The hardcore layer sits directly beneath a concrete base and acts as the load-bearing foundation that supports everything above it — from insulation and concrete to vehicles, buildings, or foot traffic. Get it wrong, and you’ll be dealing with movement, cracks, and failures down the line. Done right, it forms a rock-solid platform that stays stable for decades. In this guide, we’ll walk you through how to lay hardcore properly — including what type to use, how thick it needs to be, and how to compact it the way professionals do.
For most slabs — whether residential or commercial — the go-to material is MOT Type 1. It’s a well-graded mix of crushed limestone or granite up to 40mm in size, with fines that bind together when compacted. Avoid loose rubble, round gravel, or skip waste — these don’t lock in or compact properly. Recycled crushed concrete or crushed brick can be used if it’s certified and clean, but you’ll need to ensure consistency in size and compaction performance. Hardcore needs to be strong, inert, and stable. If it squashes, crumbles, or breaks under foot, don’t use it.
Depth depends on what the slab will support. A typical patio or shed base might need 100–150mm. Driveways or garages should go 150–200mm or more. Measure from your finished slab level down — and subtract the depth of any insulation or concrete that’ll go above. Use pegs, laser levels, or string lines to maintain consistent depth across the area. If the ground is soft, add extra depth or switch to a geotextile-reinforced base. Never guess — a shallow, uneven hardcore bed can cause cracking or sagging in the slab later on.
Always build up hardcore in thin layers — around 75mm per lift is ideal. If you try to dump 200mm and compact it in one go, the top might seem solid, but the bottom will stay loose. Rake it evenly across the excavation and make sure it’s not bridging voids or lumps underneath. Larger areas should be laid with a tracked dumper or barrowed in with care. Compact each layer fully before adding the next. Think of it like building bricks: if one layer’s weak, the whole thing’s compromised.
Use a vibrating plate compactor (wacker plate) or a roller depending on the size of the area. Go over each layer in overlapping passes — typically two to four runs depending on the material and machine. You should see and feel the difference: fines binding together, surface smoothing out, and the plate running more smoothly. Listen for bounce or hollow sound — it’s a sign of poor compaction. Test the surface with your foot: it should feel solid and show little to no indentation. Hardcore that’s not properly compacted will shift under pressure once the slab goes down.
Once your final layer is compacted, measure from your string line or laser level to confirm the depth is spot-on for the next layer (usually DPM or insulation). Rake and recompact any high or low spots as needed. If you're installing a damp-proof membrane, the hardcore should be smooth enough to avoid punctures — or add a sand blinding layer on top. The goal is a solid, flat, well-compacted base with no sharp debris, ready for whatever comes next. Take the time to check — it's the last chance before your concrete or insulation locks everything in place.
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