What are the Layers of Road Construction?

There are typically six layers in road construction, although some sources often cite four, as they don’t count the prepared formation and capping layer that may be required.

The six layers of road construction are (from the top down):

• Surface course
• Binder course
• Structural base
• Sub-base
• Capping layer (where required)
• Prepared formation

In this guide, we’ll explain the role each layer plays, why thickness matters and the standards and laws on road specification.

When is a road a pavement?

Throughout this guide, you will see various references to pavement. It shouldn’t be confused with footpath surfacing. In road construction, a pavement is the part of the road that carries traffic. It has a set of layers (that we’ll explore shortly) placed over the natural ground, known as the subgrade.

The various pavement layers spread vehicle load so the strength capacity of the underlying subgrade isn’t compromised.

What are the types of pavement in road construction?

There are two main types of pavement in road construction: rigid pavement and flexible pavement. Each has specific differences, but you’ll often find them made of asphalt, concrete, cobblestone, bricks or flagstone.

Rigid pavements

Rigid pavements are constructed by placing a reinforced or unreinforced concrete slab over a subbase. The loads the pavement carries are supported by transference across a wider area of subgrade. To control cracking, there are usually joints within the concrete. They are built to withstand heavy traffic in terms of weight and flow and are often found in areas such as ports or other locations where heavy vehicles may move slowly.

Flexible pavements

Flexible pavements are the type most commonly found on UK roads. They are made of multiple layers, with the surface layer made of asphalt. The pavement is supported by and protects the subgrade beneath the pavement.

Understanding important changes in terminology

In many guides, terms such as roadbase, base course and wearing course may be found. These are no longer valid, and since 2008, base, binder course and surface course are commonly used. Council highway specs currently in circulation use these newer terms, and below, we’ll show you how they fit into the structure for UK roads.

The layers of road construction explained

Formation

Formation is the prepared layer at the top of the subgrade. It provides the foundation level for the road surface and plays a crucial role in influencing whether capping is required, how thick the pavement (road surface) should be and the stability of construction, among other things.

If this layer is particularly weak, capping often becomes necessary.

Capping

Capping is used if the formation layer is weak or requires added protection. It’s not necessary for all roads but will be considered if the soil condition is inadequate, subgrade strength isn’t sufficient, or there is excessive groundwater. Other factors may also necessitate a capping layer, and its thickness and material will be determined by the needs of the site rather than a one-size-fits-all approach.

Sub-base

The sub-base will be installed above the prepared formation, or capping where it has been used. It’s there to provide a stable platform for the structural courses above it.

The sub-base plays a vital role by distributing loads, supporting drainage, protecting the formation, and ensuring consistent levels. Like the capping we referenced earlier, there is no universal thickness. Depth depends on the foundation, traffic, and design. Crushed stone and gravel are often used due to their strength and durability. Drainage measures may also be incorporated to protect the road from water damage and reduce erosion risks that could harm the road's lifespan.

Base course

The base course is the main load-carrying course that distributes and carries traffic loads down to the foundation. It will commonly sit above the sub-base and below the binder course. In some places, it is seen as loose crushed stone, but a modern base is actually a bound structural layer, usually made of asphalt, and in some designs, a cement-bound material, compacted to a high density for load-bearing.

It plays a crucial role in road structure by supporting the binder course and controlling permanent deformation, among other things.

Binder course

The binder course is an asphalt layer that sits above the base and below the surface course. As with the other layers, it plays a crucial role in a road surface as, among other things, it:

• Protects the base
• Supports the final layer
• Creates consistent levels
• Transfers surface stresses

Surface course

The surface course is the layer that endures vehicles, footfall, weather and more. As a result, it needs to be skid-resistant, provide suitable water removal, control tyre noise and be a suitable texture for the traffic it encounters. Asphalt and concrete are among the most widely used materials thanks to their durability and finish.

As it endures the most wear and tear, this is the layer that requires the most maintenance to keep the overall road structure in good condition.

Road surface construction layer thickness

There is no universal thickness for a road surface. It is ultimately determined by design, and two factors play an important role: the strength of the ground beneath the road and the traffic the road is expected to carry.

• Ground strength: Shown as a CBR value, a simple measure of how much load a subgrade can bear.
• Expected traffic: Calculated over the road’s design life and often given in MSA (million standard axles).

In simple terms, weaker ground or heavier traffic requires thicker and stronger construction, and is often what determines whether a capping layer is needed. Strong ground that carries lighter traffic, on the other hand, can be built up with less.

Developers should also be aware that each layer is designed in relation to those above and below it. Reducing sub-base thickness, for example, may increase demand on the base above it. This means that a thickness that works for one road won’t necessarily work for another.

Because thickness is set by design and not a fixed rule, there are set standards that must be met, and where a road is handed over to a highway authority, specific laws also apply.

What standards and laws apply to UK road construction?

The standards and laws that apply to UK road construction are clearly defined:

• Technical standards: Set how a road is designed and built
• Laws: Determine what happens once the road is built

Technical standards

National Highways provides the main reference for technical standards in road design. The Design Manual for Roads and Bridges (DMRB) covers the full process under:

• CD 225 for pavement foundations
• CD 226 for the design of new pavement construction
• CD 227 for pavement maintenance

Additionally, the SHW, or Specification for Highway Works, part of the Manual of Contract Documents for Highway Works (MCHW), sets out the materials, compaction and workmanship for each layer.

Law

The applicable law is the Highways Act 1980. For those building a new road, Section 38 and Section 278 apply the most.

• Section 38: The mechanism by which a new, privately built road is adopted, taken over by the local highway authority and then maintained at public expense. You can learn more about adopted and unadopted roads in our blog.
• Section 278: Governs the works that affect or connect to the existing public highway.

Both the standards and the law intertwine; if a developer intends for the road to be adopted, it must be built to the highway authority's adoptable standard, underpinned by the earlier-mentioned DMRB and SHW. If the road falls short on design, materials, thickness or compaction, the road may not be adopted. This would then leave the developer or landowner responsible for maintaining it indefinitely.

For private roads that will never be adopted, standards still apply. Without the standards to match the intended traffic, the road could fail early, leading to costly and long-running repairs.

In simple terms, the standards and the law are what turn six layers of road from a general principle into something fit for purpose, legal to connect and capable of being handed over.

Trusted road surfacing and maintenance

At Tidey & Webb, we have more than thirty years of experience in road surfacing in Sussex and across the south of England. Using skilled, qualified engineers, we carefully lay new roads from the sound formation through to the durable surface course. Contact our team to discuss your plans, and we’ll organise a site visit to assess how we can help.