Perc test (percolation test)

Holes are dug where the drain field would go, filled with water, and timed as the water level falls. Many jurisdictions now rely on a soil profile evaluation instead of, or as well as, the timed test, but the name has stuck. Who performs it and what passes is set by the state and usually administered by the county health department.

Why it matters

Rural land without a sewer connection needs a septic system to be lived on, and a septic system needs soil that drains. A failed test can mean an engineered system that costs several times more, a building site moved somewhere else, or no permit at all. The USDA soil survey’s septic ratings and the presence of hydric soil tell you how worried to be; only the test on the ground settles it.