How to Seal Gaps Between Concrete Driveway Slabs Yourself

Sealing the gaps between concrete driveway slabs is a straightforward DIY job that most homeowners can tackle over a weekend. Done properly, it stops water, weeds, and debris from getting underneath your slabs and causing long-term damage. Here's how to do it right, New Zealand conditions in mind.

Why Gaps in Concrete Slabs Matter

Concrete driveways are typically laid in individual slabs with expansion joints (also called control joints) between them. These gaps are intentional — they allow the concrete to expand and contract with temperature changes, which is especially important in NZ where we can see significant seasonal temperature swings. However, over time the original filler material breaks down, leaving open gaps that can:

• Allow rainwater to seep underneath and erode the base
• Encourage weed and grass growth
• Create trip hazards as slabs shift
• Accelerate cracking in the concrete itself

Sealing these joints regularly is good preventive maintenance.

What You'll Need

Before you start, gather the following:

• Angle grinder or oscillating tool (for removing old sealant)
• Wire brush or stiff broom
• Compressed air or leaf blower
• Backer rod (closed-cell polyethylene foam rope — available at Bunnings or Mitre 10)
• Self-levelling polyurethane or silicone joint sealant rated for concrete driveways
• Caulking gun
• Masking tape (optional, for neat edges)
• Safety glasses and gloves

Choosing the Right Sealant

For driveway expansion joints, you want a flexible, self-levelling sealant that can handle foot and vehicle traffic. The two main options are:

For most NZ driveways, a self-levelling polyurethane sealant is the preferred choice — it flows into the joint and bonds well to concrete. Check the product datasheet to confirm it's suitable for horizontal joints and vehicle traffic.

Step-by-Step: How to Seal the Gaps

Step 1 — Remove Old Sealant and Debris

Use an angle grinder with a grout removal blade, or an oscillating multi-tool, to cut out any old sealant, crumbling filler, or deteriorated material. Follow up with a wire brush to scrub the sides of the joint clean. Remove all loose material, dirt, and vegetation — weeds must be pulled out by the roots.

Step 2 — Clean the Joint Thoroughly

Blow out all dust and debris with compressed air or a leaf blower. For stubborn dirt or oil stains, scrub with a concrete cleaner and rinse well. Allow the joint to dry completely — do not apply sealant to a damp joint, as this will prevent proper adhesion.

Step 3 — Install Backer Rod

If your gap is deeper than about 10–12 mm, press a backer rod into the joint first. Choose a backer rod slightly wider than the gap so it sits snugly. Push it down to leave roughly 6–10 mm of depth for the sealant above it. The backer rod prevents the sealant from bonding to the bottom of the joint (a "three-point bond" reduces flexibility and causes premature failure).

Step 4 — Apply Masking Tape (Optional)

For a tidy finish, run masking tape along both edges of the joint. This is especially worthwhile on a light-coloured driveway where sealant smears will show.

Step 5 — Apply the Sealant

Load your caulking gun and cut the nozzle at a slight angle to match the joint width. Apply the sealant in a steady, continuous bead, slightly overfilling the joint. For self-levelling products, it will settle on its own. For non-sag products, use a wet finger or a jointing tool to smooth it flush with the surface.

Work in sections and avoid applying sealant in direct hot sun or when rain is forecast — most products need at least a few hours to skin over.

Step 6 — Remove Tape and Allow to Cure

Peel masking tape off while the sealant is still wet. Allow the sealant to cure fully before driving over it — check the manufacturer's datasheet, but this is typically 24–72 hours depending on the product and weather conditions.

Tips for a Long-Lasting Result

• Don't skip the backer rod — it's the single biggest factor in sealant longevity
• Seal joints in mild, dry weather (10–25°C is ideal)
• Plan to re-inspect and reseal every 3–5 years depending on traffic and UV exposure
• If cracks have appeared in the slab itself (not just the joints), this may indicate a deeper structural issue worth getting assessed

When to Call a Professional

Sealing expansion joints is generally DIY-friendly and does not require a building consent. However, you should call a qualified driveway or concrete professional if:

• Slabs have shifted, heaved, or sunk — this points to a base failure that sealant alone won't fix
• Cracks are wide, growing, or running through the slab rather than along joints
• You're dealing with a large area or commercial driveway
• You're unsure whether the original construction met the NZ Building Code requirements for your property

A qualified driveway layer can assess whether your driveway needs repair, resurfacing, or just maintenance sealing. You can find experienced local driveway professionals on tradedirectory.co.nz.

Sources & Further Reading

• BRANZ — Concrete Driveways and Paths
• MBIE Building Performance — NZ Building Code
• Sika NZ — Concrete Joint Sealants
• Bunnings NZ — Sealants and Caulking
• Mitre 10 NZ — Concrete Repair Products