BasementSoil

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.

Excessively drained5Somewhat excessively12Well drained22Moderately well42Somewhat poorly66Poorly drained86Very poorly drained96
The seven USDA drainage classes, and the weight each carries in our moisture score. The class describes how long the soil profile stays saturated in a normal year — which is the variable that decides whether there is pressure against a wall.
ClassCounties where dominantOur risk weightMeaning
excessively drained05/100water passes through very fast and the soil holds almost no moisture
somewhat excessively drained3912/100water drains quickly and the profile stays dry most of the year
well drained249222/100water leaves readily; the profile is saturated only briefly after rain
moderately well drained30042/100water leaves slowly enough that the lower profile is wet for part of the year
somewhat poorly drained9866/100the profile stays wet long enough to restrict ordinary use without artificial drainage
poorly drained21386/100water leaves so slowly that the profile is saturated periodically through the season
very poorly drained096/100water 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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