How to calculate Roof pressure zones

Changes AS/NZS 1170.2:2021 Wind Pressure Zones:

AS/NZS 1170.2:2021 introduces noteworthy changes to the calculation of wind pressure zones, a crucial consideration when evaluating the suitability of solar installations on commercial roof structures. Among various factors, wind emerges as a pivotal element, influencing both the building and the solar array.

Understanding how wind interacts with buildings and recognising the distinct pressures experienced across different roof sections is essential. This knowledge informs the requirements for solar mounting to counteract the effects of wind effectively.

A key alteration in the 2021 edition of the standard pertains to the methodology for determining the dimensions of roof wind pressure zones. These zones, including Corner, Edge, Intermediate, and Internal Zones, play a significant role in shaping the structural and mounting considerations for roof-mounted photovoltaic (PV) arrays. As these changes directly impact the calculation of wind pressures, they necessitate a comprehensive reassessment of the solar system's compatibility with the updated standards.

In AS/NZS 1170.2:2011, the value of a was determined as the minimum of 0.2b, or 0.2d, or the height (h) of the building. Where b is the breadth and d is the depth of the building.

For AS/NZS 1170.2:2021, the value of a is the minimum of 0.2b or 0.2b, if (h/b) or (h/d) >= 0.2; or 2h if both (h/b) and (h/d) < 0.2

This change primarily impacts large-footprint, low-rise buildings such as warehouses. In the case study below we show how this change impacts the roof zones on a typical warehouse-type building.

Case Study: Typical Warehouse

While the expansion of wind zones doesn't outright prohibit solar installations, it does introduce additional complexities and costs. The need for enhanced structural support to accommodate heightened wind pressures poses challenges to the feasibility and financial viability of such installations.

For existing buildings, the situation becomes more complicated. Original roof structures were not initially designed to withstand the increased pressure affecting areas previously designated as internal/intermediate zones. Consequently, when assessing their compliance with current codes, concerns arise regarding deflection and overall structural integrity. Increased Rail requirements or structural improvements may be necessary to meet updated standards, introducing an additional layer of difficulty and cost to solar projects.