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What Sub-Base do I need

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What Sub-Base do I need for my project?

17 Sept 24

6 min read

Sub-base aggregates and materials form the foundations of various projects, from small garden paths up to car parks and motorways.

These layers are crucial to the longevity, stability, and overall performance of a structure you're building. So, you need to ensure you pick a material that can withstand traffic demands and allows adequate drainage to prevent erosion.

There are also other environmental factors and various standards to consider, too.

Simply put, choosing the right sub-base for a building project can be challenging.

That's why we put this guide together: to help you ensure that your next commercial, residential, or infrastructure project is built on the most solid foundations.

Here, we explore sub-bases, the different types available and which are most appropriate for various applications.

What is a sub-base?

A sub-base is a layer of material between the ground and the main load-bearing layer of a construction project. Its objective is to provide a stable foundation for the upper layers and ensure they do not shift or crack.

What are the different types of sub-base?

Sub-base courses are usually made from materials like crushed stone, gravel, and recycled concrete and are suitable for various applications.

The main types of sub-base are:

Type 1 MOT

This type of sub-base gets its name from the Ministry of Transport (now known as the Department for Transport (DfT)) specification for road-building materials.

Typically made from crushed hard stone or concrete, MOT type 1 includes particles from 40mm down to dust, which ensures it compacts well and forms a strong foundation.

What is type 1 MOT used for?

MOT type 1 is used for general construction applications, including public footpaths, patios, and roads, and as a base for structures like sheds.

What are the benefits of type 1 MOT?

The mix of coarse and fine particles ensures MOT type 1 can be compacted tightly to create a solid and stable base layer.

The sub-base also offers some permeability, which helps manage water runoff and reduce standing water.

Type 3 MOT

Type 3 MOT is also used in construction projects. Like MOT type 1, it's typically made from crushed stone, gravel, or recycled aggregate.

However, Type 3 MOT is different because it doesn't contain any fine dust particles. In fact, it usually consists of stones between 4mm and 40mm, which makes it suitable for sustainable drainage systems.

What is type 3 MOT used for?

Because of its superior draining properties and coarse nature, MOT type 3 is used for permeable paving in areas where standing water may be a problem, such as sports pitches, car parks, and driveways.

What are the benefits of type 3 MOT?

Like type 1 MOT, type 3 also needs to meet DfT standards to ensure its quality and reliability. Elsewhere, type 3 offers good load-bearing capacity, making it suitable for heavy-duty applications.

Due to the size of the particles, MOT type 3 doesn't compact as well as type 1. However, it is highly permeable and effective at reducing surface water, contributing to adherence to environmental regulations.

How to select the right type of sub-base

When you're planning a construction project, choosing the right type of sub-base is crucial for ensuring stability, durability, and proper performance.

To make the right decision, consider the following factors when deciding which sub-base is most suitable.

1. Understand the project's requirements

Load-bearing needs: Materials like MOT Type 1 with strong compaction properties are ideal for projects with heavy loads (e.g. roads and driveways). A less robust material may suffice for light-duty paths or gardens
Drainage requirements: If the site needs good drainage (e.g., permeable driveways, sports fields, or SuDS projects), use a free-draining material like MOT Type 3. In areas where drainage is less critical, a compact, dust-including material like MOT Type 1 works well
Environmental considerations: Projects in wet or flood-prone areas may require materials that help manage water runoff effectively. To minimise your environmental impact and contribute to sustainability goals, use recycled materials where possible
Cost and availability: Evaluate the budget and availability of materials in your area. Recycled materials may offer cost savings while being environmentally friendly

2. Evaluate site conditions

Soil type: If the soil is soft or clay-heavy, you may need a sub-base material that increases support, such as larger aggregates. Alternatively, you could investigate whether installing membranes under the sub-base could be more appropriate. For stable soils, standard materials like type 1 MOT may suffice
Terrain and topography: A well-compacted subbase is necessary on sloped areas to prevent sliding or erosion. In flatter spaces, ensure the subbase facilitates proper drainage to avoid water pooling

3. Check the material's properties

Grading and compaction: Materials like MOT Type 1 compact tightly due to fine particles, creating a strong base. Coarser materials, e.g. MOT Type 3, are less compact but provide excellent drainage
Durability: Highly durable materials like crushed limestone or granite are preferred for long-lasting projects

4. Adhere to the relevant standards

Follow building codes and regulations: Some projects will require you to use a specific type and quality of sub-base material. MOT Type 1 and Type 3 are commonly used because they comply with Department for Transport (DfT) standards

5. Seek expert advice

If you're unsure about anything, contact our product experts. We've been quarrying stone in the UK for more than a century, and our knowledgeable team is experienced in supporting projects of all sizes from concept to completion.

How to calculate the amount of sub-base material needed

It can be easy to over- or under-order sub-base materials, and this can have a knock-on effect on the workflow. Order too little and there can be delays while additional products are sourced. Order too much, and you risk wasting money.

To calculate the amount you'll need for your next project, use these steps:

Measure: find the length and width of the area in metres and multiply them to get the total area in square metres.
Decide on the depth: Choose an appropriate sub-base depth based on the project's usage. As a general rule of thumb, the following depths apply to various applications:

Patios, garden paths: 75-100mm
Driveways, public footpaths: 100-150mm
Heavy uses: 150-225mm
Highways: 150mm

Calculate the volume: Multiply the area in square metres by the chosen depth - this will give you the volume in cubic metres.
Account for compression: Multiply the volume by 1.3 to account for the compacting process.
Add wastage allowance: Make sure you've enough by multiplying by 1.1 to add a 10% wastage allowance.
Convert to weight: Lastly, multiply the final volume by 1.6 to convert cubic metres to tonnes.

Sub-base material calculation example

For a 5m x 5m patio with a 100mm sub-base depth, you would calculate the sub-base like this:

Area: 5m × 5m = 25 sq m
Volume: 25 sq m × 0.1m = 2.5 cubic metres
Compression: 1.2 × 1.3 = 3.25 cubic metres
Wastage: 3.25 × 1.1 = 3.575 cubic metres
Weight: 3.575 × 1.6 = 5.72 tonnes

You would need approximately 5.72 tonnes of sub-base material for this patio project.

Marshalls' aggregates

If you're in the planning stages of your next construction project, trust Marshalls' aggregates to provide a strong, durable and long-lasting foundation.

Our leading range of aggregates includes crushed stone rock and gravel, grit sand, block stone and building sand from local British quarries and three sites in Yorkshire and Lancashire.

Explore our range of aggregates now, including: