What Kind of Rock Should Be Used for a Driveway Slope?

Small Rocks

• Small landscaping rocks measure under 3/4 inch, with the smallest pieces broken down into a sand-like consistency. This kind of rock compacts into a dense, almost solid surface. Use small rocks to provide a strong base for medium gravel, preventing larger rocks from sinking under heavyweight loads. Avoid using this size if your soil has poor drainage as the dense material doesn't allow moisture to escape, which can lead to puddles or ice in bad weather.

Medium Rocks

• Medium rocks are ideal for a slightly sloped gravel driveway. Between 3/4 inch and 1 1/4 inches is diameter, they have irregular shapes with sharp corners. Their size and shape allow them to wedge together when compacted, forming a more stable surface. The spaces left between compacted medium rocks are large enough for water to drain and small enough to resist the pressure of vehicles. The surface of medium rocks should be shaped into a shallow crown so water will flow to the sides.

Large Rocks

• Use a layer of large rocks for the bottom of the driveway if your soil is sandy or dense like clay. Measuring over 1 1/4 inch, large rocks don't stabilize when compacted, but they do prevent smaller rocks from sinking. If the soil feels unstable even after it is compacted, place geotextile material over the dirt bottom of the driveway site to reinforce the large rocks.

Oil and Gravel

• Oil and gravel driveways, also known as tar and chip, offer the appearance of loose rocks with the stability of a paved surface. Once the medium and large rock base is compacted, hot oil is sprayed over the rocks. A 2-inch layer of medium rocks is then spread over the oil and pressed into the material by a roller. The oil dries clear and hardens around the gravel to offer a solid surface with traction.