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Calculating rafter spans using NZS 3604 Table 10.1

When selecting suitable rafters using NZS 3604:2011 Timber-framed buildings Table 10.1, the default value given is for rafters on a building in the extra high wind zone. For buildings where the design wind speeds are less, a multiplier is used to determine the permitted span for the given rafter size and spacing.
To apply the multiplier:

  1. Choose the rafter size and the spacing intended and read off the permitted span.
  2. Multiply the span given by the multiplier for the wind zone (low and medium = 1.3) or (high and very high = 1.1) to calculate the extended span for the wind zone chosen.
  3. The span can be further extended by 10% if the span is continuous over two or more supports (Note 1 in the table).

Summarised from BRANZ Guideline, May 2013.