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:
- Choose the rafter size and the spacing intended and read off the permitted span.
- 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.
- 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.