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B1/AS3, 2nd edition

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Concurrent version: Effective on 28 July 2025

B1/AS3, 2nd edition [PDF 1.1 MB]

B1 Structure Acceptable Solution B1/AS3 Second Edition provides structural design guidance for small chimneys and hearth slabs associated with solid fuel-burning domestic appliances. It outlines acceptable construction methods for chimneys made of brick, concrete, or precast pumice concrete including specifications for materials, reinforcement, foundations, and seismic restraints. The document also details requirements for hearth slabs, including dimensions, reinforcement, and installation on timber or concrete floors. It includes bracing provisions for earthquake resistance. This edition has been published in a standalone format.

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This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people:

with compliance with the Building Act, it is published under section 175 of the Building Act
with a Weathertight Services claim, it is published under section 12 of the Weathertight Homes Resolution Services Act 2006.

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