How Do You Accurately Calculate Lumber Span Distances?


Quick Answer

The American Wood Council provides an online wood span calculator at AWC.org. The tool calculates for any species or grade of softwood and hardwood lumber that is currently available on the commercial market.

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Full Answer

The Span Calculator was engineered by Cameron Knudson and Dr. David Pollock at Washington State University with development help from the American Wood Council. The calculator lists a series of lumber characteristics with corresponding drop-down menus that provide options to choose from. Enter in the species, size, grade, member type, deflection limit, spacing (in inches), any exterior exposure and the live load and dead load (both per square foot), and click Calculate.

There is also a span options calculator that functions as a comparison tool. Select multiple species and grades of lumber at the same time, and see how they compare.

Click on Help to get a more in-depth explanation as to how the span calculations are generated. The page lists the three formulas used in calculating the maximum horizontal joist and rafter spans, and additional formulas used to calculate the bearing length and loading assumptions of a piece of lumber.

Download the Span Calculator for Wood Joists and Rafters app, available for both Android and iOS devices, to use on the go.

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