![]() ![]() Many different species of tree are used for dimensional lumber. The actual length of the wood will be longer because it has to sit on something to support itself. “Span” refers to the distance between the inside of the two bearing points of a piece of horizontal lumber. It is important to know the definition of the term “span” when talking about lumber and framing. A single 2×4 is never used for a girder because it cannot bear the loads required when framing a structure. In a span table, you can only find information for a doubled 2×4 beam – also called a girder. Other commonly available species of dimensional lumber span slightly less distance, such as SPF, which can only span 6’ 1”. How Far Can You Span a 2×4 Ceiling Joist?Ī 2×4 floor joist can span 6’ 7” when spaced at 16” and using Southern Pine. ![]() How Far Can a 2×4 Span Without Support?.Note: The 140 × 70 mm rafter size permitted to span 2.9 m could also have been used (since 2.9 × 1.1 multipler = 3.19 m), but this is not a stock size so was not chosen. ![]() This indicates a 190 × 45 mm rafter size. Read the rafter size in the left column of Table 10.1 for the 3.3 m span chosen. This exceeds the rafter span proposed without using the multiplier so is suitable. This rafter is a single span, so the 10% increase in span over two supports cannot be used.Ĭhoosing 2.5 m span from the 600 mm spacing column:Ģ.5 × 1.1 multiplier = 2.75 m does not give the required span of 3.000 m. This example is in a high wind zone, so the ‘High and Very High’ multiplier of 1.1 can be used.īack to top Step 3 – Continuous rafter 10% increase? The most likely choice here would be for 600 mm spacing or less, to accommodate ceiling battens and sheet linings (see Figure 4).īack to top Step 2 – Rafter span multipliers This is a skillion roof, so consider roof cladding, how it will be fixed, whether it is to purlins, battens or sarking, and the proposed ceiling lining requirements. This is a heavy roof, so the 1200 mm column on the right side of the table is excluded – it only applies to light roofs. We’re changing the parameters in this second example to:Ĭhoose rafter spacing from Table 10.1 – the options are 480, 600 or 900 mm. (Provided by Standards New Zealand under licence 001094.) Example 2 Read off the fixing type in the same row, in this case Type E (2/90 × 3.15 mm skew nails + 2 wire dogs) or an alternative fixing of 4.7 kN.įigure 2 Example 1 using NZS 3604:2011 Table 10.1. This indicates a 140 × 45 mm rafter size. 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).
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