Why is there a gap under my slab edge or footing?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: When the edge of a slab or the bottom of a footing is exposed, or a gap opens beneath it, soil that used to support the foundation has been removed. Running water is the most common cause, but settling fill, collapsing soil and slope movement can do the same. The size of the void and where the water comes from guide what happens next.

What are you seeing?

A slab-on-grade foundation has a thickened outer beam that is meant to be buried in soil, and footings under other foundation types are meant to bear on soil. When soil is lost from beside or beneath that beam, part of the concrete normally below grade becomes visible, or a gap appears where a probe or hand can reach under the edge.

What can cause it?

Water as the usual driver

The U.S. Geological Survey lists subterranean erosion, such as piping (soil carried away by water flowing underground), as a cause of ground movement, along with water leakage from utilities and stream erosion of the base of slopes ¹. Swelling clays can crack concrete drainage channels, and the resulting voids can lead to piping and erosion ². Leaking pipes, pools and similar sources add water that can also worsen swelling damage on clay ². In Oklahoma, soil saturation from rain, lawn watering or sewer leakage is a recognized source of damage through the expansion of soils beneath sidewalks, utility lines and foundations ⁵.

Engineering guidance for houses on expansive soil recommends keeping soil moisture steady, and suggests surrounding the building with paving or removing perimeter planter beds to limit moisture changes ³. See surface drainage and grading and water ponding near foundation.

Slopes and creeks

On sloped or creek-side lots, soil on the downhill side can erode or move away. An Oklahoma Department of Transportation study found that slope, soil texture and land cover strongly affect landslide susceptibility in Oklahoma, and that at one southeastern Oklahoma site, a fat clay slope absorbed rainwater, grew heavy and slid ⁶. See erosion, creeks and slopes.

How is it documented?

1. Probe the void with a thin rod or tape and record how far it reaches under the slab and how tall it is, at several points.
2. Measure the length of exposed or undermined edge.
3. Photograph each point with a ruler or tape in view.
4. Note where downspouts discharge, where the yard slopes toward the house, and any wet spots or leaks.
5. Re-check after heavy rain to see whether the void is growing.

Home inspection standards call for reporting signs of active water penetration and of possible foundation movement ⁴.

What can it lead to?

Where support is lost, that section of the foundation can settle or crack. Check nearby walls and floors for stair-step brick cracks, horizontal cracks at the brick ledge or slab edge and sloping floors.

How is it repaired?

Repair usually has two parts: stopping the water or soil loss, and restoring support. Options include drainage solutions, void and compaction grouting, slope and erosion stabilization, and piering where the edge has already settled.

Related

• Erosion, undermining and washout voids
• Surface drainage and grading
• Soil pulled away from foundation
• Void and compaction grouting
• Drainage solutions