Laying of Park Kerbs

Kerbs go in before the paving because they form the frame that the paving is then laid into. The method is simple in principle, but the result depends on accuracy at every step.

• Setting out: Pegs and a taut string line mark the route and, more importantly, the height of the top edge of the kerb. At this stage you already need to think about the finished level of the paving and the fall that will carry rainwater away, usually away from the house.
• Digging: A trench is dug slightly wider than the kerb and deep enough for the sub-base, the concrete bed and the kerb itself. For ordinary park kerbs this is usually roughly 25 to 35 cm below the top edge, and more where cars will drive.
• Sub-base: The bottom of the trench is compacted and, where needed, topped up with compacted gravel or crushed stone.
• Concrete bed: Semi-dry concrete is spread over the sub-base, usually about 10 cm thick. Semi-dry means it holds together when you squeeze a handful but does not run.
• Placing: The kerb is pressed into the bed and tapped down to the string line with a rubber mallet. The level is checked with a spirit level as the work goes on.
• Haunching: On one or both sides the kerb is supported by a wedge of concrete, called a haunch, which reaches roughly a third to half of its height. This haunch is what stops the kerb from leaning over in time.

Before you dig, check whether underground services run along the route: water, gas, electricity, telecoms. Utility companies can usually provide plans or mark their cables and pipes on request. If the kerbs are part of a new driveway entrance, a connection to a public road or a larger change to ground levels, ask your local building authority and, for anything touching the road or pavement, the highway authority whether permission is needed. Rules differ from place to place, so do not assume no approval is required. In a block of flats, changes to the surroundings are agreed with the building management or the owners' association.

The most common choice is concrete park kerbs, usually roughly 5 to 10 cm thick and around 20 to 25 cm high. They suit paths, patios and areas around the house. Where a car will park, thicker units and a stronger bed are used. Road kerbs are heavier and belong on roads and busy car parks.

Natural stone kerbs, such as granite, are very durable and look smart. They are heavier, though, and less accurate in size, so laying and cutting them takes longer. Plastic or metal garden edging is only meant to separate lawn from gravel or mulch. It will not hold paving that people walk or drive on.

What about the bed itself? Under paving, it is almost always concrete. Setting kerbs in gravel only works for purely decorative edging in the garden, and even then the kerbs shift or tip more easily over time.

The simplest case is a straight line on flat, firm ground where a wheelbarrow and delivered concrete or gravel can get close. The job takes longer, and therefore costs more, when there are:

• Lots of curves and corners where kerbs must be cut or radius units used
• Slopes and level changes where the kerb height has to change smoothly
• Poor ground, such as clay or made-up fill, that needs replacing with gravel
• Old kerbs and concrete to break out and remove
• Difficult access, where materials have to be carried by hand through the house or garden
• Stone kerbs instead of concrete ones

The weather matters too. Concrete should not be laid when the temperature is below roughly 5 °C or on frozen ground. In hot weather it dries out quickly, so it is worth covering it or keeping it damp.

A kerb that looks fine on the day it is laid can start leaning after the first winter. The cause is usually one of these:

• Uncompacted sub-base: The kerb gradually settles and the line becomes wavy
• Missing haunch: Under pressure from the paving and passing cars, the kerb leans outwards
• Concrete too wet: The kerb "floats" in it and is hard to keep at the right height
• Wrong height relative to the paving: Puddles form near the house or water runs towards the wall
• Ignoring water: A waterlogged base loses its strength and frost moves it in winter
• Loading too early: Running a plate compactor over the paving right next to fresh concrete

In practice, the concrete haunch on the lawn side is often where corners get cut. It is the least visible part of the job, but one of the most important.

For a garden path or a patio with no cars, you can lay the kerbs yourself. You will need a spade, string line, pegs, a spirit level, a rubber mallet, a hand tamper or plate compactor, and a mixer or bucket for the concrete. Kerbs are heavy, so lift with a straight back and preferably with a second person. When cutting with an angle grinder, wear safety glasses, ear defenders and a dust mask. Wet cutting keeps the dust down.

1. Check the route for underground services.
2. Drive pegs in at each end and stretch the string at the height of the kerb top, allowing for the fall.
3. Dig the trench to the required depth and slightly wider than the kerb.
4. Compact the bottom of the trench, and add and compact gravel if needed.
5. Mix semi-dry concrete following the instructions on the bag.
6. Spread the concrete bed along the length of two or three kerbs.
7. Place the kerb and tap it down to the string line with the mallet.
8. Check it with the spirit level and make sure it lines up with the previous unit.
9. Leave a small, even joint between units.
10. Build the concrete haunch along the side and smooth it with a trowel.
11. Let the concrete harden and keep loads off it until the manufacturer allows.
12. Backfill the lawn side with soil and carry on with the paving.

When you check the work, walk slowly along the whole line and also look along it from low down at the side. Unevenness is much easier to see at a shallow angle. Check that:

• The line is straight, or the curves flow smoothly without kinks or jumps
• The top edges of neighbouring kerbs meet without steps
• The kerb height matches the planned level of the paving and the ground
• The fall takes water away from the house and no puddles form
• The joints between units are even
• The concrete haunch is continuous and the kerb does not move when you push it
• Cuts are clean and corners meet without large gaps
• The area is tidy, with no concrete left on the paving or kerbs