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Free-standing garage design – update

The February 2015 BRANZ Guideline article covering a question to the BRANZ Helpline on garage design confirmed that NZS 3604:2011 Timber-framed buildings can be used to design a garage that is larger than 30 m2. Since the article, the BRANZ Helpline has had further questions about:

  • specific engineering design (SED)
  • the importance level (IL) of buildings.

Key points

Garages and outbuildings can be built to a lower structural requirement than NZS 3604:2011 when SED is used for both IL 1 and IL 2 structures.

Buildings constructed to IL 1 are not intended for gatherings of people.

NZS 3604:2011 Table 1.1 for buildings covered by this standard, states the following for IL 1 buildings: They are those ‘structures presenting a low degree of hazard to life and other property’ such as ‘freestanding, uninhabited garages and buildings with a total floor area of < 30 m2’. This is considered confusing because stand-alone garages designed to NZS 3604:2011 automatically meet the requirements for IL 2 irrespective of size.

IL 2 buildings have no floor area limit for single storey. However, the open area of structures designed to NZS 3604:2011 will be limited by the bracing requirements and the restrictions on bracing line spacing.

Garages over 30 m2 can be constructed using NZS 3604:2011, but will be restricted by:

  • the maximum construction space of 7.5 m x 7.5 m when dragon ties are used to extend the bracing grid from 6.0 m to 7.5 m (see NZS 3604:2011 clause 8.3.3) or
  • the structural ceiling diaphragm requirements.

Garages designed to NZS 3604:2011 can be used as spaces where people congregate (see limitations of Table 1.1 for IL 2 buildings).

Read Sizing a domestic car enclosure(external link) from BRANZ Guideline, February 2015.

Summarised from BRANZ Guideline, March 2015.