A roof slab is the single largest concrete pour in a house. Estimating it properly means knowing the thickness, the mix and the reinforcement, then converting wet volume into bags, cubic feet and kilograms.
1. Fix the thickness
For residential two-way slabs with spans of about 3.5–4.5 m (11.5–15 ft), 125 mm (5 in) is typical and 150 mm (6 in) is used for longer spans or heavier loads. The structural engineer confirms it from the span-to-depth ratio. Do not reduce it on site to save cement.
2. Wet volume to dry volume
Take a slab of about 1,000 sq.ft (92.9 m²) at 125 mm thick:
- Wet volume = 92.9 × 0.125 = 11.6 m³
- Dry volume = wet volume × 1.54 (voids and bulking) = 17.9 m³
3. Split by the M20 mix (1 : 1.5 : 3)
The parts add up to 5.5.
- Cement: 17.9 ÷ 5.5 = 3.25 m³ × 1,440 kg/m³ ÷ 50 kg ≈ 94 bags
- Sand: 3.25 × 1.5 = 4.9 m³ ≈ 172 cft
- Aggregate (20 mm): 3.25 × 3 = 9.8 m³ ≈ 344 cft
4. Reinforcement
Residential slabs typically consume 70–90 kg of steel per m³ of concrete, depending on span and bar spacing. At 80 kg/m³ our example needs about 930 kg, plus 3–5% for laps and cutting wastage. The exact figure comes from the bar bending schedule, which we prepare with every structural drawing.
5. What to check on pour day
- Cover blocks under the bottom mesh (20 mm for slabs).
- Chairs holding the top steel at extra-reinforcement zones.
- A needle vibrator on site and a person assigned to use it.
- Curing arranged for at least 7 days (10 days for PPC cement).
