Paving Edging and Restraints: Concrete Haunching, Flexible Edge Restraints and Why Edges Matter for Longevity

Quick Answer: Every block, slab, or in-situ paving needs continuous edge restraint to prevent the field from spreading under load. The standard solution is a concrete-haunched edge — a precast edge block or kerb set on a 100mm C20 concrete bed with a 75mm concrete haunch up the rear face. Without it, joints widen within 2–3 winters, the laying course migrates, and the paving fails progressively from edge inward. There is no domestic situation where edge restraints can be omitted.

Summary

Edges are the structural element that paving installers most commonly underspecify. Joints, bedding, sub-base — these get attention because they're visible during the build. Edges look unimportant once the field blocks are laid, and a contractor under time pressure can skimp the haunching to save half an hour. That half-hour shows up as the dominant failure mode within 3–5 years.

BS 7533-3 explicitly mandates edge restraints for all flexibly-bedded block paving. The standard recognises that an unrestrained field cannot maintain joint integrity — vehicle wheel loads spread laterally, blocks creep outwards, joints open from 2–5mm to 8–12mm, sand washes out, and surface water reaches the laying course. Once water has access, freeze-thaw cycles do the rest of the work.

The edge restraint also matters for in-situ surfaces — concrete, tarmac, resin bound. A concrete slab without edge support cracks at the perimeter as the slab edge unloaded by passing vehicles flexes. Tarmac unsupported at the edge crumbles within 2 winters. Resin bound stones detach at the perimeter where the binder is least confined.

Key Facts

Quick Reference Table

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Edge type Domestic patio Domestic drive Vehicular Typical haunch detail
Soldier course (header block) OK OK Marginal 100mm bed, 75mm haunch
Edging block (45×100×200) OK OK No 100mm bed, 75mm haunch
Concrete edging (50×150×450) OK OK OK 100mm bed, 75mm haunch
KSL kerb (125×255) Overkill Premium Standard 150mm bed, 100mm haunch
KAL kerb (125×305) Overkill Premium Standard 150mm bed, 100mm haunch
Bullnose edge Decorative Decorative No 100mm bed, 75mm haunch
Plastic edging OK No No Pin-fixed, no haunch
Steel edging OK No No Pin-fixed, no haunch

Detailed Guidance

Why edge restraint is non-negotiable

When a vehicle wheel rolls onto a flexible block paving surface, the load on each block is transmitted to neighbours via the kiln-dried sand in joints. The interlock between blocks creates a load-spreading effect — pressure in one square metre is distributed through interlock to several square metres. This works only if the perimeter is restrained. At a free edge, blocks at the perimeter rotate outwards under load, joints widen, interlock is lost, and the failure propagates inward block by block.

The pattern is predictable: within one season, perimeter joints visibly wider; within three seasons, the outer 2–3 courses are loose; within five seasons, the field shows undulating surface as deeper blocks tilt.

The fix at year 5 is the same work that should have been done at year 0 — except now you've also got to lift and re-bed the affected field blocks. The labour cost of doing edges right at install is small; the cost of fixing failed edges is significant.

Concrete haunching specification

A correct haunched edge:

  1. Sub-base — same MOT Type 1 as the field, 150mm minimum domestic vehicular
  2. Concrete bed — C20 (1:2:4 ballast:cement), 100mm thick, slightly wider than the edge unit
  3. Edge unit set — bedded into the wet concrete, tapped level with a rubber mallet, top 5mm above intended finished paving level
  4. Haunch — additional C20 concrete pressed against the rear face of the edge unit, sloping back to ground level, 75mm minimum height from bottom of bed

The haunch is what holds the edge against horizontal load. The bed alone holds the edge vertically but not horizontally — without the haunch, the edge can rotate forward under wheel load.

For very heavily loaded edges (HGV access, paths near loading bays), a reinforced haunch with a 6mm steel bar embedded in the concrete improves rotation resistance.

Soldier courses

A soldier course is a single header row of paving blocks laid against the haunch as a final perimeter detail. It serves three purposes:

  1. Aesthetic — defines the field edge and frames the pattern
  2. Structural — adds a half-block depth of restraint against the haunch
  3. Practical — accommodates field-pattern cuts away from the visible edge

For a 200×100mm block, the soldier course is one row of blocks laid with the long dimension across the perimeter. Set in the same laying course sand as the field; bed against the cured haunch concrete.

