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Napa County, California

USDA's soil survey flags shrink-swell clay on 14% of Napa County's mapped land and a shallow water table under basements on 6%, both below the national shares of 15% and 29%. None of the county is in drought on the October 13, 2026 Drought Monitor map.

FEMA's flood data list 29 flood losses here since 2016. 57% of homes were built before 1980, against 50% nationally; the median home dates from 1976.

Data as of October 13, 2026 · sources

14%

expansive clay: shrink-swell limits slab homes

6%

high water table: limits basements

0%

in drought, October 13, 2026

57%

of homes built before 1980

Soil under Napa County

Expansive clay (limits slab homes)14.1%
High water table (limits basements)6.0%
Septic fields very limited88.2%
Clay swell (LEP, by area)4.0%
High-swell land (LEP 6%+)12.9%
Mapped soil512,684 acres
Soil seriesShare hereSeries nationally: expansive clay
Bressa15%n/a
Henneke11%n/a
Forward7%n/a
Maymen4%n/a
Dibble3%n/a
Sobrante3%n/a
Yolo County: 44%Solano County: 50%Sonoma County: 22%Marin County: 20%Contra Costa County: 48%Colusa County: 55%Lake County: 8%Sutter County: 41%Napa County YoloSolanoSonomaMarinContra CostaColusaLakeSutterNapa
Napa County (heavy outline) and nearby counties, shaded by expansive clay share.under 10%10-25%25-40%40-60%60%+

How we count · survey areas CA055 (2025-09-02)

What the numbers mean here

Rain over the 12 months to August 2026: 26.37 in, 5.37 in below the 1991-2020 average year of 31.74 in.

USDA rates 88% of the land very limited for septic absorption fields.

With shrink-swell clay on 14% and a shallow water table on 6% of the land, the soil survey points to neither problem across most of Napa County; conditions still vary lot by lot.

Drought

0% of the county is in drought (D1 or worse) on the October 13, 2026 map. No part of the county was in drought in the last 52 weeks.

October 21, 2025Share of area in drought, weeklyOctober 13, 2026
Category, October 13, 2026Share of county
D0 abnormally dry or worse27.3%
D1 moderate drought or worse0%
D2 severe or worse0%
D3 extreme or worse0%
D4 exceptional0%

Flood losses since 2016

29 flood losses since 2016, $405,663 paid. The worst year was 2017, with 11.

YearFlood losses
20164
201711
20180
20199
20200
20213
20220
20231
20240
20251
2026 to date0

Rain

Average year, 1991-202031.74 in
12 months to August 202626.37 in (-5.37)
202525.67 in (-6.07)
Wettest year since 18951983: 66.01 in
Driest year since 18952013: 6.72 in
Days with 1 inch or more, normal at ANGWIN PACIFIC UNION COLLEGE (8 mi away)12.3

Homes

Housing units55,817
Median year built1976
Built before 198057%
Built 2000 or later19%
Owner-occupied63%
Median value, owner-occupied$869,500
Single-family detached67%

What repairs cost

Published ranges from cost guides, not quotes. More sources, and where they disagree.

WorkRangeSource
Foundation repair, US $2,200 to $8,200 Fixr, January 27, 2026
Pier-and-beam repair, US $2,000 to $15,000 Fixr, January 27, 2026
Foundation inspection, US $300 to $600 Fixr, January 31, 2025

Nearby counties

PlaceDistanceExpansive clayHigh water table
Yolo County, CA 26 mi 44% 23%
Solano County, CA 27 mi 50% 21%
Sonoma County, CA 32 mi 22% 5%
Marin County, CA 39 mi 20% 6%
Contra Costa County, CA 46 mi 48% 11%
Colusa County, CA 47 mi 55% 37%
Lake County, CA 47 mi 8% 1%
Sutter County, CA 50 mi 41% 37%

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