Last Modified: February 14, 2024

What is Sub Base

the foundations of any successful paving

Sub base close up

By Ben Parrot - Landscape Gardener & Amateur Geologist

Overview

Whether for a public pavement or DIY driveway, foundations serve three purposes: they provide essential drainage while offering a stable base for the top layer. It prevents the surface materials from sinking or popping up due to weather changes. In colder climates, the foundation expands and contracts as the soil freezes and thaws. This constant movement of the foundation can lead to significant structural damage if not addressed properly, which is why a sub base is a crucial aspect of engineering and domestic construction processes. Proper preparation is always key to the installation of any landscaping feature's longevity and functionality.

In this article we bring you all you need to know about the different types of sub-base products, provide you with information about some of the issues when installing a sub base as well as complimentary materials required to successfully lay subbase in your garden. Let's take your project to the next level!

Garden gnome with plans for a subbase.

Key Takeaways

  • Foundations: Find out why a sub-base is the backbone of any paving project, from stability to load distribution.
  • Materials: Learn what materials you need build a foundation for the prevention of sinking and rut formation.
  • Drainage: Effective water drainage under construction projects reduces the risk of water damage and flooding.
  • Installation Basics: Find the basic steps for a well-constructed sub-base.
Commercial compactor on a gravel driveway. Jpg

What is a Sub-Base?

Subbase, also called subgrade, is a compacted layer of aggregate material which is laid beneath landscaping features to spread the load of the surface materials and any traffic upon it to the subgrade beneath. A proper sub-base also aids drainage and prevents undesirable settlement and subgrade rutting. It does this by distributing the weight, which prevents sinking, cracking and other types of wear and tear. 

Whether block paving slabs, gravel or asphalt on a driveway, patio or pathway, a sub-base forms the backbone of any paving project by providing support that minimises movement and the pressures of freeze-thaw cycles, significantly extending the life of the installation.

What is Subgrade Rutting

Loads are distributed through the paving structure and push down on the sub-base, usually causing subgrade rutting in the wheelpaths. This is because the top layer's structure flexes downward to accommodate the depressed foundation layer.

Sub base rutting from poor installation

When to Install a Sub-Base

Generally speaking, a sub-base foundation is not necessary for most aesthetic features built with decorative aggregates, such as ground covers or gravel gardens. However, when creating any project where traffic will impact the surface, from roads and patios to pathways and driveways, it is necessary to build a proper foundation regardless of the climate or type of material used. Without one, the traffic will quickly cause the surface to distort and deteriorate.

Understanding Construction

In this section, we cover the stages of construction when building a sub-base, which applies to all types of paving, from slate pavers or setts to pebbles, cobbles and artifical grass. If you are looking at using gravel as your surface, check out the entire range available in the UK here.

Layer Upon Layer Upon Layer..

The best type of sub-base is composed of three layers: fine gravel at the bottom, hardcore (also known as crushed rock) in the middle, and the chosen surface material on top. However, they can also be constructed from specialised blends which contain varying grades of crushed aggregates, from 60mm down to dust, such as MOT type 1 sub-base or MOT type 2. Compacting these blends is the same as establishing a fine gravel beneath a layer of larger stones, but it is more convenient and easy to install. We cover them below.

Woven Geotextile Membrane

Placing a heavy duty weed membrane on top of the sub-base layer is an essential step that is often missed to the detriment of the project. It acts as a filter that stops the movement downward of soil and gravel dust into the base, which gradually reduces its drainage ability. The fabric also helps to keep the top layer separated from the base, providing useful structure while also serving as an effective barrier that prevents the growth of weeds.

You can also add a 25mm (1-inch) layer of fine-washed sand on top of the fabric to prevent angular, crushed gravel from damaging it. They compact further and reduce surface gravel migration.

Geotextile weed membrane showing through a cracked bonded resin path

Edging

The type of edging you choose can help in keeping the geotextile fabric secure. However, if a hard edging is not desirable, such as when the gravel borders a perennial bed, you need to keep in mind that the gravel will migrate and will need more regular top-ups. To keep the fabric in place, knock in some landscaping spikes before topping it. Find out more in Edging Garden Borders.

Sub-base Installation Tips

Depth

In climates where the soil seldom freezes or only freezes 100mm (4 inches) down, the foundation need not be as deep. This means that if you are using a 50mm (2 inches) thick top layer, you will need to excavate only 150mm (6 inches) deep - 100mm (4 inches) for the sub-base and 50mm (2 inches) for the top layer of paving or gravels.

