Grinding down high spots on a concrete subfloor is a practical DIY task that can make the difference between a flat, long-lasting floor and one that creaks, cracks, or fails early. With the right equipment, preparation, and safety gear, most homeowners can tackle minor levelling work themselves — though significant unevenness or large areas are best left to a professional flooring installer.
Why a Level Subfloor Matters
Most flooring products — including tiles, vinyl planks, engineered timber, and laminate — require a subfloor that is flat to within 3 mm over a 3-metre span (a common industry benchmark; always check your specific product's installation guide). High spots cause tiles to crack, vinyl to bubble, and floating floors to rock. Grinding removes those high spots efficiently without the mess of adding self-levelling compound everywhere.
Tools and Equipment You'll Need
- Concrete floor grinder (single-disc or planetary) — available for hire from most tool hire shops
- Diamond grinding cup wheels — choose the grit based on hardness of your concrete (coarser for harder concrete)
- Dust shroud and industrial vacuum (wet/dry rated) — essential for dust control
- Straightedge or long spirit level (1.8–3 m)
- Chalk or pencil to mark high spots
- Personal protective equipment (PPE): P2 respirator, safety glasses, hearing protection, steel-capped boots, and knee pads
Safety First — Silica Dust Is Serious
Concrete grinding generates respirable crystalline silica dust, which causes silicosis — a serious and irreversible lung disease. WorkSafe NZ classifies silica dust as a significant health hazard. Always use a P2 (or higher) respirator, never a simple dust mask. Connect your grinder to a vacuum with a HEPA filter, keep the work area ventilated, and wet-grind where possible. If others are nearby, restrict access to the work zone.
Step-by-Step: Grinding Your Concrete Subfloor
1. Assess and Mark High Spots
Use your straightedge or spirit level to identify high spots across the slab. Drag it slowly in multiple directions and mark any areas that rock or show a gap underneath. Circle high spots with chalk — these are your grinding targets.
2. Prepare the Area
Remove all furniture, sweep thoroughly, and check for any protruding fixings (nails, bolts, old adhesive lumps). Chip off large adhesive blobs with a scraper before grinding — large lumps can damage diamond wheels. Seal off doorways with plastic sheeting to contain dust.
3. Set Up the Grinder
Attach the appropriate diamond cup wheel to the grinder. Connect the dust shroud to your industrial vacuum. Plug the grinder into a suitable power circuit — larger grinders (typically 15 A or three-phase) may need a specific outlet. Check the hire company's requirements.
4. Grind the High Spots
Start the grinder and move it in slow, overlapping passes over the marked high spots. Let the machine do the work — don't press down hard. Keep the grinder moving at all times; stopping in one place will create a hollow. Work in a circular or figure-eight pattern, feathering the edges so you don't create a sharp step.
Check your progress frequently with the straightedge. It's easy to over-grind, so take it gradually.
5. Deal with Low Spots
Grinding only removes material — it cannot fill low spots. Once high spots are ground flat, use a self-levelling compound or floor levelling underlayment to fill any remaining dips. Products from Ardex or Mapei are widely used in NZ for this purpose. Follow the manufacturer's mixing and application instructions carefully.
6. Clean Up and Check
Vacuum the entire area thoroughly, then wipe down with a damp mop. Once dry, do a final check with your straightedge. The surface should be flat, clean, and free of dust before any flooring adhesive or underlay goes down.
Choosing the Right Grinder
| Grinder Type | Best For | Typical Hire Cost (guide only) |
|---|---|---|
| Single-disc angle grinder with cup wheel | Small patches, edges, tight spots | Lower cost |
| Walk-behind single-disc grinder | Medium rooms, spot grinding | Moderate |
| Planetary (multi-disc) grinder | Large areas, full-floor prep | Higher cost |
Hire costs vary by region and supplier — check with your local hire shop for current pricing.
NZ Building Code Considerations
Grinding an existing concrete slab is generally maintenance work and does not require a building consent. However, if you're part of a larger renovation that involves structural changes or new wet areas, check with your local council. MBIE Building Performance has guidance on what work requires consent and what falls under Schedule 1 exemptions — always confirm with your council if you're unsure.
For flooring in rental properties, note that Tenancy Services Healthy Homes Standards require floors to be in good condition and reasonably draught-free, so proper subfloor prep is important for landlords too.
When to Call a Professional
Call a qualified flooring installer or concrete grinding specialist if:
- The slab has significant cracking, heaving, or structural movement — grinding won't fix underlying problems
- You need to grind a large area (whole house or commercial space) where a planetary grinder and professional technique are needed for an even result
- The concrete has existing coatings (epoxy, paint, old adhesive) that need specialist removal
- You're unsure about the hardness or condition of the concrete
- The project is part of a larger build requiring licensed tradespeople
A skilled flooring installer will have the right equipment, experience, and knowledge of NZ flooring standards to get the job done efficiently. You can find qualified local flooring professionals on tradedirectory.co.nz.
