Concrete Subgrade Prep: Excavation, Base Material and Compaction

This guide covers every step of concrete slab base preparation: how deep to dig, what base material to use, how to compact it, when you need a vapor barrier, and how to handle problem soils. Use our concrete calculator to figure out how much material you need once your subgrade is ready.

Excavation Depth by Project Type

Your total excavation depth equals the slab thickness plus the gravel base thickness. You also need to remove any organic topsoil—roots, grass, and decomposing material will compress unevenly and ruin your slab.

Project Type	Slab Thickness	Gravel Base	Total Excavation Depth
Patio	4 inches	4 inches	8-10 inches
Sidewalk	4 inches	4 inches	8-10 inches
Shed floor	4 inches	4 inches	8-10 inches
Standard driveway	5-6 inches	4-6 inches	10-12 inches
Heavy-use driveway	6 inches	6 inches	12-14 inches
Garage floor	4-6 inches	4-6 inches	10-12 inches

These are minimum depths assuming you hit stable subsoil immediately. If your topsoil layer extends deeper than expected (common in garden areas or recently graded lots), keep excavating until you reach firm, undisturbed earth. For specifics on slab thickness, see our guides on patio thickness and driveway thickness.

Excavation Best Practices

1. Mark your area. Extend excavation 6-12 inches beyond the slab edges on all sides. Forms need room, and the base should extend past the slab perimeter.
2. Remove all organic material. Topsoil, roots, and grass must go. Organic matter decomposes and compresses, causing settlement.
3. Check for soft spots. Walk the excavated area. If your feet sink in any spot, dig deeper there and fill with compactable gravel.
4. Grade the bottom. The excavated surface should be roughly level (or sloped to match your planned drainage). High and low spots create uneven base thickness.
5. Don't over-excavate. If you dig too deep, fill with compactable gravel rather than putting topsoil back in. Backfilled native soil won't compact properly.

Choosing the Right Base Material

Not all gravel performs equally under concrete. The shape and gradation of the stone determine how well it locks together and resists movement.

Gravel Type Comparison

Material	Compaction	Drainage	Cost	Best Use
3/4" crusher run (road base)	Excellent	Good	$25-40/yard	All-purpose slab base
3/4" crushed stone (clean)	Very good	Excellent	$30-50/yard	High-drainage needs
Recycled concrete aggregate	Good	Good	$15-30/yard	Budget projects
Pea gravel	Poor	Excellent	$30-45/yard	Not recommended for slab base
River rock	Poor	Good	$35-55/yard	Not recommended for slab base
Sand (fill)	Moderate	Poor	$15-25/yard	Not recommended as primary base

Best choice for most projects: 3/4-inch crusher run (also called "road base" or "dense grade aggregate"). It contains a mix of crushed stone and fine particles that interlock tightly when compacted. The angular pieces grip each other; the fines fill the gaps.

What to avoid: Round stones (pea gravel, river rock) look nice in landscaping but are terrible under concrete. The smooth, rounded surfaces slide against each other instead of locking. No amount of compaction will make river rock stable. For the full breakdown on gravel selection, see our gravel under concrete guide.

How Much Gravel Do You Need?

For a 4-inch base, plan on approximately 1.25 cubic yards per 100 square feet. For a 6-inch base, that increases to roughly 1.85 cubic yards per 100 square feet. Order 10-15% extra to account for uneven subgrade and settling during compaction.

Compaction Methods

Compaction is the step most DIYers skip or shortchange. Dumping gravel into your excavation and pouring concrete on top guarantees failure. Uncompacted base material will continue to settle under the weight of the slab, causing cracks, sinking, and uneven surfaces.

Mechanical Compaction (Recommended)

A plate compactor (also called a vibratory compactor) is the right tool for the job. Rental cost runs $75-100 per day at most equipment rental centers.

