What soil is my house built on?

Four ways to look up the soil under a house

Start broad and narrow down: this site's ZIP lookup for your county, Web Soil Survey for your lot, SoilWeb for the map unit at a point, and DrillerDB for a water depth in feet. The first row is this site. The other three were read on October 6, 2026 from each tool's own pages, and the facts behind them sit on our comparison of soil lookup websites.

Step by step, from ZIP code to lot

1. Look up your ZIP code. The ZIP lookup shows your county's share of shrink-swell clay and of shallow water table. For scale: Bexar County, TX (San Antonio) is rated shrink-swell clay on 51% of its mapped land and has a basement-depth water table on under 1%; Franklin County, OH (Columbus) is the reverse, with 71% water table and 12% clay.
2. Open your county or city page. It lists the soil series that cover the most land, this week's drought and the flood losses FEMA has recorded since 2016. Each series name links to a page on what that soil is, such as Houston Black.
3. Outline your lot in Web Soil Survey. Start the app, enter your address, and draw the area of interest around the lot. The Soil Map tab lists the soils on it. In the Soil Data Explorer tab, open Soil Suitabilities and Limitations, then Building Site Development, and choose Dwellings Without Basements or Dwellings With Basements (NRCS menu guide). The table gives each soil a rating and the reasons for it, with a value from 0.0 to 1.0 beside each reason (University of Florida IFAS, 2024, whose example is a septic rating).
4. Choose the rating that fits the house. For Dwellings Without Basements, NRCS assumes spread footings about 2 feet down, or at frost depth if that is deeper (NRCS). For Dwellings With Basements it assumes footings about 7 feet down (NRCS). Use the first for a house without a basement and the second for one with a basement.
5. Read the reasons, not only the rating. Each rating is the most limiting of a list of features (NRCS). Shrink-swell is one: for Dwellings Without Basements, NRCS calls the thickest layer limiting when its linear extensibility is above 6 percent. Depth to a saturated zone is another: for Dwellings With Basements, a water table within 75 centimeters (about 2.5 feet) of the surface is limiting, and one down to 182 centimeters (about 6 feet) is somewhat limiting (NRCS).
6. Put the county and the lot together. Clay on most of the county's land and a lot rated limiting for shrink-swell is the case to take to an engineer; start with how to tell if your house is on expansive clay. A shallow water table as the reason points to basement flooding and what a sump pump costs. A county with little clay and a lot with no limiting feature is the opposite case, and most results sit between the two.

Limit of this method: when we read Web Soil Survey on October 6, 2026, its ratings loaded with JavaScript and did not show without a browser. The steps above use only what its home page, the rating rules in USDA's Soil Data Access service and the University of Florida guide describe, so check the tab names against the screen in front of you.

How many homes are in clay or wet-ground counties

About 23.4 million homes are in the 766 counties where shrink-swell clay covers more than a quarter of the mapped land, and 79.1 million are in the 1,698 counties where a water table at basement depth does. The two groups overlap. USDA's soil survey rates every map unit for building a house. Adding those ratings up by county, and counting the homes in each county (Census ACS 2020-2024), gives these totals for the 3,122 counties USDA has rated, which hold 143.5 million homes.

What the lookups cannot tell you

• Mapped, not measured. USDA's Web Soil Survey home page says onsite investigation is needed in some cases, such as soil quality assessments and certain conservation and engineering applications (USDA, read October 6, 2026). Foundation Risk Registry's data notice says its soil data are typically mapped at a scale of 1:12,000 to 1:63,360. It adds that they "may not accurately reflect sub-surface conditions at a precise residential address level" (Foundation Risk Registry, read October 6, 2026).
• Who to ask. The same home page points readers to their local USDA Service Center or their NRCS State Soil Scientist for more detailed information (USDA, read October 6, 2026).
• Fill and grading do not show. HUD lists fill that was not compacted in layers beside highly expansive soil among the poor foundation soils (HUD Rehab Guide, 1997). A soil map shows the ground as surveyed, not how a lot was filled.
• Soil is one factor. USDA's Soil Survey Manual notes that reinforced concrete slab-on-ground construction has markedly reduced the shrink-swell limitation for small buildings (NRCS Soil Survey Manual). Texas engineers name other factors too: design and construction, landscaping practices, leaking plumbing and slope instability (ASCE Texas Section, 2022).
• The numbers move at different speeds. The soil ratings are survey versions to August 5, 2026. The drought map changes every week, and each county page shows the date of the map it uses.

When you need a real test

A lab test is the only one that settles whether soil is expansive, because the building code defines expansive soil by test, not by map. The 2024 International Residential Code calls a soil expansive when it meets all four provisions. The plasticity index is 15 or more, more than 10 percent of its particles pass a No. 200 sieve, and more than 10 percent are smaller than 5 micrometers. The expansion index is above 20 (ASTM D4829). Where the expansion index test is run, the first three are not required (International Residential Code). HUD advises consulting a structural or geotechnical engineer when settlement is suspected (HUD Rehab Guide, 1997). What an engineer's visit costs is in the foundation inspection cost guide, and the signs that make it worth booking are in signs of foundation problems.

