‘Perc’ Test: What It Is and How It’s Done

Reader Contribution by Jennifer Kongs
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The Small Home, Big Decisions series follows Jennifer and her husband, Tyler, as they build a self-reliant homestead on a piece of country property in northeastern Kansas. The series will delve into questions that arise during their building process and the decisions they make along the way. The posts are a work in progress, written as their home-building adventure unfolds.

In an earlier post about getting our septic system installed, I noted that soil type will determine whether a standard “perc” (short for percolation) test or a more elaborate soil profile survey will be required. By reviewing our soil map (the image is available via the link above), we were pretty sure that our chosen house site would require the fuller soil profile description. But, in a surprising burst of good luck, our septic installer found some nice soil that was high-quality enough to only need a perc test.

So, what does that mean exactly? No matter which waste system we had decided to install — composting toilets for example —— we cannot obtain a building permit without installing an inspected and approved septic system. In order to obtain a building permit, we must provide the county with a completed percolation test sheet, the name of a county-licensed installer, the site plan of the proposed septic system, and the number of bedrooms we plan to have in our home.  Our installer is handling all of the details with our contractor and the county, but most perc tests will follow the same basic process that ours did.

A perc test is, most basically, a test of how quickly water will drain, or percolate, down through the soil; it’s basically a test of soil texture. The septic installer came out with a back hoe and a borer one sunny morning, and he dug several deep trenches (see evidence in photo above, with Tyler modeling for scale) and bored a few smaller holes. After evaluating the soil texture, the installer determined the best site to run the official perc test. In Kansas, this requires digging a total of six holes no more than two days before a scheduled test and inspection. The holes must be dug 8 to 12 inches in diameter and 24 inches deep, at least 25 to 50 feet apart, and distributed evenly over the area of the proposed lateral field. The edges of the holes are then “roughed up” with a sharp implement. Here’s a photo of one of the bored holes, flagged so the county sanitarian could easily find it when completing the test and inspection:


Twenty-four hours before the test, the installer filled the holes with water. (Depending on how quickly the water drains, the holes may have to be refilled to keep them saturated until the county sanitarian arrives.) The sanitarian from the county arrived the next day to do the official perc test, which involved recording how much the water level drops at specified time intervals. Our perc test was successful, meaning the water drained at a sufficient rate to safely house our septic tank’s lateral field. The length of the laterals that our installer will bury was determined by the rate of percolation.

So, ultimately, this means we can build our house at the site on our land we had hoped. And, now we will begin reading more about septic system maintenance. This city-raised couple still has a lot to learn!

Photo of Tyler modeling how large the trenches were dug in order to test if we needed a full soil description of our property; taken by Jennifer.

Next in the series:Green Building Materials, Part 1: Shopping at the Habitat for Humanity ReStore
Previously in the series:  Finalizing Our Passive Solar House Design: Minor Tweaks and Major Planning Tips

Jennifer Kongs
 is the Managing Editor at MOTHER EARTH NEWS magazine. When she’s not working at the magazine, she’s likely in her garden, on the local running trails or in her kitchen instead. You can connect directly with Jennifer and Tyler by leaving a comment below!