Slab deflection limits as per IS 456 are serviceability checks that control bending under load. Excessive deflection can damage finishes and partitions even when strength requirements are satisfied.
Limits under IS 456
IS 456:2000 Clause 23.2 specifies a general final deflection limit of span/250, considering all loads and long-term effects such as creep, shrinkage and temperature. Deflection occurring after finishes and partitions are installed, due to imposed load, should not exceed span/350 or 20 mm, whichever is less.
Checking procedure
- Determine effective span and actual support conditions.
- Establish slab thickness, effective depth, reinforcement and concrete properties.
- Calculate immediate deflection using suitable section stiffness.
- Include creep, shrinkage and sustained loading effects.
- Check total final and post-finish imposed-load deflection separately.
Span-to-depth method
IS 456 permits span-to-effective-depth ratios as a simplified control. Basic ratios depend on whether the member is simply supported, continuous or a cantilever. Apply the code modification factors and assumptions; do not treat a basic ratio as a universal slab thickness rule.
Illustrative calculation
For a 4,000 mm effective span, span/250 = 16 mm. Span/350 = 11.43 mm, which is less than 20 mm. Actual deflection still requires calculation for the correct loading stages.
Factors and common mistakes
Span, continuity, slab depth, reinforcement, cracking, concrete modulus, creep, shrinkage, finishes and construction sequence affect deflection. Common errors include ignoring long-term effects, using the wrong span definition and skipping serviceability checks.
Related reading and references
See one-way slab steel requirements and flat slab overview. Refer to IS 456:2000 and the NPTEL reinforced concrete design course.
FAQs
What is the final deflection limit?
The general limit is span/250, considering the effects specified in the code.
What is the post-finish limit?
It is span/350 or 20 mm, whichever is less, for the specified imposed-load deflection.
Does passing deflection prove safety?
No. Flexure, shear, anchorage, durability and other design checks are also required.