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). 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. 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.

  2. I am in the early planning stages of construction in the Teton Valley of Idaho. I would like to do a geothermal loop and I am wondering if I could use the same trenches for the leech field. Since the GT lines are approximately 6-7 feet in the ground I could cover with a few feet of dirt and add the drain pipe, then cover the remaining 3-4 feet. Thoughts?

    • buildingadvisor says:

      Interesting idea and could save you some money, but I’m not sure it will fly. First off, I would make sure it passes muster with both the septic system designer/installer and geothermal contractor – as well as the local code officials, assuming you have building codes in your area.

      A couple of key issues occur to me:

      1. The soil around the geothermal loop gets chilled all winter and could freeze in your area over the course of the winter (which is why most ground loops use antifreeze). Since freezing can be an issue with septic systems in cold climates, it’s possible that combining the two systems would increase the likelihood of septic system freeze-ups.

      2. Leach fields are generally placed on well-drained, undisturbed soil. The percolation rate of the soil (along with the usage) determines the size of the leach field. If you are building your leach field on backfill, you will need to take that into account and use suitable fill. In general, geothermal systems like dense and moist soil, while septic systems like dry and granular soils, so finding a soils type that works for both could be tricky.

      Anyone else have experience with similar installations?

      You can read an interesting discussion on this topic at:

  3. 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.

  4. 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?



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