Understanding Trus Joist Pro Ratings and Minimum Deflections by Code

When designing floor joists engineers must check several design properties including moment, shear, bearing and deflection.  In this article we are going to focus on deflection since it is a common complaint from homeowners.

For any structural member deflection limits must be satisfied based on the building code.  For floor joists a live load deflection limit of L/360 and a total load deflection limit of L/240 must be met.  For example, a floor joist spanning 20ft (240 inches) a deflection limit of 2/3inch for live loads and 1inch for total loads.  This is the maximum deflection the designer should meet but it is only a minimum requirement.  Some homeowners may notice this deflection and complain about bouncy floors.

To provide higher quality floors for homeowners a deflection limit of L/480 is commonly used.  For the 20ft floor joist example the new deflection limit is now 1/2inch.

iLevel has taken this one step further by creating their TJ Pro Rating.  The TJ Pro Rating is based on satisfaction percentages of homeowners.  Below is some information provided by iLevel.

TJ-Pro™ Ratings factor in the human element in designing floors.
How a floor feels to a home buyer has always been subjective. Yet making floor performance predictable is exactly what we at iLevel have done. We invested over 10 years of research into “real world” floor situations and tested hundreds of floors that were built to precise deflection characteristics. Then we had thousands of people walk on them. We captured their preferences, put the information into a computer model and analyzed the data. TJ-Pro™ Ratings are the result. Based on the number of people who preferred one floor to another, we assigned a performance number. Our proven software programs use this information to analyze your design and assign it a performance number that is typically between 25 and 65. The higher the number, the more likely a home buyer will be happy.


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