Floor Deflection Standards

For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360.

Floor deflection standards.

The china in the sideboard rattling as you walk across the dining room floor is not a mark of quality construction. 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. The l 360 standard means that the floor should not deflect more than the span divided by 360. If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6.

For an older home the deflection usually exceeds the current code requirements. Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection. Many tile contractors can judge a floor s deflection by walking around. Similarly the allowable deflection for a criterium of l 240 would be 144 inches divided by 240 which is 0 6 inch.

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. 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.

Figure 2 live loads and deflection limits are set by code. To calculate a specific value the tile council of north america tcna offers a standard formula. Consequently better built homes use stiffer deflection standards often l 480 or l 720. An l 720 floor would deflect only half what an l 360 floor does and would be far less bouncy to walk on.

The irc says that floors must deflect no more than 1 360 of the floor s span. Ft plus any. For example for a 30 foot span l deflection of up to 1 5 inches in a single floor member would be permissible. Codes also specify how much floors are allowed to bend under load a measurement called a deflection limit.

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. Frequently there is misunderstanding regarding deflection between joists. 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.