Moderate risk About 6% of Boone County's soil area is high shrink-swell (expansive) clay — far below the Iowa average of 24%, and far below the national average of 17%. That places it #74 of 99 Iowa counties for foundation soil risk.
Share of the county's ~367,200 acres of USDA-mapped soil with linear extensibility ≥ 6% in the top meter (SSURGO).
| High shrink-swell (expansive) clay | 6% |
| Moderately expansive | 59% |
| Low / non-expansive | 35% |
| Foundation risk tier | Moderate |
| Rank in Iowa | #74 of 99 counties |
| Higher-risk than | 45% of all U.S. counties |
Expansive clay swells as it takes on water and shrinks as it dries, and that repeated movement is what lifts and drops a foundation unevenly — opening stair-step cracks, racking door and window frames, and, left unmanaged, cracking slabs and footings. Boone County's exposure is low-to-moderate. At 6% high-expansive soil, Boone County carries real but uneven risk — trouble concentrates on lots with poor drainage, cut-and-fill grading, or aging plumbing leaks rather than striking every home. A soil-aware inspection beats assuming the worst.
Boone County's shrink-swell risk is concentrated in the Okoboji soil series alongside Luther and Kossuth — clays the USDA maps as strongly expansive, swelling and shrinking with every wet–dry cycle. Homes built on these series most need the drainage and moisture discipline above; a lot-level soil report (or the county NRCS survey) shows which one sits under a given address.
| County | High-risk soil | |
|---|---|---|
| Higher risk → | Story County | 6% |
| This county | Boone County (#74 of 99) | 6% |
| Lower risk → | Clinton County | 6% |
For context, the average Iowa county is 24% high-expansive soil and the average U.S. county is 17%.
Foundation problems get more expensive the longer they wait. Get an assessment and repair quotes from independent local pros.
Get repair quotes →Costs follow the same structure everywhere — from a few hundred dollars for a single crack injection to $8,000–$25,000+ for pier stabilization on a settling home. The right fix depends on the actual cause of movement, so get a diagnosis before committing to clay-specific work. See the full foundation repair cost guide for method-by-method pricing.
Risk metrics are computed from USDA SSURGO soil survey data (linear extensibility of soil components, area-weighted by county). Soil varies lot to lot — this is county-scale context, not a substitute for a site-specific geotechnical or structural assessment.