What is a Perc Test?

What is a Perc Test

Are you wondering what a perc test is? If you’re buying a new piece of land, it’s something you may want to know about.


A percolation test, or perc test, is a procedure performed to review water drainage in different soils. It is important for many uses, such as when designing a leach field for a septic system, planning a building design, or considering agricultural use of the land. If the water cannot drain properly, a swamp of sewer water will result. This is a foul-smelling health hazard that could cost you thousands of dollars to correct, or ruin your house or farm entirely.



Perc Tests and Septic Systems


Some types of soil require a percolation test to ensure sufficient drainage before a septic system can be installed. If you’re planning on building a house on a new piece of land, it is highly recommended that a perc test be performed to ensure that it passes state and local regulations. The results may determine how many bedrooms are allowed to be developed in the house. Soil that fails a perc test could require expensive septic alternatives.


Shoots Veis of Interstate Engineering has the lowdown on how an underground wastewater treatment system is designed. “(It’s) designed based on two factors: household water usage and percolation rate of the soil where the underground system is located.  Using those two pieces of information, the size of drainfield area is determined.”


“If a household used a lot of water and it is discharged to a soil with a slow percolation rate, then a large drainfield area will be required.  A household with small water usage and well drained soil will be able to utilize a smaller drainfield footprint.”



Leach Field System pulls water from the rain water cistern and delivers water directly to the roots of the plants, avoiding the evaporation that makes surface irrigation wasteful.  Peforated pipe wrapped in landscape fabric and surrounded by gravel is connected by solid ABS distribution manifold and return.

How To Do a Perc Test


You can perform a perc test on any soil. Depending on your soil, the test results could be different on various parts of your property.  Local building regulations may require a test be performed by a licensed professional, so check with your local zoning department to find out if this applies to you. If not, a personal perc test may do the trick.


You can perform a simple test by picking up a handful of soil and compressing it in your fist. If the soil is dry, dampen it with a little water. Soil that falls apart easily will allow water to drain quickly; soil that holds together has a large amount of clay that will not allow rapid draining.


You will need a shovel, stopwatch, yardstick, gravel, and water. Dig several holes, each 1 foot wide and up to 3 feet deep, depending on local ordinance requirements. Soak the holes overnight; remember, you must perform the test in damp soil. Place 2 inches of gravel at the bottom of the holes.


Cover the gravel with 6 inches of water and measure the depth of water every 30 minutes. Local codes will indicate passing scores for your septic system. See if your water depth makes the grade.  A licensed professional may be required to perform the test and requirements vary from town to town.



Perc Tests and Building Design


Perc tests play a vital role in building design and stormwater management, just ask David Businelli, AIA. Owner of Studio 16 Architecture in Staten Island, NY.


“When designing a building, architects have to consider and deal with storm water drainage. This means that we have to calculate the amount of water from a 2” rainfall that will fall on the entire site, especially hard surfaces like the roof or paved areas.”


“Here in NYC, we have either storm sewers or combined storm and sanitary sewers into which rainwater can be discharged. But, there are many sites – especially on Staten Island that do not have storm sewers available. There are many municipalities in the country that don’t have storm sewers. Lacking them, storm water must be retained and drained on site, i.e. draining it into the ground.”


“Once an area of the site is selected, soil borings are done in order to determine the soil types and where permeable soil is. Then a percolation test is performed in order to determine how much water and at what rate the soil can absorb, in gallons per minute. This data allows us to determine the type of drainage system that is appropriate for the project, such as drywells, bioswales, retention ponds, sheet drainage over grassy areas, permeable pavement, etc. Stormwater management is an essential part of any building project, and contributes to the sustainability of the building by using the site properly.”


What is a Perc Test 3

Perc Test – Vineyards and Growing Crops


Scott Bergman, VP at Bergman Vineyards, has years of experience selling vineyards and wineries in Napa and Sonoma Counties.


“When someone buys a vineyard, a winery with a vineyard on it, or land that is ‘plantable’ for more vineyard, they do a detailed soil test to find the quality of the soils that the vines are growing in and the soils where future vines will be growing. This test normally takes a MINIMUM of 60 days to get the soils from the ground, categorize the soil samples, send them to the mid-west for the tests, get the results back, and then have the specialist perform a write up on the results.”


“Thankfully, a perc test can be a quick solution for someone that isn’t able to wait for the soil results 2 months later. “If there is a short (less than 60 days) timeframe to do the necessary due diligence (time for inspections of the property and improvements), a perc test can be performed much faster. The perc test will let us know how the clay is in the ground. If the water doesn’t draw down fast enough, we know that the vines could drown (not worthy to purchase).”


Any other Perc Test tips? Tell us in the comments below or on our social channels!


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Photo Cred: flickr | Natural Resources Conservation Service | Jeremy Levine

Photo Cred: wikimedia commons | Robert Young Vineyards




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