Cutting through a concrete slab is achievable as a DIY project for smaller jobs, but it demands careful planning, the right equipment, and a solid understanding of what might be lurking beneath or within the slab. Done carelessly, it can result in serious injury, damaged services, or a job that requires expensive remediation.
Understand What You're Cutting Into First
Before you pick up a saw, you need to know what's inside and underneath the slab. Concrete slabs — particularly in NZ homes built from the 1960s onwards — often contain:
- Reinforcing steel (rebar or mesh) – hitting this unexpectedly can kick back the saw or damage the blade
- In-slab heating pipes – common in modern homes with underfloor heating
- Electrical conduit – cutting through live wiring is extremely dangerous
- Plumbing pipes – especially in bathroom or kitchen slabs
- Post-tension cables – found in some commercial and newer residential slabs; cutting these is extremely hazardous and must only be done by a specialist
If you're unsure what's in the slab, hire a professional to scan it first using a concrete scanning service (ground-penetrating radar). This is money well spent. You can also check your original building plans if they're available — your local council may hold copies.
Choosing the Right Saw and Blade
For most residential slab cutting, you have two main options:
Angle Grinder with Diamond Blade
Suitable for smaller, shallower cuts (up to around 60–70 mm deep depending on the disc size). Widely available at Bunnings or Mitre 10. Use a segmented diamond blade rated for concrete.
Walk-Behind Concrete Cut-Off Saw (Floor Saw)
For cutting through full-depth slabs (typically 100 mm or more), a petrol or electric floor saw is the right tool. These can be hired from most tool hire companies. Use a diamond-tipped blade matched to the aggregate hardness of your concrete — check with the hire company.
Never use a standard abrasive disc on a floor saw — these wear quickly, generate excessive heat, and are far less safe than diamond blades.
Marking Out and Setting Depth
- Use a chalk line or straight edge to mark your cut clearly.
- Set the blade depth to just slightly more than the slab thickness — avoid plunging deeper than necessary.
- Make a shallow guide cut (around 10–15 mm) first, then gradually increase depth in passes. This reduces blade wander and improves control.
Essential PPE and Safety Measures
Concrete cutting is one of the dustier and noisier tasks in construction. WorkSafe NZ is clear that respirable crystalline silica (RCS) dust from concrete is a serious health hazard that can cause silicosis — an irreversible and potentially fatal lung disease.
You must use:
- P2 or P3 respirator (not just a dust mask) — properly fitted
- Safety glasses or a full face shield
- Hearing protection — concrete saws are extremely loud
- Steel-capped boots
- Cut-resistant gloves
- High-visibility or close-fitting clothing — nothing loose that can catch
Dust Control
Always use wet cutting where possible — most floor saws have a water feed attachment. Wet cutting dramatically reduces airborne silica dust. If wet cutting isn't possible, use a saw with a dust extraction shroud connected to an H-class vacuum. Never dry-cut without extraction in an enclosed space.
Ventilation and Working Indoors
If cutting inside, ensure the area is well ventilated. Open windows and doors, and consider using a fan to direct dust away from your breathing zone. Keep other people and pets well clear of the work area.
Dealing with Rebar
If your blade hits rebar, stop and reassess. You can use a diamond blade rated for cutting both concrete and steel (sometimes called a combo blade), but it's often better to cut the concrete away around the rebar and then cut the steel separately with an angle grinder or reciprocating saw. Forcing through rebar with a standard concrete blade can cause dangerous kickback.
Disposal of Concrete Waste
Broken concrete (also called rubble) is heavy — a standard 100 mm slab weighs roughly 240 kg per square metre. Plan how you'll remove and dispose of it. Many NZ councils accept clean concrete rubble at transfer stations, sometimes at a reduced rate. Check with your local council. Concrete can also often be recycled as fill or base material.
Building Consent Considerations
Cutting a concrete slab as part of a larger project (such as adding a bathroom, extending drainage, or structural alterations) may require a building consent. Under Schedule 1 of the Building Act 2004, some minor work is exempt, but you should always confirm with your local council before starting. Visit MBIE Building Performance for guidance on what requires consent.
If the work involves plumbing or drainage, that is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board. Similarly, any work on electrical wiring is restricted and must be done by a registered electrician — check EWRB.
When to Call a Professional
Call a licensed demolition contractor or concrete cutting specialist if:
- The slab is post-tensioned (you may not be able to tell without plans — check first)
- The cut is deep, long, or structural in nature
- You're unsure what services run through or under the slab
- The job is part of a consented building project
- You don't have access to proper dust extraction or wet cutting equipment
A qualified demolition contractor will have the right equipment, insurance, and expertise to complete the job safely and in compliance with NZ regulations. You can find experienced local demolition contractors on tradedirectory.co.nz.
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