Chapter 30

Sizing Joists and Rafters using Span Tables

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

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

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)

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

Determining Size and Span (cont’d.)

• Key information on span tables:– Title– Loads– Deflection– Size and spacing of lumber

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

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

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

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

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

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

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

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