Floor Deflection Standards

An l 720 floor would deflect only half what an l 360 floor does and would be far less bouncy to walk on.

Floor deflection standards.

If the floor deflects more than the 0 4 inches or 0 6 inches the floor is deflecting more than what the standard allows and does not meet code. 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. Figure 2 live loads and deflection limits are set by code. For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360.

Frequently there is misunderstanding regarding deflection between joists. Relatively longer spans and higher loads have the potential to make differential deflection problems in floors more pronounced. The l 360 standard means that the floor should not deflect more than the span divided by 360. See the table below.

This is essential in a flat slab or plate where the mid panel deflection relative to the supporting columns is the sum of the column strip deflection and the two way panel deflection. The irc says that floors must deflect no more than 1 360 of the floor s span. The china in the sideboard rattling as you walk across the dining room floor is not a mark of quality construction. Similarly the allowable deflection for a criterium of l 240 would be 144 inches divided by 240 which is 0 6 inch.

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. For an older home the deflection usually exceeds the current code requirements. Consequently better built homes use stiffer deflection standards often l 480 or l 720. For example a floor with a span of 10 feet must deflect no more than 1 3 inch or 120 inches 360 inches.

Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection. For example for a 30 foot span l deflection of up to 1 5 inches in a single floor member would be permissible. Serviceability the actual long term deflection of a normally reinforced slab or beam in the typical floor of a multi level office building depends on a large. Ft plus any.

The 2012 international building code ibc section 1604 3 requires floor members supporting dead and live loads to not exceed an l 240 deflection limit. Many tile contractors can judge a floor s deflection by walking around. Codes also specify how much floors are allowed to bend under load a measurement called a deflection limit.