In this guide

What Is a Perc Test?​

A percolation test is a special soil test ¹ that evaluates the soil’s ability to accommodate a septic system to dispose of sewage under the ground. A perc test evaluates both the soil and the site to determine whether a septic system can be installed on the property. The percolation rate refers to the soil absorption rate, or how quickly soil absorbs water. This is important because a septic system installed in a location with poor percolation could lead to significant sewer backup, soil degradation, and structural issues for homes and buildings, in addition to health and safety concerns.

Perc tests are a priority for homeowners or homebuyers looking to build or develop a piece of land. If there isn’t already a municipal sewer system in place and the property is in a semi-rural or rural location, then a perc test is a must-do before any major work can begin. A licensed contractor, excavator, or professional inspector conducts one of these tests by digging or drilling at least one hole, if not several, into the ground and pouring in water. This allows them to observe the rate at which water absorbs or percolates in the soil. Keep in mind the local health department is typically involved in managing these tests to ensure the health of the community. You will need proof of the results before installing a septic system.

How Does a Perc Test Work?

To request a perc test, you’ll need to contact local or county health or environmental agencies. The agency will inform you if a government agent must perform the test or if you can hire an independent engineer to undertake the percolation test procedure. Before a it is ordered, an engineer will discuss any site preparation that is needed. A survey is typically done to decide where the holes should be placed. If a land survey is needed, the cost is approximately $600 on average.

To begin a perc test, holes are dug into the soil of the proposed leach or drain field ². After the holes are dug, any loosened soil is scraped away from the sides of each dig site. A professional fills the hole with gravel to a depth of about two inches.

It is then required to pre-soak the soil to mimic septic system saturation conditions. At a minimum, 12 inches of water should sit in the hole for at least four hours during the pre-soak phase. Clay soils need to soak for longer than sandier soils. Sandy soils are porous, which means water flows through them faster. In comparison, clay soils are denser with smaller pores, making water flow through them at a slower rate. A return time is usually scheduled the next day for the technician to complete the procedure.

After pre-soaking, the actual testing procedure commences. To start, the holes are filled with water to cover the gravel with six inches of water. The technician may need to wait until the water is measuring exactly six inches before recording the depth. Measurements are then taken of the water every 15 to 30 minutes to determine how low the water is.

The percolation rate is calculated by the amount of water divided by the amount of time taken for the water to dissipate into the subsoil. For example, if the water level dropped one inch every 30 minutes, the soil would have a percolation rate of two inches per hour.

Average Cost for a Perc Test by Land Size

When performing a percolation test on your soil, more holes equals better accuracy with test results. In most cases, at least two holes are dug with at least 40 feet of space between each one. Sometimes a hole can be small and dug by hand with a shovel. Other times, larger holes need to be dug with heavy equipment like excavators and backhoes. Professionals might charge a higher amount if they need to make more holes or use heavy equipment. Here are the average costs of a perc test based on the depth of the one or two holes dug on your property. The deeper the holes, the larger the equipment, and the more resources needed, such as engineers and septic designers.

Perc Test Cost by Hole Size

The standard depth for most holes will be between two and three3 feet but can reach up to six feet deep. The width starts at 6 inches but can be as wide as 3 feet. The size of each hole impacts the cost because if the depth or width requires a backhoe, the perc test cost may be increased by $200 or more. Here are some of the most common hole sizes and the cost of the perc test if two holes are dug on the property.

Perc Test Cost

Prices vary by municipality in each state, but the average labor cost for a land perc test is around $100 to $200 per hour. As an alternative, a flat fee of $300 or more may be charged by the technician performing the perc test. They take one to three hours for undeveloped sites and one to two hours for a site that needs a septic system replacement. For complicated, larger sites, it may take up to six hours.

Sometimes an engineer is needed to determine the height of the water table. The septic designer takes measurements and helps perform the test. When looking at the cost of the test, keep in mind that hiring an engineer or septic designer costs between $150 and $250 per hour. Engineers assist with issues like high water tables. Engineers can come up with strategies to de-water the leaching area. They achieve this by planning out placements for gravel-filled trenches and drain pipes. The pipes and trenches move water away from the drain field ² to make it possible for the property to pass the test.

Sometimes a septic system designer can perform a perc test. However, often the professional who performs it is a local government agent to ensure adherence to environmental regulations. The agent can bring in other professionals such as engineers or contractors. Independent contractors can be used in some cases, but the health department may require an official to be present during the perc test to confirm the legitimacy of the results.

Depending on the soil and testing sites, the testing window is usually between one and four hours. This does not include preparations such as digging and pre-soaking. If the soil is difficult to excavate for testing purposes, the cost increases due to more labor hours and the need for heavy-duty equipment. The number of people required to perform the test changes the cost. More people equal a higher labor price. Smaller properties, where fewer holes are dug, require fewer people than larger properties.

