Landscape Rock and Stone: Choosing and Calculating What You Need
Landscape rock and stone are among the most versatile materials in outdoor design. They serve as ground cover, define borders, create dry creek beds, support drainage, and provide low-maintenance alternatives to organic mulch. Unlike mulch or bark, stone does not decompose, blow away, or need annual replacement. But choosing the right type and ordering the correct amount requires understanding the differences between materials and how coverage changes with stone size. This guide walks you through every common landscape rock type, how to calculate exactly what you need in tons or cubic yards, and how to keep costs under control.
Types of Landscape Rock and Their Best Uses
Landscape rock ranges from tiny pea gravel to large boulders, and each type has specific strengths. Pea gravel, with its smooth rounded stones averaging 3/8 inch in diameter, fills walkways, patios, and play areas comfortably. River rock, typically 1 to 3 inches, creates attractive beds around foundations, in dry creek features, and as drainage material. Lava rock is lightweight and porous, making it ideal for xeriscape gardens and areas where weight on structures is a concern.
Slate chippings are flat, angular fragments prized for their rich color range, which includes blue, purple, gray, green, and russet tones. They compact well and stay in place, making them excellent for formal garden paths, ornamental beds, and contemporary landscapes. Crushed granite offers a natural, earthy aesthetic and packs down to form a semi-firm surface for pathways. Flagstone pieces laid loose or on mortar create natural-looking stepping paths and patios.
- Pea gravel (3/8 inch): walkways, patios, ground cover
- River rock (1-3 inches): beds, dry creeks, drainage
- Slate chippings (10-40mm): ornamental beds, formal paths
- Lava rock: xeriscape, lightweight applications, fire pits
- Crushed granite: paths, driveways, rustic landscaping
- Flagstone (flat): stepping stones, patios, accent paths
- Boulders (12+ inches): focal points, borders, erosion control
How to Calculate Coverage for Any Rock Type
The formula is the same regardless of stone type: length times width times depth gives you volume in cubic feet. Divide by 27 to convert to cubic yards. To get tons, multiply cubic yards by the material density. Density varies significantly: lightweight lava rock weighs about 0.5 tons per cubic yard, while dense river rock can weigh 1.3 to 1.5 tons per cubic yard.
Coverage depth depends on stone size. Small stones like pea gravel or slate chippings need 2 inches for good coverage. Medium stones (1 to 3 inches) need 3 to 4 inches to fully cover the ground. Large river rock (3 to 6 inches) requires 4 to 6 inches of depth. Boulders are typically placed individually and ordered by the piece or by the pallet rather than by volume.
Slate Chippings: A Detailed Look
Slate chippings deserve special attention because they are among the most searched-for landscape materials. Available in sizes from 10mm to 40mm, they offer a distinctive flat, angular profile that catches light beautifully. The most popular colors are blue-gray, plum, and green, though mixed-color blends are increasingly common.
For a standard landscape bed, plan on a 2-inch layer of slate chippings. One ton of 20mm slate chippings covers approximately 14 to 16 square meters (150 to 170 square feet) at a 2-inch depth. Lay landscape membrane underneath to prevent weeds and keep the chippings from sinking into the soil. Slate does not decompose and retains its color for decades, making it one of the most cost-effective long-term ground cover options despite a higher initial price.
Cost Comparison Across Rock Types
Landscape rock prices vary widely by type and region. Basic pea gravel is the most affordable at $30 to $60 per ton. River rock typically costs $80 to $150 per ton depending on size and color. Slate chippings range from $100 to $250 per ton, with premium colors and sizes at the higher end. Lava rock runs $75 to $150 per ton. Decorative polished stone and exotic imports can exceed $300 per ton.
While stone costs more upfront than organic mulch, the long-term economics often favor stone. Mulch needs replacement every 1 to 2 years at $25 to $50 per cubic yard each time. Over a 10-year period, the accumulated cost of mulch replacement can exceed the one-time cost of stone installation. Stone also eliminates the annual labor of spreading new material.
Installation Best Practices
Proper installation starts with a weed-free surface. Remove all vegetation and apply a quality landscape fabric. Use commercial-grade fabric, not the thin material sold in home improvement stores, as it lasts significantly longer under heavy stone. Secure the fabric with steel landscape staples and overlap seams by 6 to 12 inches.
Install edging to contain the stone. Without a solid border, rocks migrate into the lawn and garden beds over time. Steel or aluminum edging provides the cleanest lines. Spread stone evenly with a rake, aiming for uniform depth throughout. For paths, consider compacting smaller stones like crushed granite or decomposed granite with a plate compactor to create a firm walking surface.
Maintaining Your Landscape Rock
One of the biggest advantages of landscape rock is minimal maintenance, but it is not zero. Leaves and debris accumulate on top and should be removed seasonally with a leaf blower. Small weeds that push through fabric tears or root in accumulated organic debris between stones should be pulled promptly before they establish.
Over many years, fine soil and organic matter settle between the stones, creating a layer that supports weed growth. For small areas, removing the stones, replacing the fabric, and reinstalling is practical. For large areas, a thorough cleaning with a pressure washer and weed treatment keeps rock beds looking fresh. Unlike mulch, you rarely need to add more material; a quality stone installation looks as good in year ten as it did in year one.
Frequently Asked Questions
How much slate chippings do I need per square metre?
At a recommended 50mm (2-inch) depth, you need approximately 60 to 70 kg of slate chippings per square metre. One bulk bag (approximately 850 kg) covers about 12 to 14 square metres at this depth.
How many tons of river rock do I need for 100 square feet?
At a 3-inch depth, 100 square feet requires about 0.93 cubic yards of river rock, which weighs approximately 1.2 tons. At a 2-inch depth, you would need about 0.8 tons.
Does landscape rock attract snakes or pests?
Rock beds can provide shelter for snakes and insects, particularly in warm climates. Using landscape fabric, keeping beds clear of debris, and avoiding large gaps between stones reduces this risk. Rock is generally no worse than mulch for pest attraction.
Can I put landscape rock over existing mulch?
It is not recommended. The mulch will decompose under the rock, creating an uneven surface and a weed-friendly soil layer. Remove old mulch, prepare a clean base, install landscape fabric, and then add your stone.