The seven USDA drainage classes, and what each does to a basement
Drainage class is the single most useful public number about the ground under an American house, and almost nobody selling basement work will mention it. It is a formal USDA classification of how long the soil profile stays saturated in a normal year. That is exactly the variable that decides whether there is pressure against your wall.
| Class | Counties where dominant | Our risk weight | Meaning |
|---|---|---|---|
| excessively drained | 0 | 5/100 | water passes through very fast and the soil holds almost no moisture |
| somewhat excessively drained | 39 | 12/100 | water drains quickly and the profile stays dry most of the year |
| well drained | 2492 | 22/100 | water leaves readily; the profile is saturated only briefly after rain |
| moderately well drained | 300 | 42/100 | water leaves slowly enough that the lower profile is wet for part of the year |
| somewhat poorly drained | 98 | 66/100 | the profile stays wet long enough to restrict ordinary use without artificial drainage |
| poorly drained | 213 | 86/100 | water leaves so slowly that the profile is saturated periodically through the season |
| very poorly drained | 0 | 96/100 | water stands at or above the surface for much of the year |
Class definitions: USDA NRCS SSURGO / National Soil Survey Handbook. The risk weight is ours, derived from the definitions.
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