When heavy rain undercuts a concrete patio, walkway or driveway apron, the slab itself is rarely the problem. Runoff carries the soil out from underneath, the concrete is left bridging an empty space, and eventually it cracks or drops. Replacing the slab without fixing where the water goes means doing the same job again after the next storm. Here is how to tell whether it has happened to you.

What “undercut” actually means

Concrete slabs poured on grade are not structural bridges. They sit on compacted soil and rely on it for support across the whole underside. When fast-moving water finds a path along the edge of a slab, it carries fine soil away with it, particle by particle, and leaves a void behind.

Nothing looks wrong at first. The slab is strong enough to span a small void on its own, so it holds. Then the void grows, a car parks on the driveway apron or someone sets a grill on the patio, and the concrete cracks along the line where the support ended. That crack was months in the making even though it appeared in a second.

Why Tri-State soil does this so reliably

Two things work against homeowners here. Much of the soil across the Tri-State holds water rather than draining it,which means heavy rain sits on the surface and moves sideways instead of soaking in. And this region gets storms that drop a lot of water in a short time. In August 2026 the area took four rounds of severe storms in a single week, with flood watches across Hamilton, Warren, Clermont and Butler counties.

Water that cannot go down goes somewhere, and it follows the easiest path it can find. The seam between a concrete slab and the soil beside it is one of the easiest paths on the property.

Where it shows up first

• Driveway aprons, where the driveway meets the street or the garage floor. Vehicle weight finds the void quickly.
• Patios and walkways that sit next to a downspout, particularly ones where the downspout empties onto or beside the slab.
• Steps and stoops pulling away from the house, often with a widening gap you can see daylight through.
• Pool decks and sidewalks on any slope, where runoff naturally concentrates.
• Garage floors, usually showing up as a slab that has dropped slightly at the door.

How to check yours without digging anything up

Walk the property after the ground has dried out and look for these:

• A gap under the slab edge. Get low and look sideways along the edges. If you can see or feel a void under the lip, soil has already moved.
• A hollow sound. Tap along the slab with a hammer handle or the toe of your boot. Supported concrete sounds solid. Unsupported concrete sounds noticeably hollow, and the change is obvious once you have heard both.
• Soil or mulch that migrated. If landscaping material ended up ten feet downhill after the storm, that is the water telling you which way it ran.
• A slab now tilting toward the house. This is the one to take seriously. A patio that has settled toward the foundation is now delivering every future rainfall directly to your basement wall.
• New cracks that were not there in July. Particularly cracks running parallel to the edge of the slab rather than across it.

The slab is the symptom

The reason this matters is that a washed-out patio and a wet basement are usually the same problem wearing two different faces. Water that undercuts a slab beside the house is water that is already sitting against the foundation. Fix the concrete and ignore the drainage, and you have spent money on the visible half of the issue.

Any honest repair starts with where the water comes from. Roof runoff is the most common source by a wide margin. A single downspout can put a remarkable volume of water on one spot during a heavy Ohio storm, and if it discharges next to a slab, that slab was always going to fail eventually. Extending the downspout six to ten feet away, or piping it underground to daylight, removes the cause.

The grade is the second half. The ground should fall away from the house and away from the slab. Over ten or fifteen years mulch, settling and new landscaping quietly flatten or reverse that, and nobody notices until a storm demonstrates it.

What repair usually involves

Once the drainage is sorted, the concrete itself has two broad paths. If the slab is sound and has simply lost support, the void can be filled and the slab brought back to level. If the concrete is cracked through, badly settled, or was in poor condition before the storm, replacing that section is the more sensible spend.

Either way the sequence is the same. Deal with where the water goes, restore and compact the base, then handle the concrete. Doing it in the other order is how people end up paying twice.

Questions homeowners ask about washed-out concrete

How urgent is washed-out concrete next to a house?

A hollow patio in the middle of the yard can wait. A slab that has settled toward your house cannot, because it is now channelling rainwater at the foundation, and that is a more expensive problem than the concrete. A gap at a step or stoop that someone could catch a foot in should also be dealt with promptly.

Can I just fill the gap with sand or gravel myself?

You can, and it will wash out again in the next storm if the water still runs the same way. Filling the void without changing the drainage treats the hole rather than the cause. If the void is small and the drainage is genuinely fine, filling and compacting it is reasonable.

Does the whole driveway need replacing?

Usually not. Undercutting tends to affect specific sections, most often the apron and the edges, and those sections can be dealt with on their own. An inspection tells you how far the void actually extends, which is often smaller than homeowners fear and occasionally larger.

Who repairs concrete undercut by storm runoff in Cincinnati?

Call Huy Contracting at 513-521-4489. Huy Contracting repairs concrete patios, walkways, driveway aprons and steps undercut by storm runoff across Hamilton, Clermont, Butler and Warren counties, and deals with the grading and drainage that caused it rather than just the slab.

Will this happen again?

Only if the water still runs the same way. Concrete that is properly bedded on compacted base, with roof water directed away and a grade that falls away from the house, holds up fine through Ohio storms. The slabs that fail twice are the ones where only the concrete got fixed.

If you want someone to look at it

Huy Contracting is a veteran-owned residential contractor with more than 25 years of experience, based on Galbraith Road in Cincinnati and working right across the Tri-State. Call 513-521-4489 and someone comes out the same day to walk the property, and the assessment is free. If the storms also got into the house, our page on storm damage repair in Greater Cincinnati covers the rest, and water in your basement after a Cincinnati storm is worth reading if any of it ended up downstairs.