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Why Do a Perc Test Before Buying Land

why do a perc test before buying land

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Published date:

July 03, 2024

Last updated date:

July 04, 2024

By Laura Mueller

It’s important you do your due diligence when purchasing raw land. This includes making sure the property is well-suited to any sort of plans you might have for it – a process that, for many, will include a perc test. 

A perc test, or percolation test, is a way to check the drainage rate of the soil. The primary goal of this test is to tell you whether your land can stand up to a septic system, though it’s also useful for gaining insight into general building and agricultural capabilities.

Here’s what else you need to know about perc tests, including who does them and what the process looks like.

What is a Perc Test? Understanding the Basics

A perc test measures how well water drains in soil. Land buyers, surveyors, and builders rely on this test to make decisions about where and how to place certain structures like septic systems, trenches, and drains, among other features. 

For the purposes of buying land, a perc test gives you information on soil quality and drainage capabilities that you might not be able to gain through other means. You’ll also get insight into some other notable elements of the soil, such as what type of soil you have and how soil slopes around the property.  

You can perform a simple perc test on your own with a few tools and some basic math, but official perc tests (such as those that need to be performed before installing a septic tank) can only be done by licensed professionals.  

How To Do a Perc Test

To do a perc test, you’ll need to follow a several-step process that includes:

  • Digging a hole
  • Filling the hole with water
  • Measuring water absorption after a set period of time
  • Calculating the percolation rate

Get a bit more detail on these steps below, and call in a perc specialist if your interest in how soil drains goes beyond general curiosity. 

Step 1: Digging a Hole

Use a shovel to dig a hole roughly five feet deep and six to eight inches in diameter. You may want to dig multiple holes to test the soil in various parts of the property, focusing on areas with minimal slope.

Step 2: Filling the Hole

Place a wooden stake in the hole to serve as your measuring tool. It should be at least as tall as the hole itself, with measurements in six-inch intervals. Presoak the hole to saturate the soil, waiting until all the water is absorbed. Then fill the hole with water, being sure to write down your starting measurement. 

Step 3: Wait and Measure Again

Set a timer for thirty minutes. When your timer goes off, check your measuring stake again and record the new measurement.

Step 4: Calculate the Perc Rate

To get your perc rate, subtract the final water depth from your initial water depth, then divide 30 by that number. 

For example:

  • Initial water depth: 30 inches
  • Final water depth: 18 inches

30 -18 = 12

30/12 = 2.5

This tells you that your percolation rate is 2.5 minutes per inch.

To Sum-Up

The ideal perc rate for your soil depends on what type of soil you have, your county requirements, and your intended use. Refer to local building codes to determine whether or not your perc results are good enough for your plans. 


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