BC Update: No.96 - Guidance on the classification of freestanding, uninhabited garages
4 March 2009: The Department of Building and Housing has moved to clarify the classification of freestanding, uninhabited garages.
The Department considers that freestanding, uninhabited garages should be classified as Importance Level 1 (low consequence of failure) under Compliance Document B1/VM1 and AS/NZS 1170:20021.
AS/NZS 1170 came into effect in December 2008 as a cited reference in the compliance document B1/VM1 Structure. This Standard, like the previously referenced NZS 4203, differentiates buildings based on the risk that their failure would present to the safety of people, as well as any social, economic and environmental consequences that might arise. This differentiation is expressed through ’importance levels‘, which determine the applied design loads that buildings should be able to withstand. The levels include:
Level 1 (low consequence of failure)
This level is defined as low consequence for loss of human life, or small or moderate economic, social or environmental consequences. It covers minor structures where their failure is not likely to endanger human life. Examples include structures smaller than 30m2, farm buildings, isolated structures, towers in rural situations, fences, masts, walls and in-ground swimming pools.
Level 2 (ordinary consequences of failure)
This level is defined as medium consequences for loss of human life, or considerable economic, social or environmental consequences. Examples include single-family dwellings and car parking buildings.
The classification of freestanding, uninhabited garages is ambiguous, as they are not explicitly mentioned under any importance level.
The Department has considered each of the criteria used to establish the importance level, and has concluded that collectively the economic, social and environmental consequences of freestanding, uninhabited garage failure are lower than those of a dwelling. The Department has considered the risk of loss of life in this context, recognising that, while cladding materials could be blown around in an inhabited area in very high winds, the level of risk is still low.
Building importance levels reflect the necessary balance of a variety of factors, including consequences of failure and the cost of designing buildings to withstand applied loads. In the case of freestanding, uninhabited garages, the Department considers the low risk that their failure poses does not justify the added cost of constructing them to Importance Level 2. Therefore, in the Department’s view, freestanding, uninhabited garages should be classified as Importance Level 1 for the purpose of applied design loads.
1. This document is issued as Guidance under Section 175 of the Building Act 2004.