A kerb is the raised edge that separates the carriageway from the footpath, median, or verge, and it does far more than mark the boundary of the road. Kerbs channel stormwater into drainage inlets, keep vehicles on the carriageway, protect pedestrians, and give the pavement a finished edge that resists fretting. The profile, height, and material chosen for a kerb line depend on the road class, the traffic speed, and the drainage layout.
Kerb design looks simple, but the details control how well the element works. The top of a standard kerb sits about 100 mm above the road surface, the kerb is laid on a concrete bed at least 100 mm thick, and at least 150 mm of backing concrete provides lateral support behind the unit. Getting these dimensions wrong produces kerbs that crack, shift, or trap water at the pavement edge.
What Are Kerbs in Roads?
In highway engineering, a kerb is a raised or flush edging that defines the carriageway boundary and controls where vehicles and pedestrians can move. The term kerb is used in British and Indian practice, while curb is the North American spelling; both describe the same element.
Functions of a Kerb
- Channel surface runoff to gullies and inlets as part of the longitudinal drainage system
- Prevent vehicles from leaving the carriageway, especially at curves and bridge approaches
- Protect pedestrians, structures, and road furniture from encroaching traffic
- Provide a clean, finished edge that stops the pavement from breaking up
- Guide drivers visually at night when painted or fitted with reflective surfaces
Where Kerbs Are Used
Kerbs appear on urban streets, residential roads, highways, footpaths, medians, and bridge decks. On high-speed rural roads they may be omitted entirely in favour of wide shoulders, while urban sections depend on them to manage drainage, parking, and pedestrian safety in a confined space.
Types of Kerbs
Kerbs are classified by how they interact with vehicles, and the class determines the height above the road surface. The four main categories are mountable kerbs, low-speed barrier kerbs, high-speed barrier kerbs, and submerged kerbs.
Low or Mountable Kerbs
Mountable kerbs stand only 70 to 80 mm above the road surface, low enough that vehicles can climb over them in an emergency. They suit residential streets, driveways, and footpaths where emergency vehicles may need to mount the kerb, and they help drain the carriageway while staying out of the way of normal traffic. The shallow rise lets a car cross at low speed without damage.
Low-Speed Barrier or Urban Parking Kerbs
Urban parking kerbs rise higher, typically 150 to 230 mm, and they keep cars from mounting the footpath in parking areas and city streets. They also protect pedestrians at crossings and junctions where vehicle speeds stay low.
High-Speed Barrier Kerbs
Barrier kerbs for high-speed roads are taller still, up to about 300 mm on major highways and bridge decks, with a steep face that a vehicle cannot climb without serious damage. They prevent lane departures at speed and shield pedestrians on the far side of the barrier.
Submerged Kerbs
Submerged kerbs sit flush with or slightly below the pavement surface and are used where the road is wide enough that vehicles rarely reach the edge. They preserve the edge of the pavement and provide a reference line for drainage without creating a trip hazard.
The choice is not automatic, and engineers still debate whether kerbs are necessary in road pavements on rural and low-traffic roads where shoulders and drainage ditches can do the same work at lower cost. The decision usually comes down to traffic speed, pedestrian exposure, and the cost of maintaining the verge.
Shapes of Kerbs
The cross-section profile of a kerb controls how water moves and how vehicles interact with the edge. Shape selection follows the same logic as type selection: match the profile to the road function.
Barrier and Square Kerbs
A vertical or nearly vertical face forms a barrier kerb that stops vehicles from leaving the roadway. Square or nearly square profiles are common in towns and cities where a crisp edge keeps pedestrians safe and simplifies the connection between the kerb and drainage inlets.
Sloped Kerbs
Sloped kerbs let vehicles mount the edge gradually at low speed, which makes them popular on suburban thoroughfares where driveways and parking access are frequent. The gentle ramp also sheds water more evenly than a vertical face.
