Short answer
Expansive clay swells when it gets wet and shrinks when it dries. A slab sitting on it rises and drops unevenly, most at the edges, which cracks the slab and the house above. In our USDA soil calculation, 70% of Dallas County's rated soil area is high or very high shrink-swell, versus about 1% in Franklin County, Ohio.
Key facts
- In southeast Texas, foundation movement damage to homes is most often due to changes in the moisture content of expansive soils, per FPA's repair committee.[1]
- Our calculation from USDA soil survey data: Dallas County 70%, Tarrant 60%, Bexar 47%, Travis 45%, Harris 42% and Cook County, Illinois 37% of rated soil area has linear extensibility of 6% or more.[3]
- Jones and Holtz (1973) estimated that shrinking and swelling soils cause at least $2.3 billion a year in US damage to houses, buildings, roads and pipelines.[5]
How clay moves a slab
Some clay minerals take water into their structure and swell, then shrink as they dry. Soil scientists measure this as linear extensibility (LEP): the % change in a soil clod's length between moist and oven-dry[2]. USDA classes it as low (under 3%), moderate (3% to 5.9%), high (6% to 8.9%) and very high (9% or more)[2]. USDA notes that moderate to very high shrink-swell "can damage buildings, roads, and other structures"[2].
A slab on grade sits right on this soil. The soil under the middle of a house stays fairly steady. The soil at the edges wets and dries with rain, drought, sprinklers, gutters and tree roots. So the edge moves more than the center. In wet seasons the edge can rise (edge heave). In droughts it can drop (edge settlement). Leaks under the slab can make one area swell on its own. FPA lists broken sewer or water pipes, trees close to the slab, extreme seasonal moisture changes and poor drainage as reasons repairs fail on clay[1].
Builders design for some of this. NAHB guidance says most foundations are designed for up to 1 inch of soil movement, and slabs on highly expansive soil can be designed for up to about 4 inches[4].
Where expansive clay is worst in our launch states
We pulled USDA soil survey data for 18 metro counties and computed the share of soil area rated high or very high shrink-swell[3]. These are our numbers, not a figure USDA publishes.
| County (main city) | High or very high (LEP 6% or more) | Very high (LEP 9% or more) |
|---|---|---|
| Dallas, TX (Dallas) | 70% | 45% |
| Tarrant, TX (Fort Worth) | 60% | 33% |
| Bexar, TX (San Antonio) | 47% | 21% |
| Travis, TX (Austin) | 45% | 34% |
| Harris, TX (Houston) | 42% | 21% |
| Sangamon, IL (Springfield) | 40% | 0% |
| Cook, IL (Chicago) | 37% | 0% |
| Lucas, OH (Toledo) | 21% | 0% |
| Hamilton, OH (Cincinnati) | 14% | 0.3% |
| Franklin, OH (Columbus) | about 1% | 0% |
| Cuyahoga, OH (Cleveland) | about 1% | 0% |
The full table for all 18 counties, with acreage, is on our soil shrink-swell by county page.
How we calculated it. We queried USDA NRCS Soil Data Access, the public interface to the SSURGO soil survey[3]. For each soil component, we took the highest LEP in any layer within the top 150 cm. We weighted each component by its map unit acres times its share of the map unit. The share is the rated acres with LEP of 6% or more, divided by all rated acres. Urban-land and water components have no LEP rating, so they drop out. The share describes the soil-rated part of the county, not every parcel.
Two honest caveats. First, Ohio's low share for high LEP does not mean Ohio soil is inert. In Franklin County, 95% of rated area is moderate or higher, and USDA says moderate can still damage structures[2]. Second, the Texas pattern matches what Houston-area engineers report: FPA's repair committee says damage there "is most often due to changes in moisture content of expansive soils"[1].
Myth: "Expansive soil causes $15 billion a year in damage"
This number is repeated across contractor websites, usually with "more than floods, hurricanes, tornadoes and earthquakes combined." We could not find a primary source for it.
The study those claims trace back to is Jones and Holtz, "Expansive Soils: The Hidden Disaster," in ASCE's Civil Engineering magazine, August 1973. It estimated at least $2.3 billion a year in damage to houses, buildings, roads and pipelines, in 1973 dollars[5]. It is a real figure, but it is more than 50 years old. Nobody should quote a current national total without a current study. The same goes for "1 in 4 homes" claims we could not trace.
What repair can and cannot do on clay
Piers handle settlement, not heave. FPA states that the repair piers most used on Houston-area homes are not tied to the grade beams and "cannot prevent foundation uplift due to heaving soils." Heave after repair "is not generally considered to be a failure of the piers"[1]. If heave is the concern, FPA describes lifting the whole ground floor above the soil's maximum potential rise, with piers deep enough to resist uplift[1]. That is a bigger, engineer-designed job.
Piers must get below the active zone. The active zone is the depth where soil moisture still swings with the seasons. Shallow piles can keep moving as moisture changes, and drought can release the grip of shallow piles[1]. See pier types compared.
Moisture control is maintenance, not repair. FPA classes these as maintenance systems[1]:
| Measure | What it does | FPA caution[1] |
|---|---|---|
| Root barrier | Blocks roots from drying soil under the slab | Chemical barriers lose effect after about 10 years; cutting big roots can cause local heave or settlement as they decay |
| Vertical or horizontal moisture barrier | Evens out moisture under the slab edge | A leak trapped between barrier and slab can move the slab |
| Perimeter watering | Keeps edge soil from drying out | Continuously saturated soil harms the foundation and piers |
| Drainage (gutters, grading) | Moves stormwater away | Needed for most repairs to perform |
When to call a pro
- New cracks and sticking doors after a drought or an unusually wet season: call a structural engineer.
- Your engineer sees heave rather than settlement: ask for a design that addresses uplift before buying piers.
- You plan to plant or remove a large tree near the slab: ask an engineer or arborist first.
- A slab leak is suspected: call a licensed plumber.
More questions
Should I water my foundation?
FPA lists perimeter watering as a maintenance system that can help keep soil moisture even. It also warns that continuously saturated soil harms the foundation and repair piers. Aim for steady, not soaked, and follow your engineer's advice.
Does a high county share mean my lot has expansive clay?
No. A county share is an average across mapped soils. Your lot may differ, and fill dirt from construction can change it. A geotechnical test or an engineer's review answers it for one house.
Sources
- FPA-RC-01-0: Post Foundation Repair Performance of Residential and Other Low-Rise Buildings on Expansive Soils (Jan 2004), Foundation Performance Association, Repair Committee. Accessed 2026-10-06.
- National Soil Survey Handbook, Part 618.42 (linear extensibility), USDA Natural Resources Conservation Service. Accessed 2026-10-06.
- Soil Data Access (SSURGO) tabular service, queried by Strive LeadGen on 2026-10-06; calculation is ours, USDA Natural Resources Conservation Service. Accessed 2026-10-06.
- Misconceptions about the common crack (Builder Education Series), National Association of Home Builders (Walt Keaveny, PE, PG). Accessed 2026-10-06.
- Expansive Soils: The Hidden Disaster (Jones Jr., D.E. and Holtz, W.G., Civil Engineering 43(8):49-51, Aug 1973), bibliographic record, Transportation Research Board, TRID database. Accessed 2026-10-06.
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