How to Cut Expansion Joints in an Existing Concrete Slab

Cutting expansion (control) joints into an existing concrete slab is a practical way to manage cracking caused by thermal movement, moisture changes, and ground settlement — all common concerns in New Zealand's varied climate. While the job is DIY-friendly for small areas, it requires the right equipment, careful planning, and a solid understanding of safe working practices.

What Are Expansion Joints and Why Do They Matter?

Expansion joints (more precisely called control joints when saw-cut into a slab) are deliberate, straight grooves cut into concrete to create a weakened plane. When the slab moves or shrinks, it cracks along this line rather than randomly across the surface. Without them, unsightly and structurally problematic random cracking is almost inevitable, particularly in larger slabs exposed to NZ's sun, frost, and moisture cycles.

Tools You'll Need

• Angle grinder with a diamond blade (suitable for small jobs or tight areas)
• Concrete cut-off saw (walk-behind) — ideal for longer, straighter cuts
• Chalk line or straight edge for marking cut lines
• Tape measure
• PPE: safety glasses, hearing protection, P2 dust mask or half-face respirator, steel-capped boots, gloves
• Wet-cutting attachment or vacuum to manage silica dust (see safety note below)

For larger areas, hiring a walk-behind concrete saw from a hire centre such as Bunnings or Mitre 10 is more efficient and produces cleaner cuts than an angle grinder.

Planning Your Joint Layout

Before you cut, plan your joint pattern carefully:

• Spacing: As a general guide, control joints are spaced at intervals of roughly 24–30 times the slab thickness. For a 100 mm slab, that's approximately 2.4–3.0 m apart. Always check against your specific slab design or consult a concreter.
• Pattern: Aim for square or slightly rectangular panels — avoid long, narrow strips, which are more prone to curling.
• Existing features: Work around drains, posts, and penetrations. Joints should run toward corners and penetrations where stress concentrates.
• Snap a chalk line along each planned cut before you start the saw.

How Deep to Cut

The standard rule of thumb is to cut to a depth of one-quarter of the slab thickness. For a typical 100 mm residential slab, that means cutting approximately 25 mm deep. This creates a weakened plane without cutting through reinforcing mesh, which should remain intact to hold the slab together.

Important: If you're unsure of your slab thickness or whether it contains reinforcing steel, probe a small area carefully or consult a professional before cutting.

Step-by-Step Cutting Process

1. Mark your lines clearly with a chalk line.
2. Set up dust control — use wet cutting where possible, or attach a vacuum shroud to your grinder. Concrete dust contains crystalline silica, which causes serious lung disease. Check WorkSafe NZ guidelines on silica dust before starting.
3. Set blade depth on your saw to the required depth (e.g., 25 mm for a 100 mm slab).
4. Make a steady, single pass along each chalk line. Don't rush — let the blade do the work.
5. Clean out the joint with a stiff brush or compressed air once cutting is complete.
6. Seal or fill the joint if required (see below).

Filling and Sealing the Joints

Once cut, joints can be left open or filled depending on the application:

• Indoor slabs: A flexible polyurethane or polysulfide sealant is typically used to keep out debris and moisture. Products from Sika are widely used in NZ for this purpose.
• Outdoor slabs: A UV-stable, flexible joint sealant is recommended to handle NZ's temperature extremes.
• Garage and driveway slabs: A backer rod inserted before sealant helps control sealant depth and improves performance.

Always follow the manufacturer's instructions for the specific sealant product you choose.

NZ Building Code Considerations

For most residential concrete work, cutting control joints into an existing slab is generally considered maintenance and does not require a building consent. However, if you're doing broader concrete work — such as laying a new slab or altering a structure — you may need consent under the NZ Building Code. Always confirm with your local council, as Schedule 1 exemptions vary by situation.

For guidance on concrete slab construction standards, BRANZ publishes technical resources relevant to NZ conditions.

Safety Note

Cutting concrete generates respirable crystalline silica dust, which can cause silicosis — a serious and irreversible lung disease. Always:

• Use wet cutting or an effective dust extraction system
• Wear a properly fitted P2 (minimum) respirator
• Work in a well-ventilated area
• Keep bystanders clear of the work zone

Refer to WorkSafe NZ for current silica dust exposure guidelines.

When to Call a Professional

Consider hiring a qualified concreter if:

• Your slab is large (over 20–30 m²) or the joint layout is complex
• You're unsure of the slab thickness, reinforcing layout, or existing joint positions
• The slab is part of a structural element (e.g., a foundation or retaining wall base)
• You don't have access to appropriate dust control equipment

A licensed concreter will have commercial-grade wet-cut saws, experience with joint layout, and the right PPE. You can find qualified local concreters on tradedirectory.co.nz.

Sources & Further Reading

• WorkSafe NZ – Silica Dust
• BRANZ – Building Research
• MBIE Building Performance – NZ Building Code
• Sika NZ – Concrete Joint Sealants
• Bunnings NZ – Concrete Tools & Hire
• Mitre 10 NZ – Concrete Supplies