The subgrade — that’s the soil directly underneath your concrete slab — has one job: stay still. If it shifts, the slab cracks. If it sinks, the whole thing fails. That’s why compaction is a non-negotiable step in any concrete pour. Whether you're working with natural ground, made-up fill, or clay-heavy subsoil, compacting it properly means your slab stays rock solid for decades. This guide shows you exactly how the pros compact a subgrade — safely, consistently, and to the right spec.
Not all ground is ready to be compacted. The ideal subgrade is slightly moist — not dusty and dry, but definitely not saturated or muddy. If the soil is too wet, wait for it to dry out or scrape away the top layer. Remove any organic matter like roots, turf, or old topsoil, which will decompose and create voids. If the site was previously dug out, check for backfill or soft spots — these will need deeper compaction or removal. Before doing anything, walk the surface. If your boot sinks in or the ground feels spongy, it’s not ready yet.
For subgrade compaction, a petrol-driven plate compactor (also called a wacker plate) is usually enough for small residential slabs. For larger areas or deeper fill, a trench roller or ride-on roller may be needed. The key is weight and vibration — you want to force air and gaps out of the soil structure. Never try to compact with hand tools or by foot — it doesn’t work. Make sure your machine has fuel, oil, and the right vibration settings before starting. Wear ear and hand protection; these machines are powerful and noisy but essential.
Start at one edge of the slab area and work in slow, overlapping passes. Move steadily — don’t rush. Each new pass should overlap the last by at least 1/3 to ensure full coverage. For large areas, change direction after each layer (like mowing a lawn in a cross pattern) to maximise compaction. Usually, two to four full passes are enough on natural ground. If you're working on loose fill or backfill zones, you may need to do more. You’ll feel and hear the difference as the ground firms up — the plate will glide instead of bounce.
Once compacted, the subgrade should feel solid underfoot. If your heel sinks in more than a couple of millimetres, it needs more work. Look for consistent levels — no sudden dips or soft patches. You can also probe the ground with a steel rod to check resistance. In high-risk areas (like load-bearing pads or poor subsoil), a CBR test or plate load test may be required by spec. If you're unsure, better to over-compact than under-do it. You only get one shot before the slab goes on top.
Once the subgrade is compacted and firm, mark out your finished level again using string or laser. Measure down from the finished slab height to confirm depth for hardcore or insulation. This is your last chance to fine-tune the base. If needed, scrape or top up low/high spots with compactable fill. Now you're ready to start laying MOT Type 1 or crushed concrete for your sub-base. Remember: a slab is only as flat and strong as the ground below it. Nail this step, and the rest gets a whole lot easier.
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