How to Calculate the Span for Engineered I-Joists

Updated July 20, 2017

Engineered I-joists, in use since 1969, serve as typical floor joists for many homeowners. A joist is a horizontal support member that distributes load between beams or walls. The joists are pre-engineered, so the distances they can span depends on the engineering properties given by the manufacturer. Determine the maximum allowable span for an I-joist by using data tables, the product manual or a software package. Consult the manufacturer if you cannot access one of these sources.

Determine the manufacturer and the model of the I-joist you are working with by viewing the product data printed on the joist.

Calculate the loads that are being placed upon the joist. A registered professional engineer should perform this calculation. The loading scenario depends on the distance between the joists and whether or not the entire length of the joist is subjected to weight being placed on top of it, and an engineer will consider these factors when making his calculations.

Locate the table in the I-joist manufacturer's literature that relates the maximum allowable span to the loading scenario and joist type. Match the loading box in the table to the type of joist to determine the allowable span.


There are many variables to consider when calculating the maximum allowable span of an engineered i-joist. Accurate calculations are critical, and if you do not consult a registered professional engineer, you could end up with a flawed design, creating an unsound structure and/or dangerous conditions.

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About the Author

Brad Otto has been writing poetry and prose since 2000, with work featured in "Ink," a collegiate publication at the Rose-Hulman Institute of Technology. He is currently employed as a civil engineer. Otto holds a B.S. in civil engineering from Rose-Hulman and an M.B.A. from Xavier University.