Floor Deflection Standards

For example a floor with a span of 10 feet must deflect no more than 1 3 inch or 120 inches 360 inches.

Floor deflection standards. This sets an allowable first floor live load of 40 psf a dead load of 10 psf and a deflection of l 360. Relatively longer spans and higher loads have the potential to make differential deflection problems in floors more pronounced. If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6. For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360.

Similarly the allowable deflection for a criterium of l 240 would be 144 inches divided by 240 which is 0 6 inch. The l 360 standard means that the floor should not deflect more than the span divided by 360. Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection. For an older home the deflection usually exceeds the current code requirements.

Figure 2 live loads and deflection limits are set by code. 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 example for a 30 foot span l deflection of up to 1 5 inches in a single floor member would be permissible. 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.

The irc says that floors must deflect no more than 1 360 of the floor s span. Consequently better built homes use stiffer deflection standards often l 480 or l 720. Codes also specify how much floors are allowed to bend under load a measurement called a deflection limit. Many tile contractors can judge a floor s deflection by walking around.

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. To calculate a specific value the tile council of north america tcna offers a standard formula. 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 china in the sideboard rattling as you walk across the dining room floor is not a mark of quality construction. An l 720 floor would deflect only half what an l 360 floor does and would be far less bouncy to walk on. Frequently there is misunderstanding regarding deflection between joists.