Jefferson County, Colorado
Shrink-swell clay limits houses without basements on 17% of Jefferson County's mapped land, a higher share than 66% of US counties. A shallow water table limits basements on 2%.
The soil survey maps 84% of the county's land area; these shares describe the mapped part. 100% of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 65 flood losses here since 2016, 60% of them in buildings with basements. 50% of homes were built before 1980, against 50% nationally; the median home dates from 1980.
Data as of October 13, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, October 13, 2026
of homes built before 1980
Soil under Jefferson County
| Expansive clay (limits slab homes) | 16.9% |
| High water table (limits basements) | 2.1% |
| Septic fields very limited | 77.8% |
| Clay swell (LEP, by area) | 3.0% |
| High-swell land (LEP 6%+) | 17.1% |
| Urban land, not rated | 4.1% |
| Mapped soil | 413,163 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Sphinx | 6% | n/a |
| Nunn | 6% | n/a |
| Ratake | 5% | n/a |
| Denver | 5% | n/a |
| Rogert | 4% | n/a |
| Herbman | 4% | n/a |
How we count · survey areas CO031 (2025-08-29), CO640 (2025-08-29), CO641 (2025-08-29)
What the numbers mean here
Clay swell averages a linear extensibility of 3.0% by land area here, and 17% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 17% of the mapped land.
Clay shrinks as it dries: 100% of the county is in drought on the latest map, and 17% of Jefferson County's mapped land has shrink-swell clay.
Rain over the 12 months to August 2026: 13.13 in, 5.42 in below the 1991-2020 average year of 18.55 in.
USDA rates 78% of the land very limited for septic absorption fields.
Drought
100% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 52 of the last 52 weeks; the most was 100% of its area, in 41 weeks, most recently on the October 13, 2026 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 100% |
| D2 severe or worse | 100% |
| D3 extreme or worse | 44.4% |
| D4 exceptional | 2% |
Flood losses since 2016
65 flood losses since 2016, $987,888 paid. The worst year was 2023, with 21. Foundations of the buildings, where recorded: 60% basements, 16% slabs, 12% crawl spaces, 12% elevated. Median water depth outside the building: 13 in.
| Year | Flood losses |
|---|---|
| 2016 | 3 |
| 2017 | 4 |
| 2018 | 4 |
| 2019 | 3 |
| 2020 | 2 |
| 2021 | 6 |
| 2022 | 2 |
| 2023 | 21 |
| 2024 | 2 |
| 2025 | 4 |
| 2026 to date | 14 |
Rain
| Average year, 1991-2020 | 18.55 in |
| 12 months to August 2026 | 13.13 in (-5.42) |
| 2025 | 16.16 in (-2.39) |
| Wettest year since 1895 | 1969: 25.72 in |
| Driest year since 1895 | 1939: 9.99 in |
| Days with 1 inch or more, normal at EVERGREEN (5 mi away) | 1.9 |
Homes
| Housing units | 253,574 |
| Median year built | 1980 |
| Built before 1980 | 50% |
| Built 2000 or later | 20% |
| Owner-occupied | 71% |
| Median value, owner-occupied | $637,600 |
| Single-family detached | 64% |
What repairs cost
Published ranges from cost guides, not quotes. More sources, and where they disagree.
Nearby counties
| Place | Distance | Expansive clay | High water table |
|---|---|---|---|
| Denver County, CO | 23 mi | 15% | 3% |
| Clear Creek County, CO | 24 mi | 0% | 3% |
| Douglas County, CO | 24 mi | 3% | 2% |
| Gilpin County, CO | 25 mi | 0% | 3% |
| Broomfield County, CO | 28 mi | 46% | 1% |
| Boulder County, CO | 37 mi | 9% | 6% |
| Park County, CO | 41 mi | 2% | 7% |
| Summit County, CO | 47 mi | 4% | 5% |
Sources: USDA SSURGO (August 29, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status