Frost line depth in St. Clair County, Illinois
Frost reaches about 29 inches in typical soil across St. Clair County, from 26″ in the mildest ZIP code to 30″ in the coldest. Codes in this climate commonly set footing depth at 24 to 36 inches.
Moderate frostFrost depth by soil type
| Soil type | Est. frost depth |
|---|---|
| Sand / gravel (drains well) | 37″ |
| Silt / loam (typical yard soil) | 29″ |
| Clay (holds water) | 25″ |
County average. Methodology.
Verify before you dig. These are climate-based estimates. The legally required frost depth is the figure your building department publishes in its IRC Table R301.2 (climatic and geographic design criteria). Call the St. Clair County building or permitting office in Illinois and call 811 before digging.
Frost depth by ZIP code in St. Clair County
| ZIP | City | Est. frost | Code range |
|---|---|---|---|
| 62201 | East Saint Louis | 30″ | 24–42″ |
| 62203 | East Saint Louis | 30″ | 24–36″ |
| 62204 | East Saint Louis | 30″ | 24–36″ |
| 62205 | East Saint Louis | 30″ | 24–36″ |
| 62206 | East Saint Louis | 30″ | 24–36″ |
| 62207 | East Saint Louis | 30″ | 24–36″ |
| 62208 | Fairview Heights | 30″ | 24–36″ |
| 62220 | Belleville | 29″ | 24–36″ |
| 62221 | Belleville | 29″ | 24–36″ |
| 62223 | Belleville | 30″ | 24–36″ |
| 62225 | Scott Air Force Base | 29″ | 24–36″ |
| 62226 | Belleville | 29″ | 24–36″ |
| 62232 | Caseyville | 30″ | 24–36″ |
| 62239 | Dupo | 30″ | 24–36″ |
| 62240 | East Carondelet | 30″ | 24–36″ |
| 62243 | Freeburg | 29″ | 24–36″ |
| 62254 | Lebanon | 30″ | 24–36″ |
| 62255 | Lenzburg | 28″ | 24–36″ |
| 62257 | Marissa | 26″ | 24–36″ |
| 62258 | Mascoutah | 29″ | 24–36″ |
| 62260 | Millstadt | 29″ | 24–36″ |
| 62264 | New Athens | 28″ | 24–36″ |
| 62269 | O Fallon | 29″ | 24–36″ |
| 62285 | Smithton | 29″ | 24–36″ |
What this means for your project in St. Clair County
Deck footings & piers
Footings must bear below the frost line and at least 12″ below undisturbed ground. For planning in this area, budget for holes around 36″ deep until your permit office confirms the exact figure.
Fence posts
Set posts at least one-third of their above-ground height. Where frost heave is a problem, carry concrete footings below the frost line (about 29″ here in typical soil) and bell or taper the bottom.
Water & irrigation lines
Bury supply lines below the frost depth. Your water utility or plumbing code sets the minimum cover; add a margin below the estimate above for unusually cold winters.
Slabs & frost-protected foundations
Frost-protected shallow foundations (IRC R403.3) use the 100-year air freezing index — 887 °F-days here — to size perimeter insulation, letting footings sit shallower than the frost line.
Weather stations used
| NOAA weather station | Distance | 100-yr AFI |
|---|---|---|
| Belleville So Ill Univ, IL | 8 mi | 880 |
| Alton Dam 26, IL | 27 mi | 982 |
| Sparta, IL | 31 mi | 683 |
| Saint Charles, MO | 33 mi | 963 |
Nearby counties
Common questions
What is the frost line depth in St. Clair County?
About 29 inches in typical soil, based on NOAA's 100-year air freezing index. Across the county's ZIP codes the estimate runs from 26 to 30 inches.
How deep should footings be in St. Clair County, Illinois?
Codes in climates like this commonly require 24 to 36 inches. The St. Clair County building department publishes the required frost depth in its adopted code (IRC Table R301.2).
Does frost depth vary within St. Clair County?
Very little. Estimates across the county stay within 4 inches. Soil type matters more than location here.