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Perc Test and Septic Systems


A perc test (also spelled perk – both are correct) is short for a percolation test. It is a test which measures the absorption rate of the soil where a proposed septic system will be installed.


WellThe traditional method of conducting this test is to dig three holes in the area where the disposal field is to be located (6 to 8 inches in diameter and 3 feet deep, the correct depth for a disposal field). When dug, the holes would be filled to the top with water. When the water has been absorbed into the ground (this simulates the actual working conditions of the soil when the disposal is installed), the test begins.

Water is then poured into the holes to a depth of 10 inches from the bottom of the hole. Lay a board across the top of the hole and measure the distance from the bottom of the board to the top of the water. Record this distance and time. Do not add any water during the test. After one hour, again measure the distance from the board to the top of the water. The difference between the first measurement and the second is the inches-per hour absorption rate. Continue the test by recording the distance down to the water level until the test has been conducted for five consecutive hours, or until all the water has been absorbed. If the water is absorbed before the end of the five hour test period, the test will be ended.

Generally speaking, the last hourly rate of absorption is less than 1 inch in 60 minutes, a disposal field will probably not work in that area. Areas, where clay or hardpan type soils are near ground level, are not satisfactory for disposal fields.


In everyday practice, it is generally not practical or necessary to conduct a perc test in the traditional manner. Because a County Health Official is required to be present for the duration of a percolation test, it would be exceedingly expensive to hire the County for the 5 hours required to complete the traditional test.

Fortunately, there is a practical alternative. Rather than digging 3 small holes, a back-hoe is hired to make three digs of 10-14 feet deep. In this way, the County Inspector can physically examine the soil and based on his knowledge and experience make a determination as to whether or not the soil will be able to absorb the anticipated discharge.

If the determination is made that the proposed site can efficiently absorb the anticipated effluent discharge from a septic system, it is said that the property “percs;” if not, then the property “doesn’t perc.”

Roughly speaking if the dig sites are rich in gravel or sandy soil, it will likely perc. If the soil has a lot of clay or if the water table is too high it probably won’t perc.


Can I still build on it?

YES. By all means! One of the most common misconceptions about vacant land is that if it doesn’t perc it’s the end of the world. That is simply not true.

All it means is that rather than just beginning construction of your septic system you need to have a system designed by an engineer and have that design approved by the County. Hence the term “engineered septic system.”


Any time you even have any questions with regards to perc tests or any other aspect of the purchase, sale or preparation for construction of vacant land, feel free to call me at (248) 568-4021.

Don’t forget to ask me about any new land listings that may have just been made available to you since the last update of this catalog.