Deflection is the bending or "sag" caused by loading. Allowable deflection is generally expressed as a fraction of the span, in inches.
All structural members will deflect or flex under load. The amount of flex depends on the magnitude of the load applied, span of the member, and stiffness of the member. Typically for better performing floors minimal defection is desired.
Chapter 3 of the International Residential Code (IRC) provides the maximum allowable deflection for a given structural member (floor, roof, wall, etc.). See the table below. Note it gives the allowable deflection based on a fractional span quantity, so a larger denominator will yield less deflection. For example, the allowable deflection of a 12ft span floor joist with plaster (L/360) is 0.4" (12ft divided by 360). If that same joist had gypsum ceiling (L/240), the allowable deflection is 0.6".
Table R301.7 Allowable Deflection of Structural Members
L: span length H: span height
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