Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of deflection limit (l/360 etc.). No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
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.
Common misuse
Checking a beam for strength only. Deflection usually governs for floors, and a member selected on strength alone frequently fails the serviceability limit.