Ten soil series with the most mapped land

Of the ten series with the most mapped acres, seven (Valentine, Gilpin, Smithdale, Cecil, Williams, Ector and Berks) have no shrink-swell rating on any of their land. Three (Harney, Sharkey and Pullman) are rated on 99 to 100% of it. A soil series is the lowest category of the national soil classification system, and its name is the most common reference in a map unit's name (NRCS National Soil Survey Handbook, 614.6). This site has a page for each of the 160 series that cover the most land in the counties and cities it lists.

Sources

• USDA NRCS Web Soil Survey: how to search, the Soil Map and Soil Data Explorer tabs, the free cart, the onsite-investigation note. Read October 6, 2026.
• USDA NRCS, Soil Data Explorer Tab guide for Web Soil Survey: where Dwellings With Basements and Dwellings w/o Basements sit in the menus. Read October 7, 2026.
• University of Florida IFAS, How to Use the Web Soil Survey to Find Information about Your Land (Mary G. Lusk, 2024): the Soil Data Explorer path and the rating table. Read October 7, 2026.
• SoilWeb, UC Davis California Soil Resource Lab and DrillerDB: what each searches by and shows. Read October 6, 2026.
• Foundation Risk Registry data notice: the SSURGO map scale and address-level caution. Read October 6, 2026.
• USDA NRCS Soil Data Access, the rating rules "ENG - Dwellings W/O Basements" and "ENG - Dwellings With Basements" (table sainterp, design date July 19, 2018): the limits for shrink-swell and depth to a saturated zone. Read September 28, 2026, rule text re-read October 7, 2026.
• USDA NRCS National Soil Survey Handbook (amended 2024): the soil series definition. Read October 7, 2026.
• USDA NRCS Soil Survey Manual (2017), chapter 8: slab-on-ground construction and shrink-swell. Read September 28, 2026.
• 2024 International Residential Code, chapter 4: the expansive soil tests. Read September 28, 2026.
• HUD, The Rehab Guide, Volume 1: Foundations (1997): poor foundation soils, when to consult an engineer. Read September 28, 2026.
• ASCE Texas Section, Guidelines for the Evaluation and Repair of Residential Foundations, Version 3 (adopted April 1, 2022): other factors in foundation performance. Read September 29, 2026, re-read October 7, 2026.
• Foundation Soils county and series data: USDA SSURGO to August 5, 2026, Census ACS 2020-2024 housing units. Methods and every source.

Questions people ask

How do I find out what soil my house is built on?

Start with your ZIP code in the soil by ZIP lookup on this site, which shows your county's share of shrink-swell clay and of shallow water table. Then outline your lot in USDA's Web Soil Survey, which lists the soils mapped on it. Both are free. Neither tests your lot: only a lab test on a soil sample from it does that.

Is there a free soil report for a house?

Web Soil Survey builds custom reports for any area you outline, and its shopping cart is labeled "Free" (read October 6, 2026). The soil by ZIP lookup on this site is free to read, with no account. Both describe mapped soil. A lab test on a sample from your lot is the only report on the soil itself.

How can I find out the soil type in my area or on my lot?

For an area, look up the ZIP code in the soil by ZIP lookup on this site, which names the county's most common soil series and the share of land each covers. For one lot, outline it in Web Soil Survey: its Soil Map tab lists the soils on the lot, each named for a soil series, such as Houston Black or Cecil. This site has a page for each of 160 soil series with their shrink-swell and water table shares.

How can I check the water table depth at my address?

DrillerDB gives a typical static water level in feet for an address, built from nearby recorded wells, and shows how many wells are behind it. It labels the lookup free (read October 6, 2026). USDA's soil ratings give depth ranges by soil, not a measured depth for your lot. This site shows the share of a county's land with a water table at basement depth, never a depth in feet.

How accurate is Web Soil Survey for a single lot?

Treat the map unit as the likely soil, not a measurement. USDA's own home page says onsite investigation is needed in some cases. Foundation Risk Registry's data notice says its soil data are typically mapped at a scale of 1:12,000 to 1:63,360. It adds that they "may not accurately reflect sub-surface conditions at a precise residential address level" (both read October 6, 2026).

Does the soil decide whether a foundation cracks?

It is one factor. 766 US counties have shrink-swell clay on more than a quarter of their mapped land, yet USDA's Soil Survey Manual notes that reinforced concrete slab-on-ground construction has markedly reduced the shrink-swell limitation for small buildings. Texas engineers name design and construction, landscaping practices, leaking plumbing and slope instability as other factors.