Rounded Kerbs
Rounded kerbs are used along suburban residential streets with many driveways, where the kerb line is interrupted constantly and a radiused profile lets vehicles cross without catching the bumper. California standardized a 610 mm gutter design to balance initial and maintenance cost across the state.
High Containment Kerbs
High containment kerbs are specified in Great Britain near pedestrian areas, fuel station pumps, and other locations that need greater protection from vehicle traffic. Their tall, angled face deflects vehicles rather than stopping them abruptly, which reduces the severity of impacts.
Materials of Kerbs
Kerbs are made from concrete, asphalt, stone, or masonry blocks, and the material choice follows the road class, the available equipment, and the budget. Portland cement concrete has become the dominant material because it is durable, machine-layable, and easy to match to standard profiles.
Concrete Kerbs
Concrete kerbs are cast in place by setting forms by hand, filling them, letting the concrete set, and removing the forms, or extruded by a slip-form machine when the run is long. A slip-form paver places a continuous kerb without forms and is the economical choice for large contracts. Portland concrete kerbs paired with asphalt concrete roads give a highly visible barrier at the edge of the road surface.
Asphalt Kerbs
Asphalt kerbs are laid by a paving machine at the same time the road is paved, which makes them cheaper than concrete when the timing lines up. They are less durable than concrete kerbs and tend to soften in hot weather, so they are used mainly for temporary works and low-traffic roads.
Stone and Granite Kerbs
Granite kerbs are cut from natural stone and outlast both concrete and asphalt, surviving decades of traffic and weather. The higher material and labour cost limits them to prestige urban streets, historic areas, and locations where appearance matters.
| Material | Relative cost | Durability | Typical use |
|---|---|---|---|
| Concrete, precast or slip-formed | Medium | High | Most urban and highway kerb lines |
| Asphalt | Low | Low to medium | Temporary works, low-traffic roads |
| Granite or natural stone | High | Very high | Historic streets, high-value urban areas |
| Masonry blocks | Medium | Medium | Footpaths, low-speed residential areas |
Kerb Dimensions and Installation Details
Standard dimensions make kerbs interchangeable and easy to lay. Most straight concrete kerbs are 915 mm long, while radius kerbs used on curves are generally 780 mm long, and shorter units are manufactured for special conditions such as tight junctions and drainage inlets.
Standard Kerb Heights
- Mountable kerbs: 70 to 80 mm above the road surface
- Urban parking or low-speed barrier kerbs: 150 to 230 mm
- High-speed barrier kerbs: up to about 300 mm
- Submerged kerbs: flush with or slightly below the pavement
Bedding and Backing
The kerb is laid on a concrete bed at least 100 mm thick, and backing concrete at least 150 mm thick is placed behind the unit to give lateral support. Kerbs can be tapped down to the correct level while the bedding concrete is still plastic, and the line is checked against a string line before the concrete sets.
Drainage Integration
Where kerbs double as drainage elements, the profile must shed water toward gullies and inlets spaced to match the road gradient and catchment. The longitudinal drainage system depends on the kerb line staying true; a sag in the line ponds water at the pavement edge and accelerates local failure.
Kerb Installation Sequence
Laying a kerb line follows a fixed sequence that keeps the line, level, and drainage working together. The crew works ahead of the paving gang so the concrete can cure before traffic is allowed.
Step-by-Step Installation
- Set out the kerb line with string lines and pegs to the design radius and gradient.
- Excavate the trench and trim the formation to the required depth.
- Place the concrete bedding at least 100 mm thick and screed it to level.
- Set each kerb unit into the bedding and tap it down to the string line.
- Place backing concrete behind the units to a minimum thickness of 150 mm.
- Fill the joints between units with mortar and cure the concrete before traffic is allowed.
A well-laid kerb line reads as a single continuous edge, guides water to every inlet, and keeps vehicles where they belong for the life of the road. Specifying the right type, shape, material, and height for the road class is the difference between a kerb that works for decades and one that has to be replaced at the first winter.