1. Sizing Joists and Rafters using Span Tables
Chapter 30
Sizing Joists and Rafters using Span Tables

2. 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

3. 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

4. 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)

5. 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

6. Determining Size and Span (cont’d.)
Key information on span tables:
Title
Loads
Deflection
Size and spacing of lumber

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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