Yes, in many cases you can grind down a high spot on a concrete slab yourself — provided the area is small, the slab is structurally sound, and you use the right equipment and safety precautions. For larger areas, uneven slabs caused by subsidence, or work near reinforcing steel, it's best to bring in a qualified concreter.
What Causes High Spots on Concrete?
High spots typically occur when concrete is poured and finished unevenly, or when one section of a slab heaves due to ground movement, tree roots, or moisture changes beneath the surface. Before you start grinding, it's worth understanding why the high spot is there:
- Poor finishing during the original pour — the most common cause, and safe to grind back.
- Slab heave from expansive soils or tree roots — grinding may be a temporary fix; the underlying cause needs addressing.
- Settlement or subsidence — if one part of the slab has dropped rather than another rising, consult a professional before doing anything.
If you're unsure what's caused the unevenness, get a concreter or structural engineer to take a look first. BRANZ has useful guidance on slab behaviour and moisture-related movement.
Tools You'll Need
For small high spots (think a bump of a few millimetres over a limited area), a angle grinder with a diamond cup wheel is the most accessible DIY option. For larger areas, you may want to hire a concrete floor grinder (a walk-behind machine) from a tool hire outlet.
| Job Size | Recommended Tool |
|---|---|
| Small bump (under 150mm wide) | Angle grinder + diamond cup wheel |
| Medium area (up to ~1m²) | Hired 180–230mm angle grinder or small floor grinder |
| Large area (over 1m²) | Hired walk-behind floor grinder |
You can find suitable grinders and diamond cup wheels at Bunnings or Mitre 10. Always check the wheel is rated for concrete and matches your grinder's disc size.
Step-by-Step: Grinding a High Spot
1. Check the slab first
Tap around the high spot with a hammer. A hollow sound can indicate a delaminated or poorly bonded layer — grinding this may cause further damage. Solid, consistent sound means you're good to proceed.
2. Mark the area
Use chalk or a pencil to mark the high spot. Use a long spirit level or straightedge to identify exactly how far the bump extends.
3. Set up dust control
Concrete dust contains crystalline silica, which is a serious respiratory hazard. Under WorkSafe NZ guidelines, you must control silica dust exposure. Use a grinder with a dust shroud connected to a wet-dry vacuum or HEPA-filter vacuum. Wet grinding (applying water to the surface) is another option to suppress dust.
4. Wear the right PPE
- P2 or P3 respirator (not just a dust mask)
- Safety glasses or goggles
- Hearing protection
- Gloves and sturdy footwear
5. Grind gradually
Work the grinder in overlapping passes, keeping it moving — don't dwell in one spot or you'll create a hollow. Check your progress frequently with a straightedge. Aim to feather the edges so there's no abrupt transition.
6. Clean up and assess
Vacuum or sweep thoroughly, then check the surface again with your level. If the area still feels rough or porous, a concrete sealer can help protect it. Products are available from Bunnings or Mitre 10.
Does This Require a Building Consent?
Generally, grinding down a surface high spot on an existing slab is maintenance work and does not require a building consent under Schedule 1 of the Building Act 2004. However, if the work is part of a larger renovation or affects a structural element, rules may differ. Always confirm with your local council if you're unsure — MBIE Building Performance has a helpful overview of what work requires consent.
Silica Dust: A Serious Health Risk
This is worth repeating. Respirable crystalline silica (RCS) from concrete grinding can cause silicosis, a permanent and potentially fatal lung disease. Even a single heavy exposure can cause harm. Always use dust suppression and a proper respirator — not just a paper dust mask. See WorkSafe NZ for current exposure standards and controls.
When to Call a Professional
Call a qualified concreter if:
- The high spot is larger than about 1m² or the height difference is more than 10–15mm
- The slab sounds hollow when tapped
- You suspect ground movement, heave, or subsidence is the cause
- The slab is in a structural or load-bearing location
- You're grinding near reinforcing steel (reo bar) — exposing reo can lead to corrosion and structural issues
- The area will be covered with tiles or a topping screed and needs to meet a specific tolerance (typically within a flatness tolerance under a straightedge for tiled floors — check the tile manufacturer's specification and confirm the applicable NZS standard with your tiler or concreter)
A professional concreter has access to industrial grinding equipment, dust extraction systems, and the experience to assess whether grinding alone will solve the problem. You can find qualified local concreters on tradedirectory.co.nz.
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