🏗️ What Is a Subgrade Base — and Why It’s Critical for Concrete Slabs

When building any concrete slab — whether it’s a driveway, patio, or shed base — the quality of what’s underneath matters just as much as the concrete itself. That layer beneath the slab is called the subgrade base, and it’s the foundation that determines how long your concrete will last.

At K & Co Contracting, we prepare and compact every subgrade properly before a single load of concrete is poured — because we know a slab is only as strong as its base.

💡 What Is a Subgrade Base for Concrete?

The subgrade base is the compacted layer of natural soil or imported material (like road base or crusher dust) that supports the concrete slab.
It acts as the foundation that distributes weight evenly, prevents movement, and provides a smooth, stable platform for the pour.

⚙️ Why the Subgrade Base Matters

A poorly prepared subgrade is one of the most common causes of:

• Uneven settling or cracking
• Water pooling under the slab
• Loss of strength or surface integrity over time

By properly preparing and compacting the subgrade, you get a durable, level, and long-lasting slab that resists cracking — even under heavy loads like vehicles or sheds.

🪨 Materials Used for a Concrete Subgrade Base

Common subgrade materials include:

• Crushed rock or road base – excellent for driveways and large slabs due to strength and drainage.
• Crusher dust – often used under smaller slabs or pavers for a smooth, compact finish.
• Gravel or sand blends – used depending on soil type and drainage requirements.

All materials are laid in layers (known as “lifts”) and compacted using rollers or plate compactors for maximum density.

🔨 How a Professional Contractor Prepares the Subgrade

At K & Co Contracting, our subgrade preparation process typically involves:

1. Excavating to the correct depth and removing organic materials, debris, or soft spots.
2. Levelling and trimming the base to the designed falls for drainage.
3. Adding road base or crusher dust where needed for stability.
4. Compacting the base layer using mechanical rollers for a firm, even surface.
5. Final checking before formwork and reinforcement installation.

This ensures a perfectly prepared foundation before the concrete pour begins.

📏 How Thick Should the Subgrade Be?

Your concreter or engineer will determine the right thickness based on soil conditions, load requirements, and drainage.

🚫 What Happens If the Subgrade Isn’t Prepared Properly?

Skipping proper preparation can cause:

• Cracking or sinking slabs
• Drainage problems or ponding
• Trip hazards and uneven surfaces
• Reduced structural lifespan

Investing in correct subgrade prep saves costly repairs later — especially in Queensland’s reactive soils and high-rainfall areas.

🧰 Can You Prepare the Subgrade Yourself?

For small DIY projects, basic preparation may be possible, but for anything structural or load-bearing, professional preparation is essential.
Our team uses the correct equipment, and levelling techniques to ensure your concrete is built on a base that lasts decades.

🕒 How Long Does a Subgrade Base Last?

With quality compaction and good drainage, a subgrade base can last indefinitely — often far beyond the life of the first slab poured on it.

✅ Key Takeaway

A strong slab starts from the ground up. Proper subgrade preparation ensures your concrete:

• Stays level and crack-free
• Handles heavy loads
• Drains correctly
• Lasts for many years

If you’re planning a concrete driveway, shed slab, or patio in North Brisbane or Moreton Bay, contact K & Co Contracting for expert preparation and concreting that’s done right from the ground up.

📞 Call 0451 917 497

‍