positions, opening sizes, and whether the shed is enclosed or open-sided dramatically affect the structural design. A seemingly minor change (adding a roller door to a face) can significantly increase the required column and footing sizes.
Roof Uplift Forces
To put the forces in perspective, consider a typical 18m × 12m shed in Wind Region A, Terrain Category 2:
In Region C (cyclonic), these forces roughly triple. This is why cyclone-rated sheds require fundamentally heavier construction.