Perc Test in Winter

Perc tests are done seasonally—usually in the spring and summer. Performing one of these tests in the winter costs more since extra equipment is required. That’s because the contractor, excavator, or inspector must drill through cold, if not frozen, ground. Percolation is also naturally lower in the winter or after heavy rainfall when the soil has already absorbed a lot of water. Prices may be on the higher end between $1,500 and $3,000 if multiple deep holes are required, and they need to use an excavator.

Why to Perc Test a Soil?

A percolation test provides important information regarding soil drainage for any property owner. It is necessary before installing a septic tank or drainage system due to local health department permit requirements. A septic tank, unlike local city or town sewer systems, requires a drain or leach field ². Good drainage is incredibly important when installing a leach field or drain field and a necessity for the wastewater disposal system. This system removes contaminants that drain out of the septic system. Improper drainage leads to the sewage backing up and contaminating the soil. A perc test determines where the leach field should go based on the drainability of the soil.

You will also need to have one of these tests done if you plan on repairing or replacing your septic system. Before you start these renovations, a professional needs to check that your soil can drain effectively and that you have a leach field. 

Infiltration basins may also need perc tests if you are redoing existing septic systems. An infiltration basin is a low-lying stretch of land that acts as a spot for water or waste runoff. An infiltration basin must drain effectively so that runoff drains into the soil rather than into nearby roadways or waterways, hence the need to perform a perc test.

Besides septic systems, they are also used for agricultural purposes. Knowing the drain rate of your soil helps determine what type of crops you can grow and where you should grow them. For example, potatoes require drier, more loamy soil than other vegetables and fruits that require moist soil. Therefore, potatoes require higher drainability than some other crops. Fruit trees need soil that drains well since over-saturation removes the oxygen and prevents root growth. However, this type of perc test is not mandated by government agencies.

Which Type of Soil Allows Water to Drain the Quickest?

The soil type factors into your perc test results and where the test should be performed. There are general permeability values for different soils you can expect if you know what kind of soil you have on your property. Most cities and rural towns have different regulations concerning these tests, so keep this in mind when planning for your property. In general, these ranges are used to evaluate soil permeability.

If your soil is below 5 Minutes Per Inch (MPI), then your soil has one of the highest permeability rates and may be supplemented through alternative systems such as pressure pre-treatment or adding dense soil near the trenches. Soil between 5 and 60 MPI falls in the ideal permeability range for septic systems, as it permits a conventional leach field and does not require alternative solutions. Soil in the 60 to 110 MPI range is slightly less permeable and may require enhanced treatments such as mound systems or pressurized dosing. Soil that is 120 MPI or higher has extremely low permeability and could be improved with specialized wastewater treatment in some cases.

Clean gravel and sand have the most rapid permeability rates, around 5 cm/hour. A mix of sand, clay, and gravel is still permeable but not quite as much, around one to three cm/hour. Silt, clay, and stratified clay typically have moderate permeability lower than 1 cm/hour but more than impermeable soil, which is less than 0.05 cm/hour.

Percolation Test Report

In most locations, a perc test report is required to plan a new or renovated septic system, which is included in the price of the test. Once finalized, the report is provided to the local health department and becomes a public record. A perc test report provides standard data such as testing date, location of the test, technician name, and under what conditions the perc test was performed.

A standard perc test report is two pages. The first page features a graph with times of each reading, water depth, percolation rate, and any comments about the testing process from the examiner. The second page features a scaled drawing of the site plan. Within the report, it will list whether the site passed or failed the perc test. Your engineer may provide site recommendations, but that would appear on a different report, not in the results.

A good perc rate for a septic system is between 1 and 30 minutes per inch. Between 30 and 60 minutes per inch might require hydraulic analysis for installing a septic system. Anything under 1 minute per inch or over 60 minutes per inch is not an ideal perc rate.

Failed Perc Test

Every city and regional area should have specific perc standards that all properties need to meet as part of the test. Soil fails a perc pest if it doesn’t drain the water at all or the water doesn’t drain fast enough. It can also fail if water drains too rapidly. Some properties are more likely to fail a perc test, particularly those built on steep slopes ³. A property with a low water table has a higher chance of passing it than those with a high water table. A high water table indicates the presence of water in the soil that doesn’t drain properly. The site may need to go through minor or major modifications to pass another perc test ordered at a later date.

Failed Perc Test Alternatives

If your soil fails a perc test, this doesn’t necessarily mean your soil is unusable or unhealthy. You can explore a few options further regarding the quality of your soil and how much it absorbs water. A second opinion can make a big difference to the eventual installation of a septic drain field on your property.

