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 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). he 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.

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. 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.

  2. 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?

  3. I did my perc hole tests last week and had great results so this week I hired an engineer and a contractor to perform the deep hole test. we hit clay at around 3′ and when we got to 6′ there was a presence of water. My engineer told me that I would not get a BOHA with these results in Westchester County NY. is there anything I can do to get approved?

    • buildingadvisor says:

      As you probably know, septic system regulations are governed at the town or county level, sometimes with overriding state laws. There is enormous variation, so I can’t comment on the particular laws in your jurisdiction.

      In many areas, a site that fails for a conventional system may still be suitable for an “alternative” septic system. The most common type is a mound system which essentially raises the ground level at the leach field with suitable soils to provide the required clearances. A mound system can add $10,000 or more to the cost of your system and requires monitoring and annual maintenance, but is a proven system that is widely used. A wide variety of other systems are out there, but tend to be more complicated and less likely to be approved locally.

      See Also: Are Alternative Septic Systems Allowed?

      There is also the possibility that another area on your site would pass the deep hole test – if you have more than one possible locations for your leach field.

      Your best bet, at this point, is to meet in person with the building and/or health department that has jurisdiction. Bring your test results and ask what options you might have to build on the site. In my experience these people are knowledgeable and helpful if approached as a resource prior to building.

      If you are unable to get anywhere with local officials, a good real estate or construction lawyer in your area would be your last resort. It is possible that a “hardship” variance could be granted allowing you to build on the site despite your test results.

  4. A piece of land I am considering purchasing already has a septic system installed for a 3 bedroom 2 bath structure. Should I still request a soil test for a foundation be done and make that contingent on the purchase? I have been told that since a standard septic was allowed that the land should be suitable to build on as it is. If the owner refuses this, are they expensive to have done?

    • buildingadvisor says:

      Typically, the limiting factor for building is a perc test, which apparently has already been done if an approved septic system has already been installed. Assuming that you are in an area that requires perc test and approved septic system designs, I would just ask for certification that the town/county building department (or health department) has approved this system for a three-bedroom house.

      If you will need well water, that is another contingency that you might want to include in the contract – although you would have to pay to drill a test well. I would only recommend this in areas with questionable well water. A local well driller can be a good source here.

      As for testing to see whether the land is suitable to build on, this is usually not a problem unless you have a special problem such as buried toxic waste, filled land, organic soils such as peat, solid ledge (a problem if you want a basement), wetland areas where building is prohibited, or a site with extreme water problems. While these problems are relatively uncommon, they are certainly worth considering when looking at land.

      Almost all of these problems can be solved with money. Even building on legal wetlands is sometimes allowed if you create replacement wetland areas. Unless you suspect one of these types a problems, a site inspection – typically by a civil engineer – is not usually required. However, if you have reason to suspect problems, or just want to sleep easier, by all means get an engineer to examine the soil conditions in the area where you intend to locate the house.

      I can’t tell you what it will cost, but, around here, I’ve have engineers do limited inspections of structural plans or land issues for well under $1,000. Find a small engineering firm, describe what you need done, and get cost estimate before proceeding. The more you can narrow his scope, like inspecting for just for water issues, the less you will pay.
      Read more about PROBLEM SOILS

  5. Jim Nosek says:

    Looking at 40 acres in Northern Arizona. Is the perc testing the seller”s responsibility or is that in the negotiation price for the land?

    • buildingadvisor says:

      Who pays for the perc test and deep hole test is a matter of negotiation. The specifics of the test – how it is done, who must do it, time of year, and so on – vary greatly from one location to another, so make sure you check with the local authorities (typically the health and/or building department) on what is required in your area.

      I would definitely make any offer to purchase contingent on the lot passing a perc test. If the seller refuses to sell on that basis, it would probably be wise to walk away.

  6. A building lot we are considering has failed perk tests for over 35 years but suddenly passes a perk test. How is this possible?

    • buildingadvisor says:

      Assuming the soil in the area has not been removed and replaced, the biggest variables are the water table and moisture content of the soil. For example, a site may fail in the rainy season (or after the snow melts) but pass in drier weather. Also one part of a site may fail while another will pass since soil and groundwater conditions can vary a great deal within the same lot.

      The perc test itself varies from a pretty formal test conducted by a soils engineer to a guy with a bucket and a wristwatch – depending on the requirements of your local government. So some tests are more accurate than others.

      I can’t say why this particular site would suddenly pass suddenly after 35 years of failure. Did it pass by a good margin or just squeak by? If I were buying this lot, I would definitely want a repeat test and would get the opinion of a septic system designer or soils engineer. In addition to a perc test, most cities and towns also require a deep hole test to examine the soil type and seasonal high water table. I would want a complete picture of the soil’s capacity to process sewage by an authorized inspector, and assurances from the town that the lot is buildable, before proceeding.

  7. Anonymous says:

    We bought 6.82 acres wooded land in 2000. The previous sellers had provided us the
    perc test that was done in 1996. Now we want to sell the land. Do we need to obtain
    the soil test again? Please advise.

    • buildingadvisor says:

      In most localities, perc tests don’t last forever, but how long they are valid is up to the municipal government (town/city/county) with jurisdiction over the lot. In many places, perc tests can be “updated” for a fee without doing additional testing. Check with your local building department (or health department in some locales).

      While you do not need to have current perc test results to sell a lot in most places, it will certainly make the lot easier to sell and potentially more valuable.

  8. I live on 18 acres, mostly rock/clay and the perk tests were originally for three-bedroom house. Next door is 18 acres, just sold to a company which plans to expand a 3-4 bedroom to 10-12 for commercial use. Is this possible? In this day and age are we going backwards? Or is payola still as good as Bitcoin?

