Do You Need Gravel Under Concrete? (When and Why)
**Yes, in most cases.** A 4-6 inch layer of compacted gravel under concrete provides essential drainage, prevents settling, and significantly extends the lifespan of your slab. Skipping the gravel base is one of the most common shortcuts that leads to cracked, uneven, or failed concrete.
Do You Need Gravel Under Concrete? (When and Why)
Yes, in most cases. A 4-6 inch layer of compacted gravel under concrete provides essential drainage, prevents settling, and significantly extends the lifespan of your slab. Skipping the gravel base is one of the most common shortcuts that leads to cracked, uneven, or failed concrete.
The gravel base isn't just a recommendation—it's a critical part of a properly installed concrete slab. Here's why it matters and when you might be able to skip it.
Why Gravel Under Concrete Matters
A proper gravel base serves three essential functions:
1. Drainage
Water is concrete's enemy over time. When water collects beneath a slab and freezes (in cold climates), it expands and pushes against the concrete from below. This freeze-thaw cycle causes heaving, cracking, and eventual failure.
Gravel allows water to drain away from the underside of the concrete rather than pooling beneath it. Even in warm climates, proper drainage prevents moisture-related problems like efflorescence and subgrade erosion.
2. Uniform Support
Concrete needs consistent support across its entire surface. Native soil is rarely uniform—you might have soft spots, clay pockets, or areas with different densities. These inconsistencies cause differential settling, which leads to cracking.
A compacted gravel base creates a uniform surface that distributes the concrete's weight evenly. The gravel acts as a buffer between variable native soil and your slab.
3. Compaction Stability
Loose soil compresses over time under weight. If you pour concrete directly on uncompacted ground, the soil beneath will continue to settle—taking your concrete with it.
Gravel compacts into a stable base that won't compress further. Properly compacted gravel gives your concrete a foundation that won't move.
Recommended Gravel Depth
| Project Type | Minimum Base | Recommended Base |
|---|---|---|
| Patios | 3 inches | 4 inches |
| Sidewalks | 3 inches | 4 inches |
| Driveways | 4 inches | 6 inches |
| Garage floors | 4 inches | 4-6 inches |
| Shed floors | 3 inches | 4 inches |
| Heavy-use areas | 4 inches | 6+ inches |
For most residential projects, 4 inches of compacted gravel is the standard. Driveways and areas with vehicle traffic or heavy loads benefit from a full 6 inches.
For complete thickness guidance including concrete depth, see our guide on how thick concrete should be.
Type of Gravel to Use
Not all gravel works equally well as a concrete base. You want material that compacts firmly and allows drainage:
Best options:
- 3/4-inch crushed stone – Angular pieces lock together when compacted
- Road base (crusher run) – Mix of crushed stone and fines that compacts densely
- Recycled concrete aggregate – Crushed concrete works well and may cost less
What to avoid:
- Pea gravel – Round stones don't lock together and shift under load
- River rock – Same problem as pea gravel; won't stay compacted
- Large stones (2"+) – Too much space between stones; don't compact well
The key is angular material that interlocks when compacted. Round or smooth stones roll against each other and won't provide stable support.
Compaction Is Critical
Dumping gravel and pouring concrete on top isn't enough. The gravel must be compacted to prevent settling.
Proper compaction requires:
- Adding gravel in 2-inch lifts (layers)
- Compacting each lift with a plate compactor or hand tamper
- Achieving 95%+ compaction density
- Moistening the gravel slightly to aid compaction
Uncompacted gravel will settle over months and years, taking your concrete with it. This is where cheap contractors cut corners—and why their work fails prematurely.
Equipment needed: A plate compactor ($50-100/day rental) is essential for any project over 50 square feet. Hand tamping works for very small areas but is exhausting and less effective.
When You Might Skip Gravel
In rare situations, a gravel base isn't necessary:
Existing concrete subbase. If you're pouring a new slab over old concrete (an overlay), the old concrete serves as your base. Make sure it's clean, stable, and properly bonded.
Sandy, well-draining native soil. In some regions, native sandy soil drains well and provides stable support naturally. This is uncommon but does exist. Consult local contractors about soil conditions in your area.
Very small, non-structural projects. Individual stepping stones or decorative pavers set in soil may not need a formal base—but they'll shift more over time.
Important: Even when gravel might be optional, it's rarely a bad idea. The cost is minimal compared to the total project, and the benefits are significant.
What Happens Without Proper Base
Skipping the gravel base—or not compacting it properly—leads to predictable problems:
Settling. The slab sinks unevenly as underlying soil compresses, creating trip hazards and pooling water.
Cracking. Differential settling puts stress on the concrete, causing cracks that grow over time.
Heaving. In cold climates, water trapped beneath the slab freezes and pushes the concrete upward.
Shorter lifespan. A slab that should last 30 years might fail in 10-15 without proper base preparation.
The cost to remove a failed slab and pour a new one far exceeds the cost of doing the gravel base right the first time. For budgeting purposes, see our guide on concrete cost per square foot.
How Much Gravel Do You Need?
For a 4-inch gravel base, you'll need approximately 1.25 cubic yards per 100 square feet. Gravel is sold by the cubic yard or ton:
| Project Size | Gravel Needed (4" base) |
|---|---|
| 100 sqft | 1.25 cubic yards |
| 200 sqft | 2.5 cubic yards |
| 400 sqft | 5 cubic yards |
Delivered gravel typically costs $25-50 per cubic yard plus delivery fees.
Use our concrete slab calculator to determine your concrete needs, then plan for gravel accordingly.
Frequently Asked Questions
What type of gravel goes under concrete?
Use 3/4-inch crushed stone or road base (crusher run). The angular, irregular shapes interlock when compacted, creating a stable base. Avoid round stones like pea gravel or river rock—they don't compact well and will shift under load.
How thick should gravel be under a slab?
Standard recommendation is 4 inches for patios and sidewalks, 4-6 inches for driveways. The gravel should be compacted in 2-inch lifts using a plate compactor to achieve proper density.
Can you pour concrete directly on dirt?
You can, but you shouldn't. Concrete poured directly on soil is prone to cracking, settling, and moisture problems. The soil may seem firm but will compress over time, causing the slab to sink unevenly. Proper gravel base costs little but adds years to your slab's life.
Do I need to compact the gravel?
Yes, compaction is essential. Uncompacted gravel will continue to settle after you pour, causing your concrete to crack and become uneven. Rent a plate compactor ($50-100/day) and compact the gravel in layers for best results.
Key Takeaways
- Yes, you need gravel under concrete for most projects
- Use 4-6 inches of compacted 3/4" crushed stone or road base
- Compaction is critical—rent a plate compactor
- Skipping gravel leads to settling, cracking, and premature failure
- The cost is minimal compared to fixing a failed slab
For more foundation guidance, see our complete concrete guide or browse all concrete guides.