Floor Deflection Standards

The l 360 standard means that the floor should not deflect more than the span divided by 360.

Floor deflection standards.

Codes also specify how much floors are allowed to bend under load a measurement called a deflection limit. Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection. Similarly the allowable deflection for a criterium of l 240 would be 144 inches divided by 240 which is 0 6 inch. Consequently better built homes use stiffer deflection standards often l 480 or l 720.

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. This sets an allowable first floor live load of 40 psf a dead load of 10 psf and a deflection of l 360. Frequently there is misunderstanding regarding deflection between joists. An l 720 floor would deflect only half what an l 360 floor does and would be far less bouncy to walk on.

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. 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. If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6. Many tile contractors can judge a floor s deflection by walking around.

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. For example for a 30 foot span l deflection of up to 1 5 inches in a single floor member would be permissible. To calculate a specific value the tile council of north america tcna offers a standard formula. Relatively longer spans and higher loads have the potential to make differential deflection problems in floors more pronounced.

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. See the table below. The irc says that floors must deflect no more than 1 360 of the floor s span.

For example a floor with a span of 10 feet must deflect no more than 1 3 inch or 120 inches 360 inches. Figure 2 live loads and deflection limits are set by code. 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.