For climates where soil freezes more than 150mm (6 inches) deep, the foundation needs to be 300mm (12 inches) below the surface, which will generally require a thicker layer of subgrade and topping materials.

Layers and Compaction

When establishing the base, each layer of the sub-base material should be no thicker than 150mm (6 inches) before thoroughly compacting in order to minimise the risk of settlement.

Layers of a sub base and tarmac driveway

Check Levels and Compaction

It is important to make sure that each layer of the compacted sub-base does not deviate from the correct level by more than 10mm at any point. This helps to achieve uniform thickness for subsequent layers.

Drainage

A properly constructed, permeable surface, like a gravel driveway, will generally improve most drainage problems rather than cause any harm because water will simply pass through the gaps and into the sub-base level below. However, if you are building in a place that is prone to flooding, it might be a good idea to create a soakaway or French drain at the lowest point. Essentially, a soakaway is a hole filled with gravel where water can accumulate and slowly spread out.

You can also lay perforated drainage pipes into gravel bedding under the feature in places where water is likely to run. It's very cost-effective and important to install this drainage when you are excavating the bed ready for laying the foundation materials. Otherwise, you will need to destroy the sub-base and start over.

Poor sub base and weed membrane can lead to flooded gravel garden. Jpg

The Materials of a Foundation

In this section, we cover the materials required for building a solid foundation, such as the different types of aggregate products, ranging from MOT type 1 through to CBM and HBM. We also cover the other components which are vital to a sturdy and long-lasting floor, including edging and membranes.

One of the most common sub-base materials is crushed granite stone, often referred to as grano dust or 4-6mm dust. This type of base aggregate is well-suited to the laying of astroturf or as an artificial grass base. The following  list of materials covers the most common types of crushed aggregates used for creating a sub base.

DTp1 or MOT Type 1 (Ministry of Transport Type 1)

MOT Type 1 sub-base is the UK equivalent of a DTp1 (Department of Transport Type 1) sub-base. Also known as 'hardcore', it is a high-quality unbound aggregate used to provide a stable foundation for roads, pathways and patios. It is well-graded, compactable, and provides excellent load-bearing qualities.

  • Usage: MOT Type 1 is a primary sub-base material used extensively beneath highways, roads, pavements, and driveways. It is well-graded and provides excellent load-bearing and compaction properties, making it suitable for high-traffic areas.
  • Composition: It typically consists of crushed rock, slag, or concrete, with sizes ranging from 60mm down to dust to ensure tight compaction.

Examples:

  1. 1
    MOT Type 1 Limestone: This product is a classic example of DTp1, offering a reliable foundation for all paving projects.
  2. 2
    Type 1 Recycled Aggregate: An eco-friendly alternative made from crushed concrete and other reclaimed materials, suitable for use as a DTp1 sub-base.
Patio pavers laid flat

MOT Type 2 (Ministry of Transport Type 2)

MOT Type 2 sub-base is an unbound aggregate that is also used as a sub-base material but is generally considered to be slightly lower quality than Type 1.

  • Usage: MOT Type 2 is a primary sub-base material suitable for less critical applications such as paths or base layers where lower traffic volumes are expected.
  • Composition: Similar to MOT Type 1, it is made from crushed stone, slag, or concrete but with slightly different grading specifications. The aggregate sizes for Type 2 range from 63mm down to dust, providing good compaction but with slightly less stability than Type 1.
Mot type 2 close up

MOT Type 3 (Ministry of Transport Type 3)

MOT Type 3 sub-base is a lower grade of compactable material primarily used for permeable paving surfaces such as driveways and paths where water drainage is important.

  • Characteristics: MOT Type 3 is known for its permeability, featuring a more open gradation that allows water to pass through more easily than MOT Type 1 or 2. It's made from crushed aggregate with sizes from 38mm down to dust.
  • Applications: Specifically designed for use in areas requiring effective drainage, such as sustainable urban drainage systems (SUDS), permeable pavements, and in landscaping projects where water accumulation can be an issue.

Examples:

  1. 1
    MOT Type 3 Limestone - Available from local building suppliers, it's specifically graded for permeable surfaces.
  2. 2
    Recycled MOT Type 3 - An environmentally friendly option made from crushed concrete, brick, and reclaimed aggregates.

Need to work out how much gravel your project needs? Try our free gravel calculator.

CBM (Cement-Bound Material)

CBM refers to a mixture of aggregates bound together with cement, used for road bases and surfaces where higher strength is required. It's less permeable than unbound materials, providing a firm and high-strength base.

