Grinding a concrete subfloor flat is a practical DIY task that can save you money before laying tiles, vinyl, or timber flooring — but it does require the right equipment, proper preparation, and strict attention to safety. Done correctly, you'll end up with a smooth, level surface that gives your new floor the best possible start.
Why Flatness Matters
Most flooring manufacturers specify a flatness tolerance for concrete subfloors — commonly no more than 3–5 mm variation over a 1.8 m straightedge. High spots, ridges, or old adhesive lumps can cause tiles to crack, vinyl to telegraph bumps, or timber boards to squeak and flex. Grinding removes these high points efficiently. For low spots or widespread unevenness, grinding alone won't help — you'll also need a self-levelling compound (see below).
Tools and Materials You'll Need
- Angle grinder (125 mm or 180 mm) with a diamond cup wheel or turbo grinding disc — available from Bunnings or Mitre 10
- For larger areas: a walk-behind concrete grinder (hire from most tool hire shops)
- Long straightedge or spirit level (1.8 m minimum)
- Chalk or marker to identify high spots
- P2-rated dust mask or half-face respirator (concrete dust contains silica — see safety note below)
- Safety glasses and hearing protection
- Heavy-duty work gloves
- Vacuum with a HEPA filter, or a wet/dry vac
- Self-levelling compound (for low spots after grinding) — brands like Ardex or Mapei offer NZ-stocked products
Step-by-Step: Grinding Your Concrete Subfloor
1. Clear and Clean the Area
Remove all furniture, skirting boards if needed, and any old adhesive or paint. Sweep thoroughly. A clean surface lets you see exactly what you're dealing with.
2. Identify High Spots
Use your straightedge or spirit level to sweep across the floor in multiple directions. Mark any high spots, ridges, or humps with chalk. Pay particular attention to doorways, joins between slabs, and areas where old adhesive has built up.
3. Set Up Dust Control
Concrete grinding produces respirable crystalline silica dust, which is a serious health hazard with repeated exposure. Open windows and doors, seal off other rooms with plastic sheeting, and connect your grinder to a HEPA vacuum if possible. WorkSafe NZ has clear guidance on silica dust controls — read it before you start.
4. Grind the High Spots
Fit your diamond cup wheel to the angle grinder. Work in overlapping circular or sweeping passes over each marked high spot. Don't dwell in one place — keep the grinder moving to avoid creating dips. Check your progress frequently with the straightedge. For long ridges or large areas, a walk-behind grinder will give a more consistent result and is much less tiring.
5. Vacuum as You Go
Stop regularly to vacuum up dust and debris. This lets you see the surface clearly and keeps silica exposure down.
6. Check Flatness Again
Once you've ground all the marked areas, re-check with your straightedge across the whole floor. Re-grind any remaining high spots. Take your time here — it's much easier to fix now than after your flooring is down.
7. Address Low Spots
Grinding only removes material — it can't fill hollows. For low spots greater than about 3 mm, apply a self-levelling or floor-levelling compound following the manufacturer's instructions. Products from Ardex and Mapei are widely used by NZ flooring installers and are available through PlaceMakers.
8. Final Clean
Vacuum the entire floor thoroughly, then wipe down with a damp mop. Allow to dry completely before laying any flooring or adhesive.
Safety Note: Silica Dust Is a Real Risk
This is the most important part of this job. Concrete contains crystalline silica, and grinding releases fine particles that can cause serious, irreversible lung disease (silicosis) with repeated exposure. Always wear a P2-rated respirator as a minimum — a basic dust mask is not sufficient. Ideally, use dust extraction connected directly to your grinder. Review WorkSafe NZ's guidance on managing silica dust on worksites before you begin.
NZ Building Code Considerations
Subfloor preparation itself doesn't typically require a building consent, but the flooring system above it may need to comply with the NZ Building Code — particularly clause E3 (Internal Moisture) if you're in a ground-floor slab situation. If you're unsure whether your concrete slab has adequate moisture protection, check with your local council or a flooring professional before laying moisture-sensitive products like timber. BRANZ publishes useful guidance on subfloor moisture assessment.
When to Call a Professional
Consider hiring a qualified flooring installer or concrete grinding specialist if:
- The floor has widespread unevenness across a large area (a walk-behind grinder and professional technique will give far better results)
- You find cracks, heaving, or structural movement in the slab — this needs assessment before any flooring goes down
- The existing floor has old vinyl or adhesive that may contain asbestos (common in NZ homes built before the mid-1980s) — do not grind or disturb this material; get it tested and removed by a licensed asbestos removalist first
- You're not confident working with power tools or managing dust safely
You can find experienced local flooring installers and concrete grinding specialists on tradedirectory.co.nz.
