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Gravel Base for Concrete Slab: Best Types, Installation Tips, and What to Avoid

A proper gravel base for a concrete slab is one of the most critical components in any slab installation. It supports the concrete by distributing weight, improving drainage, and preventing long-term damage such as cracking or sinking. Without the right base, slabs are more likely to shift, settle unevenly, or fail prematurely. Choosing the right material and installing it correctly ensures the slab stays strong, stable, and low-maintenance for years.

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What Gravel Does Beneath a Slab

Gravel serves as the structural buffer between the ground and the concrete. Its primary role is to distribute loads evenly, reducing pressure points that could cause the slab to crack or sink. Additionally, it improves drainage by creating a porous layer that allows water to move away from the concrete. This helps to prevent moisture buildup that can lead to erosion or freeze-thaw damage. In colder climates, gravel also minimizes frost heave by limiting the amount of moisture that remains in the soil beneath the slab. Without this layer, even a well-poured concrete slab can shift, settle, or break down over time.

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Best Gravel Types for Under a Concrete Slab

Crushed Stone (Top Choice)
Crushed stone is the most widely recommended material for slab bases. It’s produced by mechanically breaking quarried rock (such as limestone or granite) into angular fragments that compact tightly and provide excellent load-bearing support. The angular edges interlock when compacted, forming a dense and stable subbase. Standard practice utilizes ¾-inch crushed stone (sometimes labeled as ASTM #57), which strikes a balance between compaction and drainage. For these reasons, it’s the best choice for most slab installations.

Recycled Concrete Aggregate (RCA)
RCA is made by crushing reclaimed concrete. It can serve as a reliable alternative to virgin crushed stone if the material is clean and well-graded. RCA compacts well and drains similarly to traditional crushed stone. However, it may contain small traces of old reinforcement (rebar) or fine particles. Therefore, it must be sourced from a reputable supplier and used only in non-structural or light-duty slabs—unless approved by an engineer.

Dense-Graded Base (DGB) / Crusher Run
Also known as crusher run or road base, this material includes a mix of crushed stone and stone dust. It compacts extremely well, forming a dense, stable layer. As a result, it’s often used for heavily loaded slabs, industrial pads, and roadways. However, the fine particles can retain moisture, so proper drainage and compaction are crucial to performance.

⚠️ Avoid rounded materials like pea gravel or river rock. They don’t compact well and can lead to slab failure over time.

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Key Gravel Selection Factors

Gravel Size
For concrete slabs, the ideal gravel size is typically ¾ inch (ASTM #57), as it offers a good balance of drainage and compaction. On the other hand, gravel that’s too small or fine—like screenings or stone dust—should be avoided unless it’s part of a properly compacted dense-graded base. Oversized stone can also leave voids and compromise stability.

Compaction
The gravel must be compactable to create a solid, uniform base. Angular materials, such as crushed stone or crusher run, interlock under pressure and compact more effectively than rounded or loose-fill materials. In contrast, poor compaction leads to slab settlement and cracking.

Drainage
Good drainage is essential to prevent water accumulation beneath the slab. Excess water can cause frost heave or erosion. Therefore, choose gravel that promotes vertical drainage, and ensure the subbase is graded to direct water away from the slab.

Cost and Availability
Crushed stone is widely available and affordable in most regions. Recycled concrete aggregate may be cheaper and more sustainable. However, it should only be used when it meets quality standards. Always factor in transportation costs when sourcing material to stay within budget. Here is another article on best practices for slab foundations.

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Ready to Build with Confidence?

Choosing the right gravel base for your concrete slab is one of the most important steps in achieving long-lasting structural performance. If you’re unsure about the type of base material suitable for your project, we can help. Use our free gravel calculator or request a quote, and we’ll connect you with the right material for your job—fast, reliable, and delivered directly to your site.