| Primary value | 130 mm (for 4.0 m span (L / 30)) |
| Applies to | One-way slabs spanning over 3+ supports · Typical apartment floor slabs continuous over beams · Aspect ratio ly / lx > 2 |
| Exceptions | End span (treat as simply supported) → L / 26 |
| Cantilever overhang → L / 7 | |
| Heavy live load (> 5 kN/m²) → Reduce ratio by 10–15% | |
| Measured as | Effective span = clear span + effective depth, capped at centre-to-centre. Use the longer interior span when sizing — exterior end spans usually need a separate check at L/26. |
| Source | IS 456 — Clause 23.2.1 ✓ Verified |
64 related items across IS codes, knowledge articles, design rules, maps and tools
Continuity reduces mid-span moment by ~25% versus a simply-supported slab, so the same span allows a thinner section. L/30 is the IS 456 ceiling — going thinner needs an explicit deflection calculation. For 4 m bays this gives 130 mm effective, ~155 mm overall.
Most multi-bay apartment slabs are 150 mm overall — comfortably above the L/30 floor and avoids needing a separate end-span calc. Premium builds use 175 mm to keep finishes flat and reduce vibration.