When pouring a concrete slab—whether for a driveway, patio, or building foundation—one of the most common questions we hear is: “Do I really need crushed rock underneath?” The short answer is yes—and for good reason. Properly preparing the base beneath your concrete is essential to achieving a strong, long-lasting slab.

Why Crushed Rock is Essential

Concrete is only as stable as the ground beneath it. Placing it directly on soil often leads to shifting, settling, or cracking over time due to moisture changes and soil compaction issues. Crushed rock (also called crushed stone or gravel) solves these problems by:

• Providing a Stable Base – Compacted crushed rock creates a solid foundation that evenly supports the concrete slab.

• Improving Drainage – The rock layer allows water to pass through instead of pooling under the slab, reducing the risk of heaving and cracking.

• Preventing Soil Movement – A properly compacted base helps lock the underlying soil in place, minimizing shifts caused by freeze-thaw cycles or erosion.

• Enhancing Longevity – With proper support and drainage, concrete lasts longer and is less likely to require costly repairs.

Recommended Thickness

For most residential concrete projects (such as driveways and patios), contractors typically recommend:

• 4–6 inches of compacted crushed rock for standard slabs

• 6–8 inches for heavy loads (e.g., garage floors or areas supporting vehicles)

The exact thickness can vary based on soil conditions, climate, and slab design.

Choosing the Right Crushed Rock

The ideal material for the sub-base is well-graded crushed stone (commonly ¾-inch minus) that compacts tightly while still allowing for drainage. CrusherWorks offers a range of crushing and screening equipment to produce high-quality aggregates that meet these specifications for contractors and material suppliers.

Bottom Line

While it may seem like an extra step, using crushed rock under concrete is critical for structural integrity and long-term performance. It ensures proper drainage, prevents uneven settling, and provides a strong foundation for your project.

Looking to produce your own high-quality crushed rock for concrete preparation? CrusherWorks offers leading crushers and screens to help you process raw material into the perfect base aggregate. Contact our team today to learn more!

If you’re in the market for already-crushed rock material, or on-site/remote C&D waste recycling services, reach out to our partners at Recovered Construction Materials (RCM) Alabama, also located in the Birmingham, AL area.