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Round Rock, Texas
Inside Round Rock's city limits, shrink-swell clay limits houses without basements on 56% of mapped land, higher than 62% of Texas cities. A shallow water table limits basements on under 1%.
That clay share is close to Williamson County's 60%. FEMA's flood data list 11 flood losses since 2016 in the 3 ZIP codes overlapping the city. Of its 51,143 homes, 11% were built before 1980; the median dates from 2002.
Data as of September 22, 2026 · sources · in Williamson County
expansive clay: shrink-swell limits slab homes
high water table: limits basements
flood losses since 2016 in 3 overlapping ZIPs
of homes built before 1980
Soil under Round Rock city limits
| Expansive clay (limits slab homes) | 55.9% |
| High water table (limits basements) | 0.1% |
| Septic fields very limited | 85.1% |
| Clay swell (LEP, by area) | 7.0% |
| High-swell land (LEP 6%+) | 49.4% |
| Mapped soil | 25,240 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Austin | 15% | 93% |
| Eckrant | 14% | 8% |
| Houston Black | 13% | 91% |
| Georgetown | 7% | n/a |
| Castephen | 6% | n/a |
| Heiden | 6% | 80% |
What the numbers mean here
Clay swell averages a linear extensibility of 7.0% by land area here, and 49% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 56% of the mapped land.
Clay shrinks as it dries: 100% of Williamson County is in drought on the latest map, and 56% of Round Rock's mapped land has shrink-swell clay.
USDA rates 85% of the land very limited for septic absorption fields.
Drought in Williamson County
100% of Williamson County is in drought (D1 or worse) on the September 22, 2026 map. County detail.
Flood losses by ZIP code
FEMA flood records since 2016 in the ZIP codes that overlap Round Rock. ZIP areas reach past city limits.
| ZIP | Share of city land | Flood losses | Paid |
|---|---|---|---|
| 78665 | 39% | 5 | $7,708 |
| 78664 | 32% | 4 | $156,147 |
| 78681 | 28% | 2 | $0 |
Homes in Round Rock
| Housing units | 51,143 |
| Median year built | 2002 |
| Built before 1980 | 11% |
| Built 2000 or later | 55% |
| Owner-occupied | 56% |
| Median value, owner-occupied | $418,600 |
| Single-family detached | 64% |
Nearby cities
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Pflugerville | 7 mi | 66% | under 1% |
| Hutto | 7 mi | 70% | under 1% |
| Cedar Park | 10 mi | 48% | 0% |
| Georgetown | 10 mi | 42% | under 1% |
| Weir | 11 mi | 57% | under 1% |
| Leander | 12 mi | 48% | under 1% |
| Manor | 13 mi | 84% | 1% |
| Taylor | 14 mi | 81% | under 1% |
Sources: USDA SSURGO (August 5, 2026) · US Drought Monitor (September 22, 2026) · FEMA NFIP v3 (September 9, 2026) · Census ACS 2020-2024. Methods · Data status