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What Is a Perc Test?


Jun 02, 2020 by Marc Rapport

A perc test is simply a test of the drainage -- or percolation -- ability of the patch of ground being assessed.

But a perc test is not a perk of ownership for people developing land for residential and commercial use: It's usually a requirement.

That's especially true for properties out of the reach of public sewer and water systems, which often require owners to tap in instead of relying on septic systems.

As noted here by Fixr: "A septic tank, unlike local city or town sewer systems, requires a drain or leach field. Good drainage is incredibly important when installing a leach field or drain field and a necessity for the wastewater disposal system."

Why do you need one?

If a septic tank is placed in soil that can't properly drain, the consequences can be unpleasant and costly.

Perc tests determine the right and wrong locations for a septic system, and they're often required by local jurisdictions before a new one can be built or an old one replaced.

That's because septic tanks work by holding wastewater long enough to naturally separate liquids and solids. Clarified liquids then go to a leach or drain field or through a series of trenches a couple of feet below the ground. There they drain into the surrounding soil.

(Here's a succinct description of how this all works from the National Environmental Services Center.)

Among the factors being assessed are proper slope and an appropriate mix of sand and gravel content in soil that will allow the treated wastewater to leach into the surrounding soil. The more sand and gravel, the better. Clay and rock drain the worst.

Eyeing rural property? Make sure it passes this test.

"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," says BuildingAdvisor.com.

That's increasingly easier said than done as prime building sites grow scarce and/or pricier in many parts of the country and failed perc tests become more common on what's left.

What do they involve, and who does them?

The process is relatively simple: digging holes and seeing how long it takes water to drain through them. The percolation rate is simply how much water was put in the holes divided by how much time it took for it to drain. So, if it took two hours for water to drain two inches, that's a perc rate of an inch an hour.

But, it really isn't that simple. While there are home test kits available, who digs those holes, where, and interpreting the results are all important, meaning that for anything beyond lawn and garden use, this is not really a do-it-yourself job.

Check with your county health or environmental agencies to find out if they do the testing or if you can hire an independent engineer. There also are rules about how often the perc tests must be repeated, typically every two to five years.

How much do they cost?

Fixr says the national average is $750, with a low of $150 and a high of $1,500. Add another $575 on average if a survey is needed.

That includes discussion with an engineer about your plans for the property and then the labor involved in digging at least two holes 40 feet apart and then coming back to see how it all drained. Engineers and technicians also can help with issues like strategies for reducing high water tables.

And, even if you do hire a third-party tester, be aware that local officials might well require one of their folks to be on hand during the test, too, to make sure it was done legitimately.

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