The Percolation Test

Commonly known as a perc test, it is performed before selecting a site for the installation of a septic system drainfield. The test measures how rapidly water is absorbed into the soil in a test hole dug for the purpose. The absorption rate is used to determine if a site is suitable and how large the drainfield should be for the house.

Dig test holes for percolation test

Prior to running a perc test, take soil samples at a depth of at least 5 feet to determine the soil type such as sand, silt and clay at the proposed drainfield location. Generally, soils with higher sand content are better choices for the drainfield area. Once a location is selected, dig at least three holes at opposite ends of the site and do a percolation test in each hole.

The depth and size of the holes and the percolation test procedure are set by local or state regulatory authorities. The sides of the hole should be roughened to expose the natural soil surface and to make sure a smooth surface has not been formed by the shovel while digging. Loose soil is removed from the hole, and 2 inches of gravel is placed in the bottom of the hole to prevent scouring the bottom of the hole when water is added.


The hole is filled with at least 12 inches of clear water and kept at that depth for at least four hours to thoroughly saturate the soil in the hole. Only saturated soil conditions will produce an accurate measurement of absorption by the soil.

Septic system drainfield perc test

Refill if necessary to keep the water at the 12-inch level. In clay soils, water must be kept in the hole at least 12 hours for soil swelling to take place before measuring the percolation rate.

Perc test measurements are taken by observing the drop in water level in the hole over a period of time, measured in minutes per inch. The water level drop is used to calculate the percolation rate, divide the time interval in minutes by the water level drop in inches.

Check with local and state regulatory authorities in your area for information specific to your area.

Septic Perc restores percolation to drainfield soil even in clay conditions.
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