No big crack running across the driveway.

No section had dropped.

Nothing looked like it was about to fail.

But along the edge of the concrete, I noticed a gap underneath the slab.

At first, it looked like it might only be a small area where the soil had settled or washed away.

Then I pushed my tape measure into the opening.

It kept going.

All the way across under the driveway.

That small gap at the edge was the warning.

The support underneath the slab had disappeared across the entire width of the driveway.

From the top, the driveway didn't look particularly alarming.

There wasn't a major crack running through it. No section had noticeably dropped. You could drive across it every day and probably never think twice about it.

But along one edge of the concrete, I could see underneath the slab.

To get a better idea of how far the gap extended, I pushed my tape measure into the opening.

I expected it to stop.

It didn't.

I was able to push the tape measure underneath the driveway across its entire width.

The gap wasn't just along the edge.

The loss of support extended beneath the entire driveway.

Concrete is strong, but a driveway is designed to sit on a properly prepared and supported base. When that support disappears, sections of the slab can essentially be left spanning an empty space.

And yet from above, there still wasn't much to see.

No dramatic crack.

No collapsed section.

No obvious failure.

The driveway hadn't failed yet. But one of the things helping keep it from failing was disappearing underneath it.

That's exactly why catching a problem like this early matters.

The likely cause was sitting right beside the void.

The downspout was missing its extension.

Instead of carrying roof water away, it was dumping water directly beside the driveway.

Every rainfall sent more water into the same area, gradually washing away the material beneath the slab.

A small missing downspout extension was helping create a much larger problem.

If you find a gap underneath a driveway or sidewalk, don’t just fill the visible opening.

Find out why the material disappeared.

Check for:

A concrete contractor can determine how extensive the void is and whether the slab can still be supported and repaired.

Depending on the condition, mud-jacking or foam jacking may be used to fill the void and restore support beneath the concrete before more serious damage occurs.

Take a quick look at your driveway after a heavy rain.

Make sure your downspouts are carrying water away from the concrete and foundation.

Then check along the slab edges for:

If you see a gap, slide a tape measure underneath the slab to see how far the void extends. You may discover the visible opening is only a small part of the problem.

Catching the void before the concrete moves is a lot better than finding it after the driveway drops.

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Want to learn more about Driveway Gaps?

This Old House — How to Lift a Sinking Concrete Slab

Concrete Network — How to Lift a Concrete Slab

Concrete Network — 7 Signs It’s Time for Driveway Replacement

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Next week, I’ll show you what I found behind that simple symptom — and why flickering lights are sometimes worth paying attention to. ⚡