In This Article
No Perc, No House
Deep Hole Test
Perc Testing
Options if Site Fails            View all SEPTIC SYSTEM articles

Traditional septic systems only work if the soil in the leach area is sufficiently permeable that it can readily absorb the liquid effluent flowing into it. Also, there must be at least a few feet of good soil from the bottom of the leach pipes to rock or impervious hardpan below, or to the water table. The specific standards vary from town to town, but any of these characteristics can prohibit the use of a standard gravity-fed septic system. In some cases, a more expensive alternative septic system may be allowed. To determine is a building site is suitable for a septic system, a percolation test (typically called a “perc test’ or “perk test”) is required.

On rural sites without municipal sewage systems, a failed perc test means that no house can be built  — which is why you should make any offer to purchase land contingent on the site passing the soil and perc tests. As prime building sites become increasingly scarce (or prohibitively expensive) in many parts of the country, rural sites that will not pass a percolation or perc test are increasingly common.

In general, soils with high sand and gravel content drain the best and soils with a high clay content or solid rock are the worst. Most soils fall somewhere in the middle with a mix of course sand and gravel particles, small silt particles, and miniscule clay particles – the smallest. To get a rough idea before investing time and money in testing, dig below the top few inches of topsoil (loam) to the lighter soil beneath. If you can take a handful of the damp subsoil and roll it into a thin, flat shape or worm shape that holds together, and it had a sticky firm texture, the soil has a high clay content and will probably fail a standard perc test.

The two main tests used to determine a site’s suitability are a soil evaluation and percolation or perc test. Testing requirements vary greatly from state to state and often from town to town, as most states allow individual towns to establish separate rules within state guidelines. So make sure you talk to your town health officer about what tests are needed, when they can be done, and who should perform them. Whether or not a licensed professional is required, it’s a good idea to hire an seasoned expert with local experience as many of these tests have a bit of wiggle room.

Most tests start with a deep hole test dug by machine to well below the bottom of the proposed leach field – often 7 to 10 feet deep or greater. Soil samples may be taken back to the lab, or a visual test of soil layers may be sufficient. Soil tests (or observations) are used to identify the drainage characteristics of the soil, and the seasonal high water table is identified by examining splotches of color or “mottling” in the soil indicating the presence of water. The soil types, high water table, and depth of rock or impermeable hardpan are documented.

While most soil experts believe they have enough information at this point to design an effective septic system, most states today also require perc testing to directly measure the rate at which water percolates through the soil. The test measures how fast water drains into a standard-sized hole in the ground. The results determine whether the town will allow a septic system to be installed, and system designers use the results to size the leach field.

To conduct a perc test, first talk to the local health department official as requirements can vary significantly from town to town as far as who can conduct the test, the minimum number of holes, depth of holes, required absoption rates, and when the tests can be performed. In general, tests cannot be conducted in frozen or disturbed soil, and some areas only allow tests during certain months of the year – so plan ahead.

A standard septic system will only work if the soil is sufficiently permeable to water, as determined by a “perc” test. If the test fails, you may need a more expensive alternative system –or the site may be unbuildable.

Test procedure. A typical perc test consists of three or more holes dug about 30 to 40 feet apart in the proposed drain field area (see illustration). The holes are typically 6 to 12 inches in diameter and two feet deep, the typical depth of the trenches in a leach field. Two inches of clean sand or gravel are placed in the bottom of the hole. Since the perc test is meant to simulate the actual conditions in a working septic system, the soil is then “pre-soaked” for several hours.

Next, the technician fills each hole with water to a depth of 6 inches above the gravel and measures how much the water drops in 30 minutes (or less for highly permeable soil that drains quickly). The times are carefully documented and used to calculate the percolation rate – the time it takes for the water to fall one inch. This is usually expressed in minutes per inch of drop. A rate of 60 minutes per inch, meaning the water dropped one inch in 60 minutes, is often the cutoff point for a standard gravity-flow septic system, although the number varies from 30 to 120 minutes in other states. Test results are usually good for two years, but as with all things perc, check with the town health department. Some towns require that a town official be present to witness the test.

Even if your site fails a perc or deep-hole test, all is not lost. For sites with high water tables, you may be able to “de-water” the leaching area by strategically placing gravel-filled trenches and subsurface drain pipe to conduct water away from the drain field. You’ll need a highly experienced earthwork contractor, and possibly the help of a civil engineer or geotechnical engineer, to make this work.

Also, a wide range of alternative septic systems have been developed in recent years for use on almost any type of site. Find out which systems are approved for use in your area and which might be suitable for your site. In general, these systems cost more and many require pumps, alarms, and other components that require more monitoring and maintenance than a standard septic system. As these become more common and more widely accepted, formerly unbuildable lots may all of a sudden become approved building lots. As with all new building technology, however, look at products and systems with a proven track record in the field.


