Road Kerb Installation

A kerb looks like a simple block of concrete, but how long it lasts depends mostly on what you cannot see: the base, the concrete bed and the support behind it. A typical installation goes like this:

1. Setting out the line and level. The installer marks the route with pegs and a string line. A level or a laser sets the height and fall to match the finished surface.
2. Digging the trench. The trench must be deep and wide enough for the gravel base, the concrete bed, the kerb itself and the concrete haunching behind it.
3. Compacting the base. The installer compacts the bottom of the trench, lays a layer of crushed stone and compacts it again thoroughly with a plate compactor.
4. Laying the concrete bed. A semi-dry, fairly stiff concrete goes on top of the stone. It holds its shape well, so the kerb does not sink into it.
5. Setting the kerbs. Each unit goes into the fresh bed and is tapped down to the string line with a rubber mallet. The installer checks height and fall all the way along.
6. Haunching. Concrete goes against the back of the kerb, and on roads often against the front as well. This stops the kerb from tilting when a wheel hits it.
7. Joints and finishing. Narrow joints are left between units and filled with mortar where the design calls for it. The installer backfills and compacts the surrounding area, and the surface is then ready for asphalt or paving.

Kerbs always go in before the paving or asphalt. They form the solid edge that the surface pushes against, so it makes little sense to add them afterwards.

A standard road kerb is usually a concrete unit about a metre long, roughly 250 to 300 mm high and around 150 mm wide. Exact sizes vary by country and manufacturer. It is heavy and strong enough to take lorries driving over it. You will also come across these variants:

• Dropped or splayed kerbs with a sloped face, so a car can drive smoothly onto a driveway or across a pavement
• Transition units that join a full-height kerb to a dropped one without a step
• Radius kerbs for bends and corners, so the edge is not pieced together from short straight sections
• Edging and garden kerbs, which are lower and thinner and only suit footpaths and gardens, not roads

Choose by load, not by looks. If a delivery van or a bin lorry uses the area, a garden edging kerb will not last long. Good concrete kerbs are made to EN 1340. They should resist frost and de-icing salt.

Why does a kerb start leaning after a few winters when it was perfectly straight at first? The cause is almost always the base or water, not the concrete unit itself.

The stone layer must be thick enough and well compacted, otherwise it settles over time. For roads, the concrete bed is usually around 100 to 150 mm thick. The kerb should sit in the bed and haunching to roughly a third of its height. That leaves an upstand above the finished road surface, typically around 100 to 150 mm on an ordinary street, and less at driveways and crossings.

Older practice often used a dry mix that was simply watered after the kerbs were laid. This is less reliable, because the concrete may not harden evenly. A properly mixed semi-dry concrete gives a better result.

Water needs somewhere to go. The road edge should fall towards gullies or a planted area, and the kerb must not act as a dam with a puddle behind it. Water that soaks into the base and freezes in winter can push a kerb out of even a good bed.

How hard the job is, and so how much work it takes, can vary a lot. These things make it easier:

• Flat, accessible ground where a digger and a delivery lorry can get in
• Long straight runs without curves or changes in level
• Firm, sandy or gravelly soil

These things make it harder:

• Breaking out and removing old kerbs and concrete
• Clay or waterlogged ground that needs a deeper dig and more stone
• Many curves, cut units and joins to existing asphalt
• Carrying heavy units by hand where machinery cannot reach
• Working next to traffic, which calls for traffic management and signs

Setting road kerbs yourself is not a good idea. A single unit usually weighs several dozen kilograms, so it can easily trap fingers or injure your back. Without a level and a plate compactor, a straight and solid line is also very hard to achieve.

Changing an edge within your own property is usually a minor job. If the work touches a public road, a pavement or a driveway entrance onto a public road, you normally need consent from the local council or highway authority. A new dropped kerb is a common example. Building regulations differ from place to place, so check with your local building authority before you start what kind of approval your work needs. In a block of flats, the building management or the owners' association decides on changes to shared areas.

Before any digging, have the underground services located, especially gas, electricity, water and telecoms cables. If a gas pipe is damaged, leave the area at once and call the gas emergency line or 112. Digging near cables and gas connections belongs in the hands of an experienced contractor. Any repair to gas pipework must be done by a qualified, registered gas engineer.

The best time for concreting is from spring to autumn. Below about 5 °C, concrete sets slowly and can be damaged by frost. Above about 30 °C, it dries too fast and needs to be kept damp or covered. Kerbs should never be set on a frozen or muddy base.

• Poorly compacted base. Over time the kerbs sink unevenly and the line becomes wavy.
• Missing or thin haunching. The kerb gradually tilts each time a wheel runs against it.
• Wrong fall. Water stands at the edge, seeps under the paving and damages the base in winter.
• Kerb set too shallow. It feels firm at first but moves under the first real load.
• Garden edging next to a road. The thin unit cracks or breaks out.
• Loading fresh concrete. Driving over the kerbs before the concrete has hardened enough damages the bed.

When you accept the work, walk the whole length of the kerb line. Look at it from a distance and up close, and go through this list:

• The line is straight or smoothly curved, with no steps or kinks
• Neighbouring units are level with each other and the joints are even
• No unit moves when you push it firmly with your foot
• You can see concrete haunching behind the kerbs, not just soil
• The upstand above the surface matches what was agreed, and the kerb is dropped at driveways
• After rain or a hose test, water drains away and does not stand at the edge
• Kerbs are not cracked, chipped or smeared with concrete
• The contractor has told you when the area can take cars