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Chapter 30
Sizing Joists and Rafters using Span Tables
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Introduction• Structure complexity and drafter
experience determines who sizes framing members– Even if determined by an engineer, drafter
should understand how span tables are used• Standard framing practice is to place structural
members at 12", 16", 19.2", or 24" on center
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Introduction (cont’d.)• Standard tables have been developed for
sizing repetitive members– International Residential Code (IRC)– Major lumber associations
• American Forest and Paper Association• Western Wood Products Association• Southern Forest Products Association
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Determining Wood Characteristics• Common types of framing lumber
– Douglas fir–larch (DFL #2)– Southern pine (SP #2)– Spruce-pine-fir (SPF #2) – Hemlock-fir (Hem-fir #2)
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Determining Size and Span• IRC provides allowable span tables for floor
joists, ceiling joists, and rafters– Determine how the wood will be used – Proceed to proper table
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Determining Size and Span (cont’d.)
• Key information on span tables:– Title– Loads– Deflection– Size and spacing of lumber
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Determining Size and Span (cont’d.)
• Sizing floor joists– Tables divided by living areas and sleeping
areas are provided by the IRC• Refer to Figures 30-2 and 30-3
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Determining Size and Span (cont’d.)
• Sizing ceiling joists– Use the proper table and same used to size
floor joists– Because they are both horizontal members,
they will have similar loading patterns• Refer to Figures 30-4 and 30-5
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Determining Size and Span (cont’d.)
• Sizing rafters– Tables are available with and without the
ceiling attached, and based on ground snow load
• Refer to Figures 30-6 and 30-7
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Working with Engineered Lumber
• Span tables will vary slightly for each manufacturer– Most suppliers of engineered joists provide
materials for determining floor joists and rafters
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Working with Engineered Lumber (cont’d.)
• Sizing engineered floor joists– Figure 30-8 shows a span table for engineered
floor joists• Lower describes spans based on code-allowed
deflections• Upper portion describes spans based on
manufacturer’s suggested deflection limits of L/480
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Working with Engineered Lumber (cont’d.)
• Sizing engineered floor joists (cont’d.)– Steps to determine joist size required:
• Determine the deflection limit• Identify the loading condition• Select the on center spacing• Scan down spacing column until a distance that
exceeds the span is located
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Working with Engineered Lumber (cont’d.)
• Selecting engineered rafters– Figure 30-9 a table that can be used to
determine engineered rafter sizes• Deflection is limited to L/180• Table is divided into non-snow and snow load areas
and low and high sloped roofs
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Working with Engineered Lumber (cont’d.)
• Selecting engineered rafters (cont’d.)– Steps to determine required rafter size:
• Determine roof loading• Determine appropriate slope column• Move down the column into a row reflecting desired
spacing until a value is found that equals or exceeds required span