Do You Need a Sub-Base for Concrete? A Complete Guide

If you're planning a concrete project, one of the most important steps is preparing the ground properly. A common question we're asked is whether a sub-base is actually needed before pouring concrete.

The short answer is yes, a sub-base is essential.

Whether you're installing a shed base, driveway, patio, foundations or concrete slab, a properly prepared sub-base helps ensure the concrete remains stable and long-lasting.

Across East Sussex and South Kent, from Battle and Hastings to Eastbourne, Haywards Heath, Robertsbridge, Rye, Tunbridge Wells and Paddock Wood, we regularly supply concrete for projects where proper ground preparation makes all the difference.

The Quick Answer

In most cases, yes, you do need a sub-base for concrete.

A sub-base is important for several reasons:

• it provides a stable for foundation

• it improves drainage

• it prevents movement and sinking

• it helps distribute weight of the structure above, evenly

Skipping this step can lead to cracks, uneven surfaces and long-term structural issues.

What Is a Sub-Base?

A sub-base is a layer of compacted material placed underneath concrete before it is poured. The most common material used in the UK is called MOT Type 1 hardcore.

This is a mix of crushed stone and aggregate that compacts down for form a solid, stable base.

Why a Sub-Base is Important

Concrete is strong, but it relies on the ground beneath it for support. Without a proper base, even well-poured concrete can fail over time.

Prevents Movement: A sub-base reduces the risk of the ground shifting, which can cause cracking.

Improves Drainage: It allows water to drain away from the concrete, reducing the risk of water damage.

Supports Weight: It helps distribute loads evenly, which is especially important for driveways, foundations and slabs.

Extends Lifespan: Proper preparation ensures the concrete lasts longer and performs better.

When Do You Need a Sub-Base? Are There Any Exceptions?

In most concrete projects, a sub-base is required to ensure stability, drainage and long-term performance.

Shed Bases: Even small shed bases benefit from a compacted sub-base to prevent sinking or movement over time.

Driveways: A strong sub-base is essential for handling vehicle weight and reducing the risk of cracking.

Patios: Helps maintain a level surface and prevents movement caused by ground conditions or weather.

Foundations: For house extensions, garages and structural work, a sub-base is critical to provide a stable and load-bearing foundation.

Concrete Slabs: For larger slabs such as basements, agricultural yards, workshops or commercial flooring, a well-prepared sub-base is essential. These types of projects often carry significant loads, so proper ground preparation helps prevent settlement, cracking and long-term structural issues.

Any Exceptions?

In rare cases, a sub-base may not be strictly necessary. for example in areas with solid, well compacted natural ground an argumnet could be made.

However even in these situtaions, installing a sub-base is usually recommended as a precaution to ensure long-term durability and prevent issues in the future.

How to Prepare a Sub-Base for Concrete

Preparing the ground properly is just as important as the concrete itself.

1. Excavate the area: Remove soil to the required depth, allowing space for both the sub-base and the concrete.

2. Install the hardcore: Lay a layer of MOT Type 1 across the area

3. Compact the material: use a compactor to compress the sub-base into a solid and level surface.

4. Check levels: Ensure the surface is even before pouring concrete.

How Thick Should a Sub-Base Be?

The required thickness depends on the project.

Typical guidslines are:

• Shed bases / patios → 75–100mm

• Driveways → 100–150mm

• Foundations / heavier loads → 150mm+

The heavier the load, the thicker and more compacted the sub-base should be.

Common Mistakes to Avoid

Skipping or poorly preparing a sub-base can cause problems later.

Some of the most common mistakes you should avoid include:

Having no sub-base at all: Concrete may crack or sink over time

Poor compaction: Loose material can shift under weight.

Incorrect depth: A base that is too thin may not provide enough support.

Uneven Surface: This can lead to uneven concrete and structural issues.

Before ordering any concrete, it's important to undertstand how much you need. You can use our concrete calculator, which allows you to meaure in metric and imperial units. This will help ensure you order the correct volume for your project.

Frequently Asked Questions About Sub-Bases

Do I need hardcore under concrete?

Yes, in most cases hardcore such as MOT Type 1 is used as a sub-base to provide stability and drainage.

What happens if you pour concrete without a sub-base?

The concrete may crack, sink or become uneven over time due to movement in the ground.

How deep should a sub-base be?

It depends on the project, but typically ranges from 75mm to 150mm or more.

Need Concrete for Your Project?

If you're planning a project in East Sussex or South Kent, William the Concreter supplies high-quality volumetric concrete for all types of work.

From shed bases and patios to foundations and commercial work, we can help ensure you get the right concrete for your project.

Contact us today!