How to compact properly:

1. Work in lifts. Add 2-3 inches of gravel at a time. Never try to compact more than 3 inches in a single pass.
2. Moisten the material. Lightly dampen each lift before compacting. Dry material doesn't compact as well—but don't soak it. You want damp, not muddy.
3. Make multiple passes. Run the plate compactor over each lift 3-4 times, overlapping your passes by half the plate width.
4. Compact the subgrade first. Before adding any gravel, compact the exposed native soil. This is the foundation for your foundation.
5. Check firmness. After compacting, walk across the surface. If you leave visible footprints, compact more. The surface should feel solid underfoot.

Hand Tamping (Small Areas Only)

A hand tamper (flat metal plate on a handle) works for areas under 25-50 square feet—stepping stones, small pads, post footings. For anything larger, rent a plate compactor. Hand tamping cannot achieve the same density as mechanical compaction, and the physical effort for a full driveway or patio is extreme.

Compaction Testing

Professional projects use a nuclear density gauge or sand cone test to verify 95%+ compaction. For residential DIY work, the footprint test is practical: if you can walk across the compacted base without leaving impressions deeper than 1/8 inch, you have adequate compaction.

Vapor Barrier: When You Need One

A vapor barrier (typically 6-mil or 10-mil polyethylene sheeting) sits between the gravel base and the concrete. Its job is to block ground moisture from migrating upward through the slab.

You need a vapor barrier if:

• The slab will be enclosed (garage, basement, living space)
• You plan to install flooring, carpet, or coatings over the concrete
• Your area has a high water table
• The soil is predominantly clay (holds moisture)

You can skip a vapor barrier if:

• The slab is fully exterior and uncovered (most patios, sidewalks)
• The slab will remain bare and exposed to air circulation
• Drainage is excellent and the water table is well below grade

Installation tip: Overlap sheets by 6-12 inches and tape seams. Extend the barrier up the inside of the forms. Some builders place a thin sand layer over the barrier to protect it from punctures during the pour.

For complete guidance—including material types, perm ratings, and step-by-step installation—see our concrete vapor barrier guide.

Dealing with Problem Soils

Standard base preparation assumes reasonably stable native soil. Several soil types require extra attention.

Clay Soil

Clay is the most common problem soil for concrete work. It expands significantly when wet and shrinks when dry, creating cyclical movement that cracks slabs.

Solutions:

• Excavate 2-4 inches deeper than standard and increase gravel base to 6-8 inches
• Install geotextile fabric (landscape fabric rated for construction) between the clay subgrade and the gravel base to prevent clay from migrating into the gravel
• Consider lime stabilization for large projects (mixing hydrated lime into the top 6 inches of clay to reduce plasticity)
• Add control joints at closer spacing (8-foot intervals instead of 10-12) to manage cracking from soil movement

Expansive Soil

Some soils (often identified as CH or MH on soil classification charts) expand dramatically with moisture changes. If your area has known expansive soil issues (common in Texas, Colorado, parts of the Southeast), a standard gravel base may not be enough.

Solutions:

• Thicker gravel base (8-12 inches) to create a buffer zone
• Moisture barrier below the gravel to limit water reaching the expansive layer
• Structural reinforcement (rebar grid rather than wire mesh)
• Consult a geotechnical engineer for slabs over 400 square feet on expansive soil

Fill Soil

If your site was previously graded and filled with imported soil, that fill may not have been properly compacted. Uncompacted fill is the leading cause of slab settlement.

Solutions:

• Excavate through the fill to reach undisturbed native soil if practical
• If fill is too deep to remove, compact it in lifts with a plate compactor and use a thicker gravel base
• For deep fill (over 2 feet), a soil engineer should evaluate the site

Organic or Peaty Soil

Soil with high organic content (dark, spongy, swamp-adjacent areas) decompresses and compresses unpredictably.

