What is a Joist Calculator?

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  • Written By: Mary McMahon
  • Edited By: O. Wallace
  • Images By: Gord Webster, Howard Sandler
  • Last Modified Date: 14 November 2019
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A joist calculator is a tool which is used to determine an appropriate joist span for a construction project such as a deck, roof, or floor. Using a joist calculator is not a substitute for checking the appropriate code book, although such calculators usually take building codes into account. Joist calculators help people plan out construction projects so that they will have the materials they need on hand when they start and so that the project will run smoothly and as safely as possible.

Joists are structural supports which are strategically spaced. The space between joists is known as the span. The span length can vary depending on how much weight the joists are supporting, how big the supported area is, the type of wood or other material used, the size of the joists, and other factors, all of which can be considered in a joist calculator. For example, a calculator often allows people to select maximum live and dead loads for spaces, as well as to indicate the deflection limit, which will determine how springy the joists will be. Another important factor can be the site of installation, as well as the presence of conditions such as water.


Joist spans are not standardized, although there are some rules of thumb which can be used when quickly calculating the needs for a space. The joist calculator allows people to determine the appropriate width of the span for a project, which in turn can be used to figure out how many joists will be needed. It is important to space joists properly, as otherwise they may not provide enough support or could be too stiff for a given project.

Many websites have an embedded joist calculator which people can use. The calculator should include spaces for a number of variables which will be used to generate a very precise measurement for the appropriate joist span. When using a joist calculator, people should also consult the building code to confirm that the span suggested by the calculator will comply with the minimum standards set in the building code.

People may find that playing with some of the variables in a joist calculator can change the results considerably. For example, if joists have not yet been purchased for a project, people could experiment with different sizes or wood species to see how these change the specifications for the project. This can in turn be used to determine the most cost effective choice of joist material for the project.


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