Soil lookup: what the USDA has mapped under your county
Four fields decide most of what a basement does, and all four are public. Enter a ZIP code.
What each field is
| Drainage class | A formal USDA classification of how long the soil profile stays saturated in a normal year. Seven classes from excessively drained to very poorly drained. All seven explained. |
|---|---|
| Linear extensibility (LEP) | The percentage a soil sample shrinks in length on drying. The standard proxy for shrink-swell. Above 9% the ground moves footings on its own. |
| Depth to water table | Shallowest annual depth, in centimetres. A US basement floor is typically near 210 cm below grade, so anything under about 150 cm matters. |
| Hydrologic soil group | A to D, how fast water infiltrates. Group D sheds almost everything as runoff, which is why grading matters more on clay. |
What it cannot tell you
Whether your basement is wet. Soil is one of three variables and the other two are your house and your yard: where the downspouts end, whether the grade falls away from the wall for the first three metres, whether a footing drain exists and whether it is still open after forty years. Excellent soil plus a downspout emptying at the foundation beats terrible soil plus competent drainage, every time.
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