Roof Pitch Calculator

To calculate concrete for a roof slab, multiply the length, width, and thickness (in feet) to get cubic feet, then divide by 27 to convert to cubic yards. For example, a slab 24 ft long, 20 ft wide, and 4 inches thick: convert 4 inches to 0.333 ft, so volume = 24 × 20 × 0.333 = 160 cubic feet. Divide by 27 to get 5.93 cubic yards. Add 10% for waste, bringing it to about 6.5 cubic yards. Use our concrete calculator for quick results, and consider our gravel calculator for sub-base needs.

For a 24 ft by 20 ft slab at 4 inches thick, the volume is 5.93 cubic yards. An 80-lb bag of concrete mix yields about 0.6 cubic feet. Since 5.93 cubic yards equals 160 cubic feet, you would need 160 / 0.6 = 267 bags. However, for such a large project, ready-mix concrete is more practical and cost-effective. Order 6.5 cubic yards of ready-mix to account for waste. For smaller slabs, bags are fine. Check our concrete calculator for precise bag counts.

The standard thickness for a concrete roof slab is 4 inches for residential applications, but it can be 5 or 6 inches for heavier loads or commercial use. For example, a patio roof might use 4 inches, while a driveway or industrial roof might require 6 inches. Thicker slabs need more concrete: a 6-inch slab for a 24x20 area would require 8.89 cubic yards. Always consult a structural engineer for load-bearing roofs. Use our roof pitch calculator to ensure proper drainage, and adjust thickness accordingly.

To convert inches to feet, divide the number of inches by 12. For instance, 4 inches equals 4/12 = 0.333 feet. If you enter 4 inches as 4 feet, your volume will be 12 times too high. For a 24x20 slab, 4 inches thick gives 5.93 cubic yards, but 4 feet thick would give 71.1 cubic yards—a costly mistake. Always double-check units. Our calculator automatically converts inches to feet, so you can enter depth in inches directly. For other conversions, use our topsoil calculator as a reference.

For roof slabs larger than 1 cubic yard, ready-mix concrete is recommended. It saves time and ensures consistent strength. For a 5.93 cubic yard slab, you would need about 267 bags of 80-lb mix, which is impractical to mix by hand. Ready-mix delivery for 6.5 cubic yards typically costs $800 to $1,200, while bagged concrete would cost over $1,500 and require extensive labor. However, for small repairs under 1 cubic yard, bags are fine. Consider our gravel calculator for base preparation.

Order 5% to 10% extra concrete to account for spillage, uneven subgrade, and over-excavation. For a 5.93 cubic yard slab, add 10% to get 6.52 cubic yards. If you order exactly 5.93 cubic yards, you risk running short, which can cause cold joints and structural weakness. Most suppliers have a minimum order and charge for returns, so it's better to have a little extra. Use the extra to form a small pad or curb. Our concrete calculator includes a waste factor option for accurate ordering.