What is sub-base material?

Essentially, the sub-base sits between the natural ground surface and the main load-bearing layer of a construction project. The purpose of the sub-base material is to provide a stable platform for everything above. Get it wrong, and things could move or crack.

Consider the requirements and usage of your structure and opt for a sub-base that will withstand the footfall and weight demands, while allowing adequate drainage.

What types of sub-base are available?

Traditionally made from crushed stone or gravel, sub-base material is produced to standards set by the Department of Transport (DOT), formerly the Ministry of Transport (MOT). Aggregates sold displaying the DOT/MOT acronyms are approved for use in public roads, known as specification for highway works (SHW).

Products classed as Type 1 or Type 3 must comply with the DOT clauses SHW 803 or SHW 805.

Please note that while SHW replaced the MOT specification, most people refer to MOT when ordering sub-base material, and nobody really uses the Department for Transport (DFT).

Type 1 MOT

Made from crushed hard stone or concrete, Type 1 includes particles ranging from 40mm down to dust, meaning it compacts down into a strong foundation. The high dust (fines) content allows Type 1 to interlock extremely tightly, creating a stable platform but restricting water flow.

Ideal for high traffic areas, such as highways, car parks and building bases, as it features excellent load-bearing qualities.

Type 3 MOT

Like Type 1, Type 3 is used in construction projects and is made from crushed stone or gravel. However, Type 3 consists of stones between 4mm and 40mm and doesn't contain fine dust particles. The tiny air gaps make it suitable for sustainable drainage systems.

The superior drainage properties and coarse nature of Type 3 make it perfect for permeable paving as it prevents surface water (e.g. sports pitches, car parks, driveways, etc.). These qualities also make Type 3 popular in projects with strict environmental regulations.

DOT Type 1 for Gritstone

Produced to the same SHW clause 803 as Type 1, gritstone is often sourced from local quarries and is a popular alternative to limestone or granite, offering a reliable and readily available sub-base material.

How to select the right sub-base material?

To ensure the stability and long-term performance of your structure, consider these key project requirements:

Load-bearing: MOT Type 1, with its strong compaction, is designed to withstand heavy loads (e.g. highways and driveways). A less robust material should suffice for gardens.

Drainage: Permeable driveways or sports fields require a free-draining material like MOT Type 3.

Environment: Projects in wet or flood-risk areas may need sub-base materials that manage water drainage more effectively.

Cost and availability: Your budget and local availability of materials can affect your choice. Consider using a lower grade material if your project doesn’t require SHW CL803/805 specification.

Soil type: Soft, clay-heavy ground may require a firmer sub-base material like MOT Type 1. Alternatively, you could look into installing membranes under the sub-base layer.

Terrain: Use a well-compacted sub-base on sloped land to prevent sliding or erosion. For level ground, allow drainage to avoid standing water.

Durability: For long-lasting performance, look for stone materials like crushed limestone or granite.

Industry standards: Follow building and environmental regulations, as some projects may require a specific sub-base material. 

Calculating the amount of sub-base material required

Even experienced builders can over- or under-order sub-base materials, impacting both project costs and timescales. Follow these steps to calculate the right amount:

Measure: Record the length and width of the area in metres and multiply them to get the total area in square metres.

Depth: Choose a sub-base depth based on usage. Here are some typical guidelines:

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

Volume: Multiply the area in square metres by the chosen depth to calculate the volume in cubic metres.

Compression: Now multiply the volume by 1.3 to account for compaction.

Wastage: Ensure you have enough material by multiplying by 1.1 (10% allowance).

Weight conversion: Finally, 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.

Contact the experts at Marshalls’ aggregates

Marshalls have 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.

You can count on Marshalls' aggregates for supplying a top-quality, durable foundation. Our leading range of aggregates includes crushed stone rock and gravel, as well as grit sand, block stone and building sand from local British quarries and three sites in Yorkshire and Lancashire.

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