  1. otis bivens says:

    Can you have storage buildings without septic service?

    • buildingadvisor says:

      You would not need a septic system for an uninhabited storage building without plumbing. If you wanted a sink, kitchen, bathroom, or other water usage that discharges gray water or sewage, then you would need an appropriate way to process the effluent or store it for pumping. Check with your local health department to find out what rules apply.

  2. Bill Cranshaw says:

    Does Drought Help Perc Test?

    Does the current drought conditions help/hurt the perc outcome? Boston, MA area

    • buildingadvisor says:

      Parched ground will certainly drain faster (perc better) than ground that has a high moisture content. For that reason, and to avoid frozen soil, many towns limit perc tests to the wetter or warmer months of the year. In any case, most towns require that the test hole be saturated with water for a number of hours before the test. The purpose is to create worst-case conditions since the septic system is expected to perform well in both dry and wet weather. In addition to the pass-fail criteria, the perc test determines the required size of the leach field.

      Drought conditions also lower the water table, which is usually assessed by a “deep hole” test conducted along with the perc test. Technicians look for mottling in the soil to visually determine the seasonal high water table, so the actual conditions the day of the test are not that important. Of course, a test hole that fills with water is not a promising sign for permitting a conventional leach field (although an alternative system might still be allowed).

      If you game the system somehow and get better results than warranted by the soil conditions, then you could end up with a septic system that fails due to an undersized leach field or one that pollutes the groundwater in wet weather – not a good outcome for anyone.

  3. manny singh says:

    Cost of Perc Test?

    What is the cost of a percolation test, including a 15 ft. deep-hole test, and filing the results with building department. How much would the total cost be?

    • buildingadvisor says:

      It is hard to generalize as the testing procedures, number of test holes, and fees associated with obtaining a septic permit vary a great deal from one town or city to the next. Also sites with difficult access or difficult digging with shallow rock or dense clay will be more expensive. In some rural areas, all you need a shovel, a bucket of water, and a watch. In more heavily regulated areas, you may need a licensed technician, an observer from the town, and a backhoe. If you are digging a 15 ft. pit, you clearly need a services of an excavator.

      The average cost for a perc test ranges from $300 to $700, or closer to $1,000 if an excavator is needed. If a deep-hole test or is also required, the total cost is more likely to range from $1,000 to $2,000. Always better to estimate high and be pleasantly surprised if costs come in lower.

      Best of luck with your project!

  4. 5BR House on 3BR Lot

    We are interested in building a five-bedroom house in middle TN to accommodate our growing family. There is an auction coming up with land that would be the perfect location for us. However it says it has perked for a three-bedroom house. I have read you can just build whatever you want, but it would be considered a three-bedroom. My issue with that would be the potential for problems down the road if we still have six people living there. Is it possible they didn’t test to see if it would accommodate a bigger house or would there be anything we could do to be sure we wouldn’t have any problems if we were to buy and build on this property?

    • buildingadvisor says:

      The maximum number of bedrooms is not determined by the perc test, but by zoning regulations. This is usually governed by the size of the lot, the zoning district, and a host of rules each community develops to regulate growth. If the lot is approved for a three-bedroom house, you cannot legally build more than three bedrooms.

      The perc test determines whether or not you can install a septic system on the property. The results of the test are also used in designing the leach field. The septic system is designed for a certain number of bedrooms, with the assumption that each bedroom may have up to two occupants. So a three-person septic system should be OK for up to six people. However, with that level of usage, you will need to pump out the tank more frequently than with two or three occupants in the home and the leach field may not last as long.

      If you want to build more than three bedrooms, you may run into problems in permitting, or when you renovate or sell the house. Exactly what a town considers a bedroom is highly variable. Some people get around the limit by naming their extra rooms a “bonus room”, media room, home office, etc. Or they finish a room in the attic or basement after they have been issued their occupancy permit. Some municipalities are strict and any room with a standard doorway and a closet will be considered a bedroom. Other towns are a lot more lenient in this area. It really depends on the policies of the town and, in some cases, on the opinion of the individual building inspector or zoning officer.

      You can discuss this with town officials beforehand, or you may be surprised when you submit your house for a building permit and are told that your plan needs to be modified.

      Best of luck with your new home!

  5. William Barbour says:

    Will My Soil Type Perc?

    Hi, I am looking at buying 13 acres in my home town of Turkey North Carolina. A perc test has never been done on the land and after doing some research I found the land to be Rainy Loamy Sand(RA) and the other half of the land being Goldsboro Loamy Sand (GoA). I found that RA is pretty much a no-no for anything so we are going to build a fishing hole there if we get the land. GoA is found to be “Moderately Well Drained”. My question is what do I need to look at to get an even better idea if the GoA part will pass perc before shelling out the cash to go through with it? Some things are just confusing me and we are running out of time to make an offer on the land. Thank you!

