Kerb Edging Calculator — Units, Haunching & Bedding

Standard UK precast concrete kerbs are 914/915mm (just under 1 metre) long to BS EN 1340, so you need approximately 1.1 kerbs per metre of run (1 ÷ 0.915 = 1.09). This applies to HB2 half battered, BN2 bullnosed, SP splayed and flat top edging types. Allow about 10% extra for cuts and wastage.

Haunching is the concrete support placed against the back (outside edge) of a kerb to hold it in position. It is a triangular profile brought up to roughly two-thirds of the kerb height — not the full depth — and is placed on the back only, not both sides (Marshalls and Tobermore installation guides; Pavingexpert advises at least half the depth is haunched). For road kerbs use at least a C20 (ST4) mix — ST4 is the standardised prescribed grade assumed to give 20 N/mm² at 28 days, i.e. C16/20 (BS 8500-1 Annex A); for light footpath edging an ST1 (C7.5) kerb mix is acceptable. The volume depends on the kerb height, so taller kerbs need more haunch than a 150mm edging.

For light garden edging a semi-dry mortar bed (roughly 6:1 sand:cement) around 25mm thick is common, and the units are tapped down with a rubber mallet. For road kerbs the bed should be a concrete bed of at least 100mm thickness at C7.5P (ST1, "kerb mix"), and the haunching at least C20 strength — per Pavingexpert and Tobermore/Marshalls installation guidance. A C20 haunch corresponds to ST4, the standardised prescribed mix assumed to give 20 N/mm² at 28 days (C16/20); ST5 (C20/25, 25 N/mm²) over-specifies it. Heavy vehicle crossings call for a stronger C30-class trafficked mix, which is a designated/designed concrete (e.g. RC32/40 or PAV2) — no ST standardised grade reaches C30, as BS 8500-1 Annex A caps the standardised prescribed range at ST5 (assumed not greater than C20/25).

HB (Half Battered) kerbs have a sloped face and are the most common type for roads and driveways. BN (Bullnosed) kerbs have a rounded top edge. Both the standard HB2 and BN2 units share the same 125×255mm cross-section in 914/915mm lengths to BS EN 1340 — the difference is the face profile, not the size. A larger 150×305mm kerb is also available where a higher upstand is required.

Kerb edging or similar edge restraint is essential for block paving, resin-bound and gravel driveways to prevent the surface spreading. For tarmac, kerbs define the edge and provide a neat finish. Flat top edging is often sufficient for garden paths, while HB2 kerbs are standard for vehicle crossovers.