Concrete Kerb Foundation Calculator — Bed & Haunch

It depends on the kerb size and your bed/haunch dimensions. On a 100mm concrete bed with 150mm-wide × 150mm-high haunches both sides, an HB2 kerb (125 × 255mm) works out at roughly 0.05 m³/m of concrete (bed plus both haunches), and a larger 150 × 305mm bullnosed kerb a little more. A thinner footpath bed and narrower haunches will be less. The haunch is a triangular fillet against each side of the kerb — base width × rise.

For general footpath and residential kerbs the standard highway detail beds and haunches with ST1 (about C7.5) concrete (Neath Port Talbot Highway Construction Detail note: kerbs backed and haunched with ST1). For driveways and trafficked or vehicle-crossover kerbs, step up the haunch to at least C20 (ST2), or C30 (ST4) for heavy crossovers (pavingexpert.com/edging3). Bedding can be a semi-dry C7.5P “kerb mix” laid 100–150mm thick before compacting. See our Concrete Mix Calculator for hand-mix proportions.

The bed is the concrete layer the kerb sits on — at least 100mm thick (150mm for highways) per pavingexpert.com/edging5 — providing a firm, level foundation. The haunch is the concrete fillet on each side of the kerb that holds it in position and resists movement. The haunch should be at least 150mm thick, cover at least half the kerb depth, and be smoothed off 25–50mm below the top of the kerb. Note: a thin 12–25mm semi-dry mortar layer is sometimes used to fine-level the kerb on top of the concrete bed — that mortar is not the structural foundation.

Best practice is to haunch both sides (front and rear). The rear haunch prevents the kerb being pushed forward by soil or vehicle pressure. The front haunch prevents the kerb falling forward into the road. For edging (flat top) where loads are light, a single rear haunch may suffice, but both sides is always recommended.

Behind the haunching, backfill with compacted Type 1 sub-base or the existing excavated material (if suitable). On driveways, the sub-base should be continuous behind the kerb. Leave the backfill around 150mm below the top of the kerb to allow for the surface course, and compact in 100mm layers with a plate compactor. The backfill figure here is an indicative rule-of-thumb volume (a haunch-width zone filled to roughly 150mm below the kerb top) — confirm it against your actual build-up and any sub-base that runs through behind the kerb.