    • buildingadvisor says:

      Goldsboro Loamy Sand, as you have already discovered, is classified as “moderately well-drained and moderately permeable” soil type. Loamy sand falls somewhere in the middle of soil absorption capacity (see Table 2.6). In general, that bodes pretty well for passing a perc test, as long as you don’t have other problems such as a high seasonal water table or rock/hardpan too close to the surface. In that case, you could still probably get a mound system approved. The required absorption rate to pass a perc test varies with local regulations, so you really need to consult with local experts.

      In general, you want a well-drained, sandy soil for a leach field. A typical loam is 40% sand, 40% silt, and 20% clay, but these percentages can vary a great deal depending on the specific soil type – there are over 12,000 soil types identified in the US. Too much clay in the soil slows down absorption, which can lead to a failed perc test. Too much course sand or gravel (a much less common problem) can flunk the test by draining too fast, potentially threatening the aquifer.

      Short of paying for a deep-hole and perc test, you might have some luck talking to local companies that design septic systems. They should be familiar with local soil types and local regulations and may be willing to provide some insight about your building site. They might be reluctant, however, to give an off-the-cuff opinion for fear of being sued if they turn out to be wrong.

      Short of conducting a full test, you might also have luck getting someone to do a 3” test bore to take a look at a soil sample. This should give a soils scientist enough info to tell you whether the site is likely to be suitable for a conventional leach field.

      Perc testing is typically done by an engineer (civil, soils, sanitary, or geotechnical), or in some areas by a licensed septic system designer. Some jurisdictions allow owners to do their own perc tests. Contact the local government agency that oversees septic system permits for specific information. If you’re not sure who to call, start with the department of building inspection, who will refer you to the proper folks – usually the dept. of health.

  6. Do Large Lots Require Perc Test?

    If you own ten acres do you need a perc test?

    • buildingadvisor says:

      The answer to your question depends on state and local regulations. Your best bet is to contact the department that oversees septic permits in you area, typically the dept. of health, sanitation, or building inspection. They will be able to tell you who needs a perc test, who can perform it, what time of year, etc.

      In our state, Vermont, most properties of 10 acres or more are exempt from perc tests under state law. But you will still need a perc test, or at least a deep hole test to examine soil types and seasonal water tables, in order to design a proper septic system. If the site is not suitable for a conventional leach field, then it will not work well and you may end up with backed-up pluming and a soggy leach field with potential health risks.

      Bottom line: You may want to conduct a perc test even if it is not required by law in your area.

  7. After Failed Perc, Can I Try Second Location

    I own a 3.5 acre lot that sits on a hill and is nearly all wooded. It failed a perc test before I purchased it. Is it possible for it to perc somewhere else on the property?

    • buildingadvisor says:

      It is possible that you could pass the test in a different location or another time of year when the weather is drier. However, some municipalities restrict the time of year when perc tests can be performed. The logic is that you want to design a system that will work in all weather, not just in the dry season.

      The perc test needs to be performed where the leach field will be located and is used, along with the number of bedrooms, to determine the size of the leach field. In addition, the type of soil and seasonal water table are determined by observing the soil in a deep hole test. This determines how much sand or gravel needs to be brought in to keep the trenches a safe distance above the water table or impermeable soils.

      If the site has more than one suitable building site, then it may be worth testing in another location. A licensed septic system designer or engineer (civil or sanitary) could help you identify the most suitable sites. Your municipal building department or board of health can provide you with names.

      If the site cannot pass the test for a conventional septic system, it is still possible that a more expensive alternative system would be approved. The most common is “mound system” where enough granular fill is brought in to create a raised leach field. A septic system engineer can help you evaluate your options and the associated costs.

  8. Who Does Perc Test?

    Who do I call to have a perk test done? There is no way I can do that myself.

    • buildingadvisor says:

      In some areas, you can legally perform the test yourself; others require that the tester is licensed by the state (usually by the Dept. of Environmental Quality – DEQ). Check with the your local building inspector or health department to see what is required in your jurisdiction and to get names of qualified inspectors. These might be engineers, septic system designers/installers, or other building professionals. In addition, you will need to hire an excavator to dig the holes for the perk test and deep hole test, if one is required.

  9. Is 18-Year-Old Perc Test Valid?

    I am buying 8.8 acres with lakefront in Willow Ak. A perc test was done 8/97 showing less than 30 min per inch. Should this test be done again or should it be ok?

    • buildingadvisor says:

      Most likely, your perc test results will not change much over time, but the test results may no longer be valid. In jurisdictions that require perc testing before building, test results are typically valid for only one to three years. In addition to a perc test, most communities require a deep-hole test to examine the soil types and locate the seasonal high water table. While successful results are necessary to get a building permit in most places, they are also used to design the leach field. So you will probably want to do a new test whether or not it is required by local authorities.



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