Floor Deflection Rating

Frequently there is misunderstanding regarding deflection between joists.

Floor deflection rating.

Codes don t regulate floor vibration. This calculator is designed to work with dimensional lumber only. What uncle dunc said. The l 360 standard means that the floor should not deflect more than the span divided by 360.

Enter your floor joist information and then hit calculate deflection to find out your floor rating. For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360. The deflection calculator provides several engineering specifications such as the moment of inertia and yield strength to determine deflection. Get yourself a copy of simplified engineering for architect and builders by parker ambrose published by john wylie and sons.

You also have options depending on the expected configuration of your solution. Whether there will be one fixed end two fixed ends or the load will be supported on both ends. Now consider the same floor joist supporting stone tile flooring such that floor dead load is now 20 psf. However the limit on total deflection is span 720 resulting in the need for total deflection to be only 0 50 half of the 1 00 value for span 360.

See the table below. Codes also specify how much floors are allowed to bend under load a measurement called a deflection limit. If the span of the joists is 10 feet between supports then the deflection should not be more than 1 3 between the center and the end. Builders design for deflection only typically by holding deflection due to live load to a maximum of l 360 where l is the floor joist span or perhaps a more restrictive l 480.

Ilevel has taken this one step further by creating their tj pro rating. If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6. To provide higher quality floors for homeowners a deflection limit of l 480 is commonly used. Book has been around for ages and one you may find helpful.

If you have engineered truss or i beam type joists please do not use this calculator. In canada the building code includes limits on floor vibration. Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection. For example a floor with a span of 10 feet must deflect no more than 1 3 inch or 120 inches 360 inches.

Length in inches divided by 360 is the maximum allowable amount of floor deflection. Divide the total span of the floor joists in inches by 360 to determine the maximum amount the floor can give in the middle under a live load of 40 lb sq. For example the maximum deflection for a joist span of 15 is 15 12 360 1 2. For the 20ft floor joist example the new deflection limit is now 1 2inch.

If we use the same joist total deflection will be 1 50 1 25 0 25.