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Allegheny County, Pennsylvania

A shallow water table limits basements on 31% of Allegheny County's mapped land, a higher share than 50% of US counties. Shrink-swell clay limits houses without basements on 10%.

None of the county is in drought on the November 3, 2026 Drought Monitor map. FEMA's flood data list 847 flood losses here since 2016, 65% of them in buildings with basements. 77% of homes were built before 1980, against 50% nationally; the median home dates from 1957.

Data as of November 3, 2026 · sources

10%

expansive clay: shrink-swell limits slab homes

31%

high water table: limits basements

0%

in drought, November 3, 2026

77%

of homes built before 1980

Soil under Allegheny County

Expansive clay (limits slab homes)10.0%
High water table (limits basements)31.3%
Septic fields very limited79.0%
Clay swell (LEP, by area)3.0%
High-swell land (LEP 6%+)9.2%
Urban land, not rated17.1%
Mapped soil476,167 acres
Soil seriesShare hereSeries nationally: high water table
Gilpin16%under 1%
Culleoka11%n/a
Wharton7%n/a
Dormont7%n/a
Guernsey6%n/a
Upshur5%0%
Washington County: 51%Beaver County: 35%Westmoreland County: 48%Butler County: 53%Hancock County: 11%Brooke County: 9%Armstrong County: 33%Lawrence County: 67%Allegheny County WashingtonBeaverWestmorelandButlerHancockBrookeArmstrongLawrenceAllegheny
Allegheny County (heavy outline) and nearby counties, shaded by high water table share.under 10%10-25%25-40%40-60%60%+

How we count · survey areas PA003 (2025-09-05)

What the numbers mean here

On 31% of Allegheny County's mapped land, soil stays saturated within about 6 feet of the surface for part of the year, which is basement depth.

Rain over the 12 months to August 2026: 38.53 in, 2.68 in below the 1991-2020 average year of 41.21 in.

USDA rates 79% of the land very limited for septic absorption fields.

Drought

0% of the county is in drought (D1 or worse) on the November 3, 2026 map. No part of the county was in drought in the last 52 weeks.

November 11, 2025Share of area in drought, weeklyNovember 3, 2026
Category, November 3, 2026Share of county
D0 abnormally dry or worse0%
D1 moderate drought or worse0%
D2 severe or worse0%
D3 extreme or worse0%
D4 exceptional0%

Flood losses since 2016

847 flood losses since 2016, $21.2 million paid. The worst year was 2018, with 330. Foundations of the buildings, where recorded: 65% basements, 20% slabs, 2% crawl spaces, 13% elevated. Median water depth outside the building: 15 in.

YearFlood losses
201623
201774
2018330
2019160
20205
2021111
202213
20232
202492
202522
2026 to date15

Rain

Average year, 1991-202041.21 in
12 months to August 202638.53 in (-2.68)
202537.40 in (-3.81)
Wettest year since 18952018: 61.32 in
Driest year since 18951930: 24.69 in
Days with 1 inch or more, normal at EMSWORTH L/D OHIO RVR (6 mi away)6.4

Homes

Housing units606,827
Median year built1957
Built before 198077%
Built 2000 or later11%
Owner-occupied65%
Median value, owner-occupied$227,600
Single-family detached62%

What repairs cost

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

WorkRangeSource
Basement waterproofing, US $3,000 to $10,000 Fixr, February 6, 2026
Sump pump installation, US $800 to $2,000 Fixr, January 31, 2025
Crawl space encapsulation, US $2,000 to $8,000 Fixr, August 29, 2025

Nearby counties

PlaceDistanceExpansive clayHigh water table
Washington County, PA 23 mi 7% 51%
Beaver County, PA 24 mi 6% 35%
Westmoreland County, PA 29 mi 5% 48%
Butler County, PA 31 mi 1% 53%
Hancock County, WV 31 mi 1% 11%
Brooke County, WV 35 mi 1% 9%
Armstrong County, PA 36 mi 1% 33%
Lawrence County, PA 41 mi 1% 67%

Sources: USDA SSURGO (September 5, 2025) · US Drought Monitor (November 3, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status