Solutions:

• Remove all organic material entirely—no shortcuts
• Replace with compactable engineered fill (crushed stone or road base)
• This can substantially increase excavation depth and cost, but there is no alternative

Step-by-Step Base Preparation Summary

Here is the full sequence from bare ground to pour-ready subgrade:

1. Lay out the slab. Mark the perimeter with stakes and string, extending 6-12 inches beyond the slab on all sides.
2. Excavate. Dig to the required total depth (slab thickness + base thickness). Remove all topsoil and organic material.
3. Inspect the subgrade. Check for soft spots, roots, and debris. Probe with a rebar stake to find hidden soft areas.
4. Compact the subgrade. Run a plate compactor over the exposed native soil. This step is often skipped and frequently the root cause of failure.
5. Install geotextile (if needed). For clay or expansive soils, lay fabric across the compacted subgrade.
6. Add gravel in lifts. Spread 2-3 inches of crusher run. Dampen lightly. Compact. Repeat until you reach the target base thickness.
7. Grade and check level. The top of the gravel should be flat (or sloped for drainage) and at the correct elevation below your planned finished slab surface.
8. Install vapor barrier (if needed). Lay polyethylene sheeting over the gravel. Overlap and tape seams.
9. Set forms. Install form boards at the correct height, braced every 2-3 feet.
10. Final check. Verify depth from form top to gravel surface matches your planned slab thickness at multiple points.

If this is your first time working with concrete, our first-time concrete pour guide walks through the entire process from start to finish. For budgeting, check the concrete cost per yard guide.

Check Local Building Codes

Requirements vary by jurisdiction. Some municipalities specify minimum base thickness, gravel type, or compaction standards. A few key items that vary:

• Minimum gravel base depth (some codes require 6 inches regardless of project type)
• Compaction density requirements (some jurisdictions require documented compaction testing)
• Vapor barrier requirements (some codes mandate vapor barriers for all enclosed slabs)
• Setback and drainage requirements that affect excavation location

Pull your local building code or call your building department before starting. A failed inspection means tearing up work and redoing it—far more expensive than checking requirements upfront.

Frequently Asked Questions

How deep should I dig for a concrete slab?

According to SlabCalc.co, excavating for a concrete slab requires the full slab thickness plus 4–6 inches of compacted gravel base—meaning 8–10 inches deep for a standard 4-inch patio or 10–12 inches deep for a 6-inch driveway. Excavate deep enough for the slab thickness plus 4-6 inches of gravel base. For a standard 4-inch patio slab, dig 8-10 inches below finished grade. For a 6-inch driveway, dig 10-12 inches. Add extra depth if you need to remove organic topsoil—you want to build on stable subsoil, not decomposable material.

What type of gravel goes under concrete?

Use 3/4-inch crushed stone (often called "crusher run" or "road base") for the main base layer. It compacts well because the angular pieces interlock. Avoid round river rock—it shifts under load and doesn't compact. Top the crushed stone with a 1-2 inch layer of fine screenings or sand for a smooth surface.

Do I need to compact the ground before pouring concrete?

Absolutely. Uncompacted fill or base material will settle unevenly under the weight of the concrete, causing cracks and sinking. Use a plate compactor (available for rent at about $75-100/day) for best results. Hand tampers work for small areas but can't match mechanical compaction. Compact in 2-3 inch lifts, wetting the material slightly.

What do I do if my soil is clay?

Clay soil expands when wet and shrinks when dry, creating movement under your slab. Excavate 2-4 inches deeper than normal and use a thicker gravel base (6-8 inches minimum). Consider installing a geotextile fabric between the clay and gravel to prevent mixing. In severe cases, consult a geotechnical engineer.

Key Takeaways

• Excavation depth = slab thickness + base thickness. Remove all organic material to reach stable subsoil.
• Use 3/4" crusher run (road base) as your base material. Avoid round stone.
• Compact in 2-3 inch lifts with a plate compactor. Do not skip this step.
• Compact the subgrade too—not just the gravel.
• Clay and expansive soils need deeper excavation, thicker base, and geotextile fabric.
• Add a vapor barrier for enclosed slabs or anywhere flooring will be installed.
• Check local building codes. Requirements vary by jurisdiction.

For more guidance on planning your concrete project, browse all our concrete guides.