Read the rafter size in the left column of Table 10.1 for the 2.2 m span chosen. This is more than the span in Figure 1, so a 2.2 m rafter span is suitable. Look at the values in the span column for 1200 mm rafter spacing.Īpply the additional 10% permitted in Note 1:Ģ.86 + 10% (0.286) = 3.146. This rafter extends over two spans, so the 10% increase can also be used to select the most economic rafter.īack to top Step 4 – Work out rafter span Table 10.1 Note 1 also allows rafter spans to be increased by 10% where rafters are continuous over two or more spans, and are not birdsmouth seated. This example is a light roof in a low wind zone, so the Low and Medium multiplier of 1.3 can be used.īack to top Step 3 – Continuous rafter 10% increase Table 10.1 has span length multipliers for ‘Low and Medium’ and ‘High and Very High’ wind zones. Is 1200 mm rafter spacing appropriate?īack to top Step 2 – Rafter span multiplier When choosing the rafter spacing, also consider the proposed roof type and the support it requires – battens, purlins or sarking. The rafter span is 3.000 m, and because this is a light roof, the far right column of NZS 3604:2011 Table 10.1 Rafters for all wind zones (see Figure 2) can be used for rafter spacings at 1200 mm (see Note 4 in Table 10.1). rafter span 3.000 m continuous over two spans.In the first example, the building’s parameters are: single span or continuous over two spans (without birdsmouth over mid support).rafter span, taken along the pitch line of the rafter (see Figure 1).TO USE TABLE 10.1 for a building, you need to know:
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