Roof Slab Casting: The Most Critical Single Day on Site
Roof slab casting involves intense coordination: steel binders, electricians, plumbers, shuttering carpenters, and concrete pouring gangs all operate concurrently. Once concrete is poured, errors cannot be corrected without tearing down the roof. A thorough 2-hour pre-pour inspection the evening before ensures complete peace of mind.
1. One-Way vs Two-Way Slab Reinforcement
Verify whether each room's slab is one-way (length-to-width ratio ≥ 2) or two-way (ratio < 2). In two-way slabs, main reinforcement spans in both directions. Ensure the smaller span steel (which carries the greater bending moment) is placed at the bottom-most layer with maximum effective depth!
2. Cranked Bars / Top Extra Steel & Spacer Chairs
Where slab bars are cranked up near supports to resist negative bending moments, they must be supported on steel chair spacers (fabricated from 10mm or 12mm rebar). Without sturdy chairs, workers walking on the slab will trample top steel down to the bottom, completely destroying the slab's ability to resist cracking over beam supports!
3. Electrical Conduit Embedment Rules
- Electrical PVC conduits must be tied securely with binding wire to the rebar grid so they cannot float or shift during concrete vibration.
- Conduits should never be bunched tightly together in parallel bundles; a minimum 50mm concrete clearance between parallel pipes is required to prevent continuous fracture planes in the slab.
- Fan boxes and chandelier junction hooks must be tied directly to main structural reinforcement with 8mm safety dowels.
4. Slab Thickness & Level Button Marks
Before pouring begins, weld or wire level pins (metal rods cut to exact slab design thickness, e.g., 125mm or 150mm) across the rebar grid every 2 meters. This allows masons to screed concrete to an exact, uniform thickness without dangerous low spots or heavy, unnecessary over-thickness.