For driveways, the soldier course is set first, then field blocks worked from a defined starting position to produce a clean cut zone aligned with the soldier rather than the haunch.

KSL/KAL kerbs for premium and vehicular

Where the drive expects regular heavy vehicle use (skip lorries, oil deliveries, plant), upgrade from header-block soldiers to a precast kerb. The KSL (125 × 255mm) and KAL (125 × 305mm) profiles to BS EN 1340 are standard. They sit on a 150mm concrete bed and need a 100mm haunch.

KSL/KAL kerbs are also used on premium domestic drives for the cleaner aesthetic — the kerb top is above paving level, providing a visible defining edge.

Flexible edge restraints — when they're acceptable

Composite plastic/polymer edging (e.g. NetPave, Permaroad) and steel edging (Coreedge, Everedge) are used on landscape paths and pedestrian areas. They are pin-fixed (300mm pegs at intervals) into the sub-base.

They are not appropriate for vehicular driveways. The pin restraint is insufficient to resist horizontal forces from wheel load and freeze-thaw, and the perimeter blocks creep within 2–3 years.

The legitimate use of flexible edging is on garden paths, gravel borders, lawn edges — areas where horizontal loads are minimal.

Edge restraint and SuDS

For permeable paving systems, edge restraints serve the same structural role but the haunching detail can be modified to allow water to escape. A common detail is a concrete-haunched edge with weep holes (50mm uPVC pipes through the haunch at intervals) or a permeable backfill (clean angular gravel) instead of concrete behind the edge unit.

However, on a permeable drive where the edge abuts impermeable surfaces (lawn, dwelling), water collected at the edge needs an outlet — either a perforated land drain or simply a permeable border (gravel strip).

Edges in tarmac and concrete drives

For tarmac, the same concrete-haunched precast edge or block kerb is set first, then the binder course and surface course laid against the back face. The tarmac can be slightly above the edge top (3–5mm) so the surface course rolls down to flush.

For concrete slabs, the timber shutter formwork during pour serves as temporary edge restraint, then is replaced after curing by the permanent edge — usually a kerb or edging set 5mm proud of the slab. Without permanent edge, the concrete slab edge gradually breaks down under wheel load.

For resin bound, the edge is detailed similarly: concrete-haunched edge first, with the porous asphalt base and the resin matrix laid up to the edge. The edge holds the resin matrix against creep.

Movement and expansion joints

Where paving abuts a building or another paved surface, an expansion joint accommodates differential movement. For block paving, a 12mm gap with foam filler at building junctions is sufficient — the flexible block matrix accommodates most thermal movement within the joint sand.

For concrete slabs, expansion joints (full-depth gap with compressible filler at slab thickness) are required at building junctions and at slab-end joints over 6m long.

For tarmac, no expansion joints are needed for domestic areas — bitumen is flexible enough to accommodate thermal movement.

Frequently Asked Questions

Can I just rely on heavy field blocks for edge restraint?

No. The interlock pattern is what gives the field its strength, and interlock requires confinement. A single edge restraint failure propagates through the interlock pattern and the whole drive becomes unstable. Field weight does not substitute for edge restraint.

How wide should the haunch be?

The haunch is concrete poured against the back face of the edge unit. Width is determined by stability — typically 100mm at the base sloping back to 0 at the top of the 75mm haunch height, giving a roughly triangular section. Wider haunching is fine but uses more concrete; narrower fails earlier.

What if the edge meets a building wall?

The wall provides a perimeter restraint already, but a 12mm flexible joint between paving and wall is standard practice to allow differential movement. No haunch is needed at the wall face — the wall itself is the restraint. Block paving against a wall just needs the side joint filler.

What about retaining walls along driveways?

Retaining walls double as edge restraints if engineered correctly. The wall must be a structural retaining design (gravity, cantilever, or reinforced) — see retaining walls article. Block paving keys against the wall directly with no separate edge unit if the wall extends to laying course level.

Does flexible edging ever work on a driveway?

Honestly, no. For domestic vehicular use, flexible edging is a false economy. Suppliers may suggest it as a cheap alternative; in service it's the most common cause of perimeter failure within 3 years.

Regulations & Standards