• Speak to a local health department representative. Ask for any records of previous perc tests that may have been done on the property. These documents are public records. This helps you determine if the entire property was searched thoroughly for an ideal septic site.
• Dig holes in another area of your property where there are different soils. Soil with a different composition might provide better drainage. It is possible to get a pass result for the property, even if the perc test failed in a different area.
• Excavate deeper. Excavate deeper to get below the frost line if the perc test failed during the winter season. If the test is not done immediately after excavating, an insulating material like hay should be placed over the hole.
• Appeal the results. You may be able to appeal the results depending on any special considerations set by your jurisdiction. For example, if a county’s Department of Health rejects the results, appeals can be made to the state Board of Health.
• Install a treatment system with the septic. With a treatment system between the drain field and septic tank, contaminants are filtered out. A sand filtered septic treatment system costs between $6,500 and $10,000.
• Install a larger septic tank. Larger tanks and pumping ⁴ them out periodically to half the tank’s volume can accommodate higher water tables by reducing waste amounts that accumulate in the tank.
• Alternative septic systems. Alternative septic systems may be required in areas with high water tables. Consider getting a different kind of septic system, such as a mound system or grazing fields.
• Check when failed perc tests occurred. Sometimes, a failed test may have something to do with when it occurred. In many locations, the water table is naturally higher or lower in certain parts of the year, usually with the biggest differences in winter and summer. The season can influence how quickly the soil drains. If your failed result came in one season, it might be better to try again in six months.
• Consider municipal water. Perc tests are important for rural properties where a septic system needs to be installed. These are areas where municipal water is not currently available, but that doesn’t mean it won’t be in the future. Consider how long you plan to leave your land undeveloped. If you don’t plan on building for several years and there’s a lot of development in the area, a municipal water and sewer system may be installed, and you won’t need to worry about putting in your own.

Septic System Alternatives Cost

There are several alternatives to traditional septic systems to consider if your property has poor percolation and you want to start building on the land. While it’s often worth it to get another percolation test after a failed result just in case an error or environmental influence affected the test. Eventually, it may be best to move toward septic system alternatives. Some popular options include storage units, plastic chamber leach fields, sand filters, aerobic treatment systems, drip irrigation, constructed wetland systems, mound systems, and pressurized dosing. Let’s take a closer look at each.

You may want to try sewer storage units. These storage tanks can be connected to the home’s plumbing and collect the wastewater until it can be picked up and emptied by the local waste management company. Although they are termed “sewer” storage units, they are not like sewer systems connected to municipal water supplies, which empty wastewater into shared pipes. Nor are they the same as traditional septic tanks, which treat wastewater underground on your property. Rather, as we said above, they are temporary holding tanks that should be treated regularly by a local waste management company.

If you have a small lot, utilizing plastic chamber leach fields is a great option. These chambers are placed in the ground so that wastewater can flow into them. There it is treated and flows back into the ground. Sand filter septic systems purify wastewater by using sand and oxygen in a sealed chamber. The chamber can be placed above or below the ground. They don’t require a leach field, making it a great option for eco-sensitive areas.

Aerobic treatment systems use oxygen to purify the effluent before releasing it into the soil. This is a good option for small lots, areas with subpar soil conditions, or soil near bodies of water that may be more sensitive to pollution. Drip irrigation, or drip distribution, is a method that distributes treated water over a large area of land. It does this by releasing small amounts of water through flexible tubing. Some wetland plants can also help with the filtration work through a constructed wetland system. After your wastewater goes through the initial septic system, it goes to a wetland plot for more purification.

A mound system requires a man-made mound as the drain field. The mound is created above the natural soil line and is usually made with sand and gravel filler. Mound systems require a designated replacement area in case the system fails. Pressurized dosing is another popular option if your septic tank has failed. It distributes effluent, or liquid waste/sewage, across an area in an even measure by using pressure. This method only focuses on dispersing the effluent, so it can be combined with any other water treatment type.

How Long is a Perc Test Good For?

Each health department has its own rules about how long perc tests are valid. Results are generally valid from two to five years before renewal is needed. At that time, the health department reviews previous perc test results and decides if additional testing is needed. Renewal helps government agencies confirm that the perc test abides by all the current regulations.

Perc Test Laws and Regulations

The laws surrounding perc tests and the requirements for conducting them vary widely from place to place. It is essential to contact your county health department to find out the local laws and regulations regarding septic installation and perc tests. Making sure you have all the information from the beginning reduces the chance of running into roadblocks after you get started.

Many counties require a health department employee to witness your perc test. That means that you will need to coordinate with both the soil professionals and the health department to find a time that works for everyone. Most counties also require the perc test to be performed by a licensed soil professional. Your health department may even have a list of approved professionals that they work with regularly.

Your county may have regulations regarding the number of tests that can be performed. Some places run two tests simultaneously in two places on the property, as soil composition and drainage can vary within a short distance. Other regions might require you to request a second test or even appeal the process if you fail your initial perc test.

Some counties require additional tests to be run on the property before approval. They may even require that extra supporting infrastructure be put in place. A common example is a leach field, a system of pipes that conducts waste away from your property. Some of these are required before the initial perc test, so be sure to inquire about the local regulations before you get started.

Unfortunately, perc tests need to be renewed every two to five years. The length of time between tests varies from county to county.

Remodeling Terms Cheat Sheet

¹ Soil test: Chemical analysis kit used to assess soil pH, and sometimes soil nutrients, for the purpose of making fertilizer recommendations (type, quantity, and frequency)

⁴ Pumping: A device used to move air, liquid, or gas by mechanical means