Concrete is one of the most durable building materials around, but it has one unavoidable habit — it cracks. Control joints are the smart way to manage that cracking, directing it to planned locations so your driveway, path, or slab stays looking tidy for years to come. Understanding how they work can save you from costly repairs down the track.
Why Does Concrete Crack in the First Place?
Fresh concrete shrinks slightly as it cures and loses moisture — this is completely normal. Add in New Zealand's variable climate, with hot summers and cold winters causing the concrete to expand and contract, and you have a recipe for random cracking if nothing is done to control it. Soil movement, tree roots, and heavy loads also contribute. Without control joints, cracks appear wherever internal stresses are highest, which is usually somewhere inconvenient and unsightly.
What Exactly Is a Control Joint?
A control joint (sometimes called a contraction joint) is a deliberately created groove or saw cut in a concrete slab. By reducing the thickness of the concrete along a straight line, you create a plane of weakness — a spot where the slab is encouraged to crack neatly underneath the surface, hidden within the joint, rather than randomly across the top. The result looks intentional and is far easier to maintain.
Control joints are different from expansion joints, which use a compressible filler material to allow two separate sections of concrete to move independently without pushing against each other. Both types are important in a well-designed concrete project.
How Deep and How Far Apart Should They Be?
As a general guide, control joints should be cut to roughly one-quarter of the slab's thickness. So for a typical 100 mm residential slab, that means a joint around 25 mm deep. Shallower than this and the joint may not be effective; the crack can bypass it entirely.
Spacing depends on the slab thickness and the concrete mix, but a commonly used rule of thumb is to space joints at intervals of roughly 24 to 30 times the slab thickness. For a 100 mm slab, that works out to joints every 2.4–3.0 metres. Thicker slabs can span greater distances between joints. Your concreter will also consider the shape of the pour — awkward corners and re-entrant angles are prime cracking spots and often need additional joints.
BRANZ provides technical guidance on concrete slab construction that concreters and builders use as a reference for good practice in New Zealand conditions.
Methods Used to Create Control Joints
Saw Cutting
Saw cutting is the most common method for residential work. After the concrete has hardened enough to hold its shape but before it has fully cured (typically within the first 24 hours), a diamond-blade saw is used to cut the joint to the correct depth. Timing is critical — cut too early and the edges crumble; too late and the concrete may already have cracked randomly.
Tooled Joints
On smaller pours or decorative work, a grooving tool can be pressed into the fresh concrete while it is still workable to create the joint. This is faster but requires skill to achieve a clean, consistent result.
Plastic or Timber Inserts
Some concreters place pre-formed plastic or timber strips into the wet concrete. These remain in place permanently and act as the joint. They are popular for decorative finishes.
Where Are Control Joints Needed?
Control joints are relevant in a wide range of residential concrete work:
- Driveways — particularly long runs and wide aprons
- Paths and footpaths — typically jointed every couple of metres
- Patios and outdoor entertaining areas
- Garage floors and workshop slabs
- Internal concrete floors — including polished concrete
For any concrete work that forms part of a building (such as a garage floor attached to a dwelling), the NZ Building Code and standards such as NZS 3109 (concrete construction) may apply. Always check whether a building consent is required for your project.
What Happens If Control Joints Are Left Out or Done Poorly?
Skipping control joints — or placing them too far apart, too shallow, or too late — almost guarantees random cracking. Common consequences include:
| Problem | Cause |
|---|---|
| Random surface cracks | No joints or joints spaced too far apart |
| Cracks bypassing joints | Joints cut too shallow |
| Crumbling joint edges | Saw cut too early while concrete was green |
| Corner cracking | No diagonal joint at re-entrant corners |
Once random cracking occurs, repairs are possible but rarely invisible. Prevention through proper jointing is always the better option.
Filling and Maintaining Control Joints
Outdoor control joints are often left open or filled with a flexible polyurethane or silicone sealant to prevent water, grit, and weeds from getting in. Products from manufacturers like Sika are commonly used for joint sealing in New Zealand. Sealant should be inspected every few years and replaced when it cracks or pulls away from the edges.
Indoor polished concrete joints are sometimes filled with a rigid epoxy filler for a seamless look, though this only works well where movement is minimal.
When to Call a Professional
While a homeowner can tool simple joints into a small DIY pour, most residential concrete work benefits significantly from a qualified concreter's experience. Getting the mix design, thickness, reinforcement, joint placement, and timing right all at once is harder than it looks. Mistakes in a concrete slab are expensive to fix.
For any structural slab, retaining wall base, or concrete work requiring a building consent, you should engage a professional. Licensed Building Practitioners (LBP) are required for restricted building work in New Zealand. You can find experienced local concreters on tradedirectory.co.nz.