The maximum allowable beam deflection, expressed as a fraction of span.
The common limits are L/360 under live load for floors, L/240 or L/180 for roofs, and L/480 to L/600 where brittle finishes such as tile or plaster must not crack.
These limits have nothing to do with strength. A beam can be perfectly safe and still be unusable because it bounces or cracks the ceiling below — which is why deflection, not stress, governs most floor design.
Checking a beam for strength only. Deflection usually governs for floors, and a member selected on strength alone frequently fails the serviceability limit.
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