Gravel under the slab

Why a gravel base

A layer of compacted crushed stone gives the slab an even, firm bed and lets water drain away instead of sitting under the concrete. Many patios, walks, and shed pads use about 4 inches of base. Driveways and soft or clay soils often get 6 inches or more. Ask your supplier for a base product that packs well, often sold as crusher run, ABC stone, or dense grade aggregate depending on where you live. Clean stone like #57 drains well but does not pack as tight.

How to measure

Measure the length and width of the slab in feet. Many people add a few inches on each side so the base reaches past the forms. Enter the depth you want after the stone is packed down, in inches.

The formula

• Cubic feet = length x width x (depth / 12)
• Add the compaction percent, since loose stone packs down
• Cubic yards = cubic feet / 27
• Tons = cubic yards x tons per yard

A 10 by 12 foot shed pad with a 4 inch base is 40 cubic feet, or about 1.5 cubic yards. With 15 percent extra for compaction that grows to about 1.7 yards. At 1.4 tons per yard you would order about 2.4 tons.

Compaction and weight

Loose stone settles when you run a plate compactor over it, so you need more loose material than the finished depth. Ten to 20 percent extra is a common allowance, and 15 percent is a good middle. Weight varies by stone type and moisture. Crushed stone is often around 1.4 to 1.5 tons per cubic yard. Many quarries sell by the ton, so ask what number they use.

Tips

• Compact in layers of about 2 inches. One thick pass will not pack the bottom.
• Wet the stone lightly before compacting. It packs tighter.
• Small jobs can use bagged gravel, but bulk delivery is usually cheaper past a yard or so.
• Then figure the concrete with the slab calculator and pick a depth from the thickness chart.