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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. 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.

  2. 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.

  3. 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!

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