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What Is a Septic Perc Test, and Why Do You Need One?

Updated: Jan 3

A septic perc test hole.

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In an earlier article, I discussed waste. Waste is one of the things you need to manage on your own when living an off grid lifestyle, and installing a septic system is a common management method.

But installing an onsite sewage disposal system isn’t as simple as digging a hole and attaching a septic tank to your home’s plumbing. First, you’ll need a permit. And before you can get a permit, you’ll need a perc test.

What is a perc test?

A septic perc test (or percolation test) checks how well your soil allows water to drain.

This test is usually required before a septic system is placed and installed to ensure the soil can support a drain field and allow the proper functioning of your septic system. After all, if your septic tank can’t drain, you’ll end up with costly (and smelly!) consequences in the future.

A septic tank works by separating the solid and liquid waste into layers: scum (floats on top of the other layers), effluent (the liquid layer in the middle), and sludge (solid matter that settles at the bottom of the tank). Beneficial bacteria help break down the solid matter so that it can liquefy and drain from the tank.

The liquid that drains from the septic tank goes into an underground leach field or drain field (the soil absorption system). The drain field must allow the fluids to quickly soak into the earth or risk oversaturation and backing up the entire septic system.

Perc testing is vital to ensure proper sewage disposal and protect environmental health. (If you'd like a book to have handy, try The Septic System Owner's Manual.)

Septic jargon

People in every field of business have their own jargon, and if you’re not sure what they’re talking about, you can get lost in the barrage of new words. Here are some key words and phrases you might hear from your soil engineer or perc test team.

Percolation rate: This measures the rate at which water moves through the soil. You want a rate between 10 and 60 minutes per inch. (It takes about 21 or so hours for your soil engineer to test the soil.)

Soil condition: This is the capacity of the native soil to function for its desired purpose (in this case, for perc tests).

Water table: The water table is the boundary under the soil where water saturates the cracks and crevices in the rock and soil layers. Your soil engineer will look for mottling and moisture in the perc test hole to determine the seasonal high water table.

Septic permit: This is the permit you need before installing a septic system. You’ll need the report from the perc test when you apply for your septic permit.

Leach field: The leach field, or drain field, is where the liquid from your septic system drains into a designated area, thereby protecting the surrounding soil and properly disposing of waste without contaminating groundwater or areas where food is grown for human consumption.

Using a backhoe to dig a test pit for a septic percolation test.

What to expect on perc test day

I wasn’t sure what to expect on our perc test day. I knew we were meeting with our excavator and soil engineer, and I knew they needed to dig a hole, but that was about it.

The whole process was fascinating and went by more quickly than I expected.

We talked to our excavator about where we plan to build our house, the GPS coordinates we provided to the county for our water well permit, and where we envision a future barn, livestock pens, and pasture.

From that information, he suggested the best location to place a septic system for optimal downhill flow and (hopefully) the installation of a gravity-fed system instead of using a pump. (We won’t have the final thumbs up on a gravity-fed system until the soil tests come back, but we’re crossing our fingers!)

The location he suggested wasn’t at all where we had initially imagined it would be, but we listened to his reasoning and decided to take his advice. After all, we’re paying for his expertise!

After we agreed on the best location to place the septic system, our excavator unloaded his backhoe and started digging. In our county, two holes are required, and they have to be 100 feet apart.

The first hole was eight feet deep, as wide as the bucket on the backhoe, and long enough that the soil engineer could walk down a steep slope to get to the bottom of the hole. He scooped out a few handfuls of dirt, placed the soil into plastic bags, and then signaled for the excavator to fill in the void.

In a matter of minutes, you could barely tell where the hole had been.

They repeated the process about 100 feet away, and before we knew it, it was time to go.

What happens if you fail your soil test?

A failed soil test means you can’t install a standard septic system. If you plan to build your home in a rural location without access to a city sewer system, you need a septic system to dispose of sewage and other unsanitary waste.

If you live in an area that allows gray water systems (a.k.a. greywater systems) and composting toilets, you may still be able to figure out how to build your off grid home.

If your lot is large enough, you may be able to request a new test at another dig site and reimagine where your house will sit. And in some cases, a soil engineer can design an alternative engineered system customized for your location and soil type.

But if a standard septic system design is your only option, a failed test may mean using the land for some other purpose or selling it to purchase land where you can build.

The waiting game …

Our soil test will be back in a few weeks. It could be done sooner, but our soil engineer will run all of our tests simultaneously. (We need a soil test for the building permit, too, and that one requires a core drill.) Check back in a few weeks for our update!

UPDATE: Our test came back not-so-great. We have highly expansive soil, so we will need additional support under our foundation. Called "caissons," these concrete piers will reach bedrock around 14' deep and extend a minimum of 12 more feet into the bedrock to support our Quonset hut home!

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