Can You Pour Concrete Over Grass?

Can You Pour Concrete Over Grass? Why It Always Fails

Absolutely not. Pouring concrete over grass or sod is one of the most common mistakes that leads to complete failure. The organic matter (grass, roots, soil) will rot and decompose under the concrete, creating voids that cause the slab to collapse. This guide explains exactly why it fails and what you must do instead.

Why Organic Matter Causes Failure

Grass, roots, and organic soil create catastrophic problems under concrete:

The Decomposition Process

What happens:

1. Grass and roots die when covered by concrete
2. Organic matter begins decomposing
3. Decomposition creates empty spaces (voids)
4. Concrete has no support in these void areas
5. Slab collapses into the voids

Timeline: This process takes 6 months to 2 years, but once it starts, failure is inevitable.

Why It's Catastrophic

• Voids form: Decomposition creates empty spaces
• No support: Concrete has nothing to sit on
• Settling occurs: Slab sinks into voids
• Cracks form: Uneven support causes cracking
• Complete failure: Eventually, sections collapse

The Correct Process: Remove Sod First

You must remove all vegetation and organic matter before pouring concrete.

Step 1: Remove Sod Completely

1. Cut sod: Use a sod cutter or shovel to remove the grass layer
2. Remove all roots: Dig out root systems completely
3. Check depth: Roots can extend 6-12 inches deep
4. Remove all organic matter: Clear leaves, debris, and organic material

Step 2: Remove Topsoil Layer

Topsoil contains organic matter and must be removed:

1. Identify topsoil: Dark, organic-rich soil layer
2. Remove completely: Excavate until you reach stable subsoil
3. Check for organic matter: Ensure no dark, organic soil remains

Step 3: Reach Stable Subsoil

Continue excavating until you reach:

• Stable, compacted soil: Light-colored, mineral soil
• No organic matter: No roots, no dark soil
• Proper depth: Account for gravel base and concrete thickness

Step 4: Install Proper Base

1. Compact subsoil: Compact the stable soil you've reached
2. Add gravel base: Install 4-6 inches of compacted gravel
3. Compact thoroughly: Use plate compactor for proper compaction
4. Level surface: Create level, stable base for concrete

How Deep to Excavate

Total excavation depth depends on your project:

For 4-Inch Slab

• Remove sod and topsoil (typically 4-8 inches)
• Reach stable subsoil
• Add 4-6 inches gravel base
• Concrete: 4 inches
• Total depth from original surface: typically 12-18 inches

For Driveways

• Remove all vegetation and organic matter
• Excavate to stable subsoil
• Add 4-6 inches compacted gravel
• Concrete: 4-6 inches
• Total depth: typically 12-20 inches from original surface

What Happens If You Skip This

The failure process when pouring over grass:

Months 1-6

• Concrete looks fine
• No visible problems
• Grass and roots begin dying

Months 6-12

• Decomposition accelerates
• Small voids begin forming
• First cracks may appear
• Slight settling noticeable

Year 1-2

• Significant voids form
• Major cracking occurs
• Uneven settling visible
• Structural problems developing

Year 2+

• Complete failure in affected areas
• Slab collapses into voids
• Requires complete removal and replacement
• Costs 2-3x more than doing it right initially

Common Mistakes

Mistake 1: "I'll just remove the sod"

• Removing sod isn't enough—roots extend deeper
• Topsoil layer still contains organic matter
• Must excavate to stable subsoil

Mistake 2: "I'll use herbicide to kill the grass"

• Dead grass still decomposes
• Roots still create voids
• Must physically remove all organic matter

Mistake 3: "I'll just add more concrete"

• More concrete doesn't solve the problem
• Voids still form underneath
• Thicker slab still fails

The Bottom Line

Absolutely not—you cannot pour concrete over grass. The organic matter will rot, creating voids that cause the slab to collapse. You must remove sod first, then continue excavating to remove all roots, topsoil, and organic matter until you reach stable subsoil. Then install a proper gravel base. This is the only way to ensure your concrete has a stable foundation that won't fail.

Don't take shortcuts. The time and cost of proper excavation is far less than replacing failed concrete. Proper preparation is the foundation of long-lasting concrete—literally.

Need help with proper site preparation? Contact Local Concrete Contractor. We ensure proper excavation and base preparation for concrete that lasts decades, not years.