Boulder County, Colorado
USDA's soil survey flags shrink-swell clay on 9% of Boulder County's mapped land and a shallow water table under basements on 6%, both below the national shares of 15% and 29%. 100% of the county is in drought on the October 13, 2026 Drought Monitor map.
FEMA's flood data list 55 flood losses here since 2016, 69% of them in buildings with basements. 42% of homes were built before 1980, against 50% nationally; the median home dates from 1986.
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 Boulder County
| Expansive clay (limits slab homes) | 9.1% |
| High water table (limits basements) | 6.1% |
| Septic fields very limited | 74.7% |
| Clay swell (LEP, by area) | 2.5% |
| High-swell land (LEP 6%+) | 10.6% |
| Mapped soil | 471,600 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Nunn | 6% | n/a |
| Ascalon | 4% | n/a |
| Valmont | 3% | n/a |
| Juget | 3% | n/a |
| Nederland | 2% | n/a |
| Colby | 2% | n/a |
How we count · survey areas CO642 (2025-08-29), CO643 (2025-09-02), CO645 (2025-08-29), CO651 (2025-08-29)
What the numbers mean here
Rain over the 12 months to August 2026: 14.85 in, 5.80 in below the 1991-2020 average year of 20.65 in.
USDA rates 75% of the land very limited for septic absorption fields.
With shrink-swell clay on 9% and a shallow water table on 6% of the land, the soil survey points to neither problem across most of Boulder County; conditions still vary lot by lot.
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 46 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 | 100% |
| D4 exceptional | 24.1% |
Flood losses since 2016
55 flood losses since 2016, $577,176 paid. The worst year was 2022, with 17. Foundations of the buildings, where recorded: 69% basements, 19% slabs, 10% crawl spaces, 2% elevated.
| Year | Flood losses |
|---|---|
| 2016 | 3 |
| 2017 | 4 |
| 2018 | 1 |
| 2019 | 1 |
| 2020 | 3 |
| 2021 | 15 |
| 2022 | 17 |
| 2023 | 7 |
| 2024 | 0 |
| 2025 | 3 |
| 2026 to date | 1 |
Rain
| Average year, 1991-2020 | 20.65 in |
| 12 months to August 2026 | 14.85 in (-5.80) |
| 2025 | 17.72 in (-2.93) |
| Wettest year since 1895 | 1923: 30.09 in |
| Driest year since 1895 | 1939: 11.43 in |
| Days with 1 inch or more, normal at BOULDER (10 mi away) | 3.2 |
Homes
| Housing units | 144,729 |
| Median year built | 1986 |
| Built before 1980 | 42% |
| Built 2000 or later | 26% |
| Owner-occupied | 62% |
| Median value, owner-occupied | $756,300 |
| Single-family detached | 57% |
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 |
|---|---|---|---|
| Gilpin County, CO | 18 mi | 0% | 3% |
| Broomfield County, CO | 21 mi | 46% | 1% |
| Clear Creek County, CO | 32 mi | 0% | 3% |
| Denver County, CO | 36 mi | 15% | 3% |
| Jefferson County, CO | 37 mi | 17% | 2% |
| Grand County, CO | 38 mi | 5% | 6% |
| Larimer County, CO | 39 mi | 6% | 4% |
| Summit County, CO | 51 mi | 4% | 5% |
Sources: USDA SSURGO (September 2, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status