Examples:

  1. 1
    Cemex ReadyRoad - A ready-to-use CBM, ideal for roads that require durability and resistance to weathering.
  2. 2
    Hanson Formpave - A permeable paving system that uses a specific CBM mix, designed for sustainable urban drainage systems (SUDS).
Cmb road and pavement installation. Jpg

HBM (Hydraulically Bound Material)

Definition: HBM is a mixture of aggregates with hydraulic binders (like cement or lime) and water, used for base layers of roads pavements, and for soil stabilisation. It gains strength through hydraulic or pozzolanic activity.

Examples:

  1. 1
    Tarmac Limelite - A lime-based HBM, perfect for historic building renovations or where flexibility and breathability are needed.
  2. 2
    Breedon Cement Hydralime - A hydraulic lime used for producing HBM, suitable for environments requiring durability and resistance to freeze-thaw cycles.

Edging

You will generally require some kind of edging material. Our article, Edging For Gravel, covers your options in great detail.

Cement edging for a gravel path. Jpg

Quick Step-by-Step Guide to Installation

Constructing a sub-base involves several key steps:

  1. 1
    Excavation: The first step is to excavate the area to the required depth, which varies depending on the intended use of the paved area.
  2. 2
    Compaction of the Sub-grade: The exposed ground, or sub-grade, is then compacted. This may involve the addition of a geotextile layer to separate the sub-grade from the sub-base material and prevent intermixing.
  3. 3
    Adding the Sub-Base Material: The sub-base material, typically a granular aggregate like MOT Type 1, is laid in layers. Each layer is compacted using a plate compactor to ensure a solid foundation.
  4. 4
    Levelling and Compaction: The final step is to ensure the sub-base is level and compacted to a uniform thickness, providing a stable base for the surface material.

If you're someone who enjoys DIY projects and is planning to pave an area, it's important to understand the purpose and construction of sub-bases. The sub-base is the foundation on which the success of your paving project depends. A well-built sub-base ensures that your paving looks good, functions well, and lasts for many years. Whether you're laying a new driveway, patio, or pathway, taking the time to properly install a sub-base will pay off in the long run by making your finished project more durable and attractive.

1excavated front garden driveway

Summary

If you're someone who enjoys DIY projects and is planning to pave an area, it's important to understand the purpose, materials and construction of sub-bases. The sub-base is the foundation on which the success of your paving project depends. A well-built sub-base is critical to the functionality, appearance and longevity of your feature. Whether you're installing a new driveway, patio, or pathway, taking the time to properly install a sub-base will pay off in the long run.

1installing a decorative stone front garden

FAQs

What is MOT sub-base?

MOT sub-base is a type of crushed aggregate used in construction to provide a stable base for roads, footpaths, driveways and other patios. It is graded to meet standards set by the Ministry of Transport, to guarantee that it has the necessary load-bearing and compaction properties.

What is sub-base material?

Sub-base material is a layer of sub base gravel placed between the soil layer and a constructed surface layer. It is a foundation designed to distribute loads evenly, provide drainage and prevent settlement and deformation of the paving layer.

What is Type 1 sub-base?

Type 1 sub-base is a high-quality aggregate that conforms to specific grading requirements set by the Ministry of Transport, including a range of particle sizes from dust to 60mm.

How deep should driveway sub-base be?

The required depth for a driveway sub-base is determined by the anticipated vehicle load, with a minimum of 100mm recommended for residential use, and 150mm for optimal support of car traffic. The preferred material for this layer is MOT Type 1, a crushed rock mixture graded from 40mm to dust.

What is MOT Type 1 sub-base?

MOT Type 1 sub-base is a crushed aggregate that meets the Department for Transport's specifications for highway works, characterised by its size and compaction properties. It is widely used in construction projects for creating stable foundations for roads, driveways, and walkways.

Can sub-base be too thick?

Yes, a sub-base can be too thick if not properly installed. Sub-base materials should be applied in layers, each not exceeding 150mm in thickness, and then compacted and levelled using tools such as whacker plates, hand tampers and rakes. This is vital to uniform compaction and stability, preventing potential issues such as uneven settling.

We have every type of gravel available to choose from in the UK. Check them out!

About the author 

Ben Parrot is a landscape gardener and amateur geologist. He is the founder of Outdoor Aggregates, with the mission to create the first encyclopaedia for decorative aggregates. From natural stone through to decorative bark, these aggregates are critical to garden and landscape design, universally.

The problem: There isn't any place where all the different types of decorative landscaping materials and construction aggregates can be found, nor are the specifications of each type readily available. So, enriched by his extensive experience, and inspired by the timeless elegance and raw beauty of stone and rock, etched by nature's artistry over millennia, Outdoor Aggregates was born.