Perc Testing in Knoxville, TN

What does a perc test check for?

A perc test, and the soil evaluation that comes before it, answers one question: can this ground take the water from a septic system and clean it before it reaches rock or groundwater? Under Tennessee's septic rules the evaluation looks at:

• How fast the soil takes water. A conventional drain field needs soil that absorbs at 10 to 75 minutes per inch. Slower than that needs a different kind of system; faster than 10 minutes per inch is too fast for a conventional one.
• The water table. The seasonal high water table normally has to sit at least 4 feet below the bottom of the field.
• Rock. Rock normally has to be more than 4 feet below the bottom of the trenches.
• Slope. Ground steeper than 30% is usually unsuitable, and steeper than 50% always is.
• Things to stay away from, such as sinkholes, drainage ways, wells, springs and streams.
• Room for a second field. The lot has to have space for a reserve field, in case the first one ever has to be replaced.

How deep is a perc test?

The rule leaves the hole depth to the type of system being planned, so there is no single number. The holes are 6 to 12 inches across, soaked for at least four hours (usually overnight), and the water level is then measured over a full four hours. Separate borings to check for rock and the water table can go down 6 feet or more.

Why Knox County lots usually need a bigger field

Most of Knox County is red clay over limestone. Clay takes water slowly, so the same house needs far more drain field here than it would on sand. On the sizing model used on this site, a three-bedroom house needs about 2,250 sq ft of field on clay and about 375 sq ft on sand.

Soil can change across a short distance. Ridge tops tend to perc slowly and the river bottoms faster, so two houses on the same road can get very different results.

Who performs a perc test in TN?

Three people are usually involved:

• A licensed soil consultant maps the soil on the lot. For a conventional system, Tennessee decides whether a site is suitable from a high or extra-high intensity soil map drawn by a state-approved soil consultant. Alternative systems need the more detailed extra-high intensity map.
• A licensed engineer, surveyor, approved soil consultant or registered environmentalist runs a perc test, when one is needed. Tennessee only allows a perc test after the soil consultant has found at least 24 inches of undisturbed soil, a slope of 30% or less, and no water problem.
• The permit office reviews it and issues the permit. Knox, Blount, Sevier and Jefferson counties run their own septic programs under contract with the state. In Anderson, Loudon, Union and Grainger counties, a TDEC environmental scientist does the site evaluation.

On our jobs, the soil mapping and perc testing are done by Tennessee-licensed soil professionals we work with. We line them up and coordinate the rest, so you deal with one company from the first call.

How much does a perc test cost in Tennessee?

There are two separate costs: the people who test and map the soil, who set their own prices, and the permit office's fee. TDEC's fee for a new conventional system permit is $400 for up to 1,000 gallons a day, and $500 for an alternative system. A repair permit has no fee. The contract counties set their own fees: Sevier County, for example, charges $300 for a septic permit. Check your county's office for its current fees.

Who pays for a perc test?

The person applying for the permit, which is usually the landowner. If you are buying land, agree in the contract who pays and make the sale depend on the result.

How long does it take to get septic approval?

TDEC says its review generally takes about 10 days, and a permit has to be granted or denied within 45 days of a complete application. The contract counties run their own queues. Knox County says its staff usually finish the first review and pre-site inspection within 3 to 5 working days, and Sevier County currently quotes about three weeks once everything is in. The soil map and any perc test come first, so book them early.

How long does a septic permit last in Tennessee?

Three years from the day it is issued. In Sevier County an expired permit cannot be renewed, and the application starts over. If the approved area is graded or changed before the system goes in, it has to be evaluated again.

Tennessee perc test requirements

Tennessee leans on soil maps more than on perc holes. A perc result on its own does not prove a site is suitable, and perc tests are only allowed on top of a soil evaluation. Where they are used, the holes go on a 50-foot grid for a conventional system and a 25-foot grid for an alternative one, the state has to be told at least three days before testing, and the lot has to be at least 20,000 sq ft with public water or 25,000 sq ft on a private well.

Once the soil work is done, the permit application goes to the county office or TDEC. No septic system can be built without that permit.

What to do if your land fails a perc test

A failed test does not always mean the lot cannot be built on. Tennessee approves several alternative systems for difficult soil, including low pressure pipe, drip and mound systems, and each needs a more detailed soil map. We cover the options in what to do if your land fails a perc test.

Looking for a perc test near you?

We arrange perc tests on building lots and existing homes in Knoxville and across Knox, Blount, Sevier, Anderson and Loudon counties. Buying land? Get the soil checked before you close, because the soil decides what kind of system the lot can take and what it will cost. See the full service area.