5 in. vs 6 in. Slab and a Concrete Order
You are pricing the foundation package for a 60 ft by 40 ft single-story shop. The architectural sheet shows a 5 in. slab-on-grade. The structural sheet shows a 6 in. slab with the note "slab thickness to be verified in field." Two grade beams, 12 in. wide by 18 in. deep, run the full 60 ft length with tops flush with the slab top. Slab reinforcement is #4 at 12 in. on center each way, with 10% added for laps and waste. The supplier's minimum order is 10 cy, 4,000 psi concrete is $168/cy, and short loads under 8 cy carry a $45 charge. You plan 7% waste, and the pump hopper holds a 1 cy residual that cannot be placed. The structural engineer is on vacation until Monday; the pour is scheduled for Thursday.
1. Which slab thickness governs, and what do you do before you order?
Read the worked answer
Do not simply pick one. On a conflict between disciplines, the structural sheet governs the structural element - slab thickness on grade with beams framing into it is structural - so 6 in. is the working assumption. But the architectural note and field-verify language create a legitimate ambiguity, and the cost delta is small enough that guessing is not worth the risk.
Send an RFI same-day with a sketch, state your assumption and the cost consequence, and ask for written confirmation by Tuesday. Most contracts make the RFI response the controlling document and start a clock the owner must meet. Price the bid at 6 in. - never bid the thinner reading and hope. If the engineer is unreachable, escalate to the owner's representative in writing; "we couldn't reach him" is not a defense for a short slab. A 1 in. error over 2,400 sf is 7.4 cy of concrete and, worse, a deflection and cracking claim later.
2. Using 6 in., compute the concrete order quantity to the nearest whole cubic yard.
Read the worked answer
Slab: 60 × 40 × 0.5 ft = 1,200 cf.
Grade beams: the top is flush with the slab top, so the beam volume below the slab is the depth to count - 18 in. − 6 in. = 12 in. deep by 12 in. wide. Each: 1 ft × 1 ft × 60 ft = 60 cf. Two beams = 120 cf.
Total: 1,200 + 120 = 1,320 cf ÷ 27 = 48.9 cy.
Waste: 48.9 × 1.07 = 52.3 cy.
Pump residual: + 1.0 cy = 53.3 cy.
Order 54 cy. Rounding down to 53 cy leaves 52 cy placeable against a 52.3 cy need - you run the pump dry mid-pour, and a cold joint in a slab is far more expensive than the $168 the last yard costs.
Sanity check on the supplier's rules: 54 cy clears the 10 cy minimum easily and can be delivered in full 8 cy loads with no short-load charge. For contrast, the 5 in. reading gives 41.9 cy gross, 45.8 cy with waste and residual - an 8 cy, ~$1,350 difference. That is precisely why you do not resolve the drawing conflict by arithmetic.
3. Compute the slab reinforcing tonnage, and identify what the takeoff still lacks.
Read the worked answer
Bars running the 40 ft direction (spaced along the 60 ft dimension, at 12 in. o.c.): 60 spacing intervals + 1 = 61 bars, each 40 ft = 2,440 ft.
Bars running the 60 ft direction (spaced along the 40 ft dimension): 40 + 1 = 41 bars, each 60 ft = 2,460 ft.
Total: 4,900 ft. With 10% for laps and waste: 5,390 ft.
Weight: #4 bar = 0.668 lb/ft → 5,390 × 0.668 = 3,601 lb, or about 1.8 tons. Quote 2 tons with the bar lap and chair allowance priced separately and ask the supplier for a bar list.
What is missing:
- Grade beam reinforcement entirely. The setup gives slab mats only. An 18 in. deep beam has longitudinal bars and stirrups; pricing this package on slab steel alone understates the job badly.
- Dowels and slab edge thickening at the overhead doors and column pads.
- Vapor retarder, mesh chairs, and saw-cut joints - separate line items, easy to leave out when you are focused on cubic yards.
The takeoff error to guard against is not arithmetic; it is scoping the package from one drawing and never opening the beam details.