Deflection is the bending or “sag” caused by loading. Allowable deflection is generally expressed as a fraction of the span, in inches. … For example, the allowable deflection of a 12ft span floor joist with plaster (L/360) is 0.4″ (12ft divided by 360).

What causes floor deflection?

The most common cause of deflection in a floor joist system is undersized joists that span too great a distance for their thickness and width.

How much floor deflection is normal?

The L/360 standard means that the floor should not deflect more than the “span” divided by 360. 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.