    • buildingadvisor says:

      In general, what your new neighbors can build depends primarily on zoning and health department regulations. Zoning would cover the size and type of building and its usage; the health department would regulate on-site sewage.

      If they are building outside of the zoning rules, they would need a variance. As an abutter, you should have been notified if they applied for a variance, and been given a time-frame to appeal.

      If their planned use is allowed by zoning, they still would need approval for the commercial-sized septic system, unless they are tying into a municipal system). Maybe they got approval for an “alternative” system, which are allowed now in many areas.

      All of this is public record, so you might want to pay a visit to the local zoning and building department to find out what the deal is. The folks there can be a great help.

  9. Bob Frazier says:

    Does the temperature have to be within a certain range for a perc test?

    • buildingadvisor says:

      When and who can perform a perc test can is governed by local regulations, so it’s always best to check with the building or health department in the municipality (town, county, or city) that has jurisdiction. During cold winter weather, the ground may be frozen deeply, which could effect the results since frozen soil will not absorb water without the proper precautions. During the wettest time of the year, the a high water table can be an issue. In addition to town officials, septic system designers, installers, and inspectors can be good sources of insight into local conditions and regulations. See more on the Best Time of Year for a Perc Test.

  10. What if a home was built with a septic tank system and purchased without the buyer knowing the land didn’t perc. The perc test was “fudged”. Is this illegal?

    • buildingadvisor says:

      This is really a legal question and I’m not a lawyer, so I cannot give you a definitive answer.

      In general, most states now have pretty strong disclosure laws requiring sellers (and their real estate brokers) to inform buyers about known defects in a property. Some states have a mandatory disclosure form which the seller must sign disclosing such defects and mold, radon, wet basements, etc. A failed or inadequate septic system, not in compliance with building codes, would certainly qualify as a defect that should have been disclosed.

      If the septic system met code when the house was built, but no longer did when the house was sold, different rules may apply. Some states require that septic systems be brought up to code (along with smoke detectors and certain other items) upon sale of a property.

      The laws governing this are different in every state so you really need to check with a local real estate lawyer. Many lawyers will answer a few questions for free – or you can pay them for an hour or two of their time to review your situation and offer advice (don’t forget to get a cost estimate from the lawyer). A licensed real estate broker – a broker, not an agent — should also be very familiar with the disclosure laws in your state and may be able to point you in the right direction.

      One hitch is you would need to prove that the owner knew about the defect. If you have solid evidence that the perc test was “fudged,” and that the seller knew this and lied to you, it sounds like a case of fraud, regardless of local real estate laws. A call to your state’s office of consumer affairs and/or the attorney general’s office might also be helpful and won’t cost you anything. Best of luck!

  11. My question is whether Alcovy and Santuc are drier soils, which would allow for a drip septic system vs. more moist soils? If a lot does not perc, does the soil scientist take into consideration that heavy rains that may be coming, and the water that should be flowing through the culvert pipe across the street, but doesn’t because the pipe is broken? Also is the flow of a natural spring-fed stream taken into account? What if it’s a drought when soil testing is done? Does the soil scientist still have to consider the other mentioned items above?

    • buildingadvisor says:

      Alcovy and Santuc “soils series” which are large groups of related soils, so it’s hard to say whether yours will perc without testing. In general, these are moderately well-drained soils, so there’s a good chance they will perc.

      Weather conditions do effect perc tests, which measure how fast water drains though the soil indicating how well it can treat sewage. For example, in very dry weather, water may drain faster than normal. Therefore, many towns specify what time of year a test can be done – typically during the winter or spring when soil is more saturated (but not when soil is frozen). Allowable times may vary from year to year, or during unusually wet or dry weather.

      Most towns also require a “deep hole” test to observe the soil layers and determine the seasonal height of the water table, which must be safely below the leach field.

      If a lot does not perc, then the locations of culverts, streams, and other surface water won’t help you. However, they would limit where you could build as the septic system must be a certain distance from streams and other surface water.

      You can hire a soil scientist to do a quick analysis before you invest in a full perc test and deep hole test.

      • When purchasing land, who pays for the testing, seller or buyer (normally)

        • buildingadvisor says:

          Who hires the tester and pays for the test is negotiable like most things in a real estate transaction. In an improved lot, the test has usually been completed before the land is offered for sale. In this case, the buyer needs to confirm that the test was done properly and that the results are recent enough that they are still valid. The town building, zoning, or health department should be able to provide that information.

          If the lot is unimproved, who pays is negotiable. From my perspective, it’s best is for the buyer to hire the tester and pay for the test so that he gets the full results and any relevant information about the soil and drainage conditions. If possible, write the offer with a contingency that requires the seller to pay if the test fails. You can factor the cost of the test, if successful, into your offer amount.

  12. I read about the Failed Perc test solutions and did not find any reference to one of the most easily installed, extremely cost effective and most simple solution — its called the Self Composting Toilet. This toilet is an all in one unit — check out Sun-Mar or Envirolet for more details. I had bought one way back in the early 70’s and used it at our cottage, for a family of 5 it was absolutely excellent, cheap (they sell for approx $1000 and up. If the perk test is negative and you want to dig your yard out and add mucho shale, gravel etc etc it would cost in the range of tens of thousands of dollars while a composting toilet is environmentally friendly – cost effective and self contained. check it out mel

    • In some states no septic system equals not being allowed to have running water on the site. For instance, in Tennessee if you can’t fit a septic system on a lot without sewer access you are not allowed to have running water on that property. Graywater must go through a septic system before being released into the surrounding environment.

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