| Primary value | 450 mm (G+4) (thumb rule: 200 + 50 × storey count) |
| Applies to | Solid raft foundations on weak / variable bearing soil · Sites where isolated footings would overlap (column spacing < 3 m) · Basement raft slabs |
| Exceptions | Cellular / inverted-T raft → 300–400 mm slab + 600+ mm rib |
| Mat under tall building (G+10+) → 1.0–2.5 m | |
| Punching shear governs (heavy column) → Increase by 100–150 mm or add pedestal | |
| Measured as | Overall thickness of the raft slab (top of raft to bottom of raft, excluding PCC). Two-way reinforcement on top and bottom is mandatory; check punching shear at every column. |
| Source | IS 2950 — Clause 7.2 📚 Cross-referenced |
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Raft thickness is set by punching shear at the most loaded column, not by bending. The 200 + 50n rule sizes the slab to keep punching shear under 0.25√fck without shear reinforcement — the cleanest detailing scenario. Going thinner forces shear stirrups or column drops and almost always costs more.
Most G+3 to G+4 raft slabs land at 400–500 mm. Basement rafts get an extra 50–100 mm because of buoyancy and earth pressure on the wall they support. Tall buildings (15+ storeys) use 1.5–2.5 m mats and accept the concrete volume because it's still cheaper than piles.