Chapter 1 · Contractor
Construction quantities
Cubic yards, board feet and squares
Takeoff questions follow one pattern: convert every dimension to the same unit, multiply, then convert to the unit the material is sold in.
Concrete is sold by the cubic yard, lumber by the board foot, roofing by the square. Each section works one of them on a drawing to scale.
1.1 · Concrete in cubic yards
Feet × feet × feet, then ÷ 27
Multiply length × width × thickness, all in feet, for cubic feet; divide by 27 for cubic yards; then add the waste allowance.
cu yd = L × W × (T in ÷ 12) ÷ 27 × (1 + waste)
Why: A cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet, and concrete is ordered by the yard.
A 40 × 30 slab, 4 inches thick: 40 × 30 × 4/12 = 400 cubic feet, ÷ 27 = 14.81 cubic yards.
With 10% for waste, order 16.3 cubic yards.
Watch out: Convert inches to feet before multiplying. Using 4 instead of 4/12 makes the slab twelve times too big.
1.2 · Board feet
Thickness × width in inches, × length in feet, ÷ 12
A board foot is a piece 1 inch thick, 12 inches wide and 1 foot long. Use nominal sizes: board feet = thickness (in) × width (in) × length (ft) ÷ 12.
BF = T″ × W″ × L′ ÷ 12
Why: Lumber is priced per board foot, so the takeoff has to end there.
One 2×10, 16 feet long: 2 × 10 × 16 ÷ 12 = 26.67 board feet.
40 of them: 1,066.67 board feet.
Watch out: Mixing units: the length is in feet and the cross-section in inches. Forgetting the ÷ 12 overstates the order twelvefold.
1.3 · Roofing squares
The plan area, stretched by the pitch
A sloped roof is larger than the area it covers. Multiply the plan area by the pitch factor, √(1 + (rise ÷ 12)²), then divide by 100 for squares.
squares = plan area × pitch factor ÷ 100
Why: Shingles cover the sloped surface, not the footprint.
A 40 × 30 plan is 1200 square feet. A 6-in-12 pitch has a factor of 1.118.
1200 × 1.118 = 1,341.64 square feet: 13.42 squares, before waste.
Watch out: Ordering by the plan area leaves the job short; the steeper the roof, the bigger the gap.
Practice · Set 1
Each question sets the figure to its own numbers
A slab is 60 feet by 24 feet and 6 inches thick. How many cubic yards, to two decimals, before waste?
Practice · Set 2
Each question sets the figure to its own numbers
A continuous footing is 120 feet long, 2 feet wide and 1 foot deep. How many cubic yards of concrete is that, to two decimals, before waste?
- Slab · 4″ thick