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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 September 22, 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 September 22, 2026 · sources

10%

expansive clay: shrink-swell limits slab homes

31%

high water table: limits basements

0%

in drought, September 22, 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 September 22, 2026 map. Part of the county was in drought in 5 of the last 52 weeks; the most was 62.6% of its area, on the October 21, 2025 map.

September 30, 2025Share of area in drought, weeklySeptember 22, 2026
Category, September 22, 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 (September 22, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status