High spots in a concrete slab aren’t just annoying — they’re a liability. Whether you're laying new flooring or levelling a workshop floor, grinding down raised areas ensures proper drainage, even coverage, and safety. But it’s not just about switching on a grinder and hoping for the best. In this guide, we’ll show you how to properly assess, mark, and grind down uneven slabs without damaging the surface or exposing aggregate where you don’t want it.
Begin by walking the full surface of the slab. Use a 2-metre straightedge, laser level, or long spirit level to identify where high points occur. You're looking for anything that rises 3 mm or more above the surrounding area — this can cause problems for levelling compound, tiling, or coatings. Chalk, tape, or spray paint is ideal for marking the perimeter of each raised spot. In some cases, especially on older slabs or overlays, curling or heaving may cause widespread unevenness — which will change your approach from isolated grinding to full surface profiling. Take notes of elevation changes and whether the area is indoors or exposed to weather (as this may affect your tooling).
Your grinder choice will depend on how much material you’re removing and the size of the area. For patches under 1 m², a handheld 125–180 mm angle grinder with a diamond turbo cup wheel will work. For anything larger or where precise levelling is required, use a 3-phase planetary floor grinder with metal bond segments. Always choose 30–40 grit diamonds for aggressive removal, especially for older or hard concrete. Never use polishing pads or resin-bond diamonds for this purpose — they’re designed to smooth, not remove. For tough surfaces, you may also need weights added to your grinder to increase pressure. Hook up to an M-class or H-class dust extractor for health and visibility.
Begin on the outside edge of your marked high spot. Keep the grinder flat — tilting will cause gouges or uneven wear on the tooling. Move in a slow, overlapping circular motion, allowing the diamonds to cut without forcing the tool. Focus on shaving down thin layers at a time, rather than trying to “dig” the spot out too quickly. After every few passes, vacuum the area, wipe the surface, and re-check with your level. This helps avoid going too deep or creating a dish in the slab. If you’re using a large grinder, make sure the machine tracks evenly and doesn’t tilt or bounce — a common problem near slab joints or edges.
One of the most important (and often skipped) steps is feathering. Once the high point is reduced, you need to gradually taper the ground surface into the surrounding area so there’s no ridge or sudden change in elevation. Switch to a slightly less aggressive grit (such as 60–80) and make wider sweeping passes across the border between ground and untouched slab. Your goal is to ensure that foot traffic or floor finishes won’t “read” the repair visually or physically. In commercial or polished concrete settings, failure to feather properly can make the repair obvious and unacceptable. For overlay prep, a well-feathered surface helps the new material sit level.
After final grinding, clean the area thoroughly. Use an industrial vacuum to remove all dust, then wipe with a microfibre cloth or tack cloth. At this point, consider your next step: If you’re overlaying or applying leveller, use a penetrating primer or bonding agent to enhance adhesion. If leaving exposed, apply a densifier and/or sealer to reduce surface dusting and absorbency. If polishing, now is the time to step down through finer grit levels. For outdoor slabs, consider applying a silane-siloxane sealer to protect your now-exposed surface from moisture and freeze-thaw damage. Never leave a freshly ground slab unprotected — it will wear or stain quickly.
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