Fixing and Refreshing a Metal Driveway with GAP20

July 13, 2026 · 2 min read

Winter rain is hard on metal driveways. Water sits in low spots, tyres churn them into potholes, and by spring the surface is rutted and loose. The good news is that a few loads of Daltons GAP20 and a bit of compaction will put it right.

What is GAP20?

GAP20 is a crushed aggregate with a maximum stone size of about 20mm, graded all the way down to fine material. That mix of sizes is what makes it so useful: the fines fill the gaps between the larger stones, so when it's compacted it locks together into a firm, stable surface.

It's the go-to product for:

refreshing metal driveways and filling potholes
a base layer under pavers, block walls and garden edging
a base under small concrete slabs and paths
levelling and stabilising uneven ground

Filling potholes

Clear it out. Scoop out standing water, mud and loose stones. A pothole full of slurry will just sink again.
Square up the edges. Cut the sides back to firm material with a spade so the new metal has something to lock against.
Fill in layers. Add GAP20 about 50mm at a time, compacting each layer with a hand tamper (or the back of a sledgehammer) before adding the next.
Finish slightly proud. Leave the patch a little higher than the surrounding surface; traffic will settle it the rest of the way.
Dampen it. Slightly damp GAP20 compacts far better than bone-dry material. A light spray with the hose helps.

Resurfacing a tired driveway

If the whole driveway is loose and rutted, a fresh top layer is the answer.

Rake or grade the existing surface to remove ruts and high spots.
Fix drainage first. Make sure water can run off the driveway rather than pooling on it, ideally with a slight crown in the middle or a gentle fall to one side.
Spread 50–75mm of GAP20 evenly over the surface.
Compact with a vibrating plate compactor (available from most hire centres), working in overlapping passes. Lightly dampen the metal if it's dry.

Building a base for pavers or a slab

For new paths, patios or small slabs, a well-compacted base is what stops the surface cracking or sinking later.

Remove topsoil and soft material until you reach firm ground.
Lay GAP20 in layers of no more than 75–100mm, compacting each layer thoroughly.
For a garden path or patio, 75–100mm of compacted base is typical. Driveways and areas carrying vehicles need more.
For anything structural, or where building consent applies, follow your builder's or engineer's specifications.

Laying pavers on top? See Laying Pavers: Getting the Base Right with Crusher Dust and Sand.

How much do I need?

Work out length × width × depth in metres, then add about 20–25% because GAP20 compacts down. For example, resurfacing a 15m × 3m driveway with 50mm of metal: 15 × 3 × 0.05 = 2.25 m³, plus 25% = about 2.8 m³, so order 3 m³.

GAP20 is heavy, so check your trailer's limit before loading. See How Much Do I Need? for more on calculating volumes.

Quick answers

What's the difference between GAP20 and crusher dust? GAP20 has stones up to 20mm and is the stronger base for driveways and sub-bases. PAP7 (crusher dust) is finer (0–7mm) and is used as a smooth, firm bedding layer under pavers, slabs, pipes and water tanks.

Do I need to compact GAP20? Yes. Uncompacted metal stays loose and moves under tyres. Compaction is what turns it into a firm surface.

When is the best time to do driveway repairs? Late winter and spring, once the worst of the rain has passed but the ground is still slightly damp, which helps compaction.