Considering installing a septic system on your property? Then you’ve most likely already come across the term:
“Perc test” (short for percolation test)
At Wholesale Septic Supply, we help landowners, contractors, and homeowners every day who are beginning to navigate the early stages of septic system design and installation, and the first step is usually a perc test. Let’s explore what a perc test is, how it works, why it matters, and what to do if your soil doesn’t pass the perc test.
A perc test (short for percolation test) is a soil evaluation used to determine how well your soil absorbs water. It’s required by most counties before approving a septic system installation.
Why it matters:
Septic systems will rely on the drain field to naturally filter and absorb wastewater after it exits the septic tank system. If your soil is not up to par and cannot absorb water properly, wastewater may back up or pool on the surface. The backed-up or pooled water may even begin to contaminate groundwater. A successful perc test will indicate your soil can safely support a leach field, which is essential for any healthy, functional septic system.
Perc tests evaluate:
How quickly water drains through the soil
Soil texture and composition (sand, clay, silt, loam)
Depth of the water table
Presence of rock or hardpan layers
Slope and site drainage
These factors will help to determine whether a conventional septic system installation is possible, or if you’ll need an alternative system.
Perc tests are typically required when:
Buying raw or undeveloped land
Building a new home in an area without city sewer
Adding bedrooms to an existing home
Replacing or upgrading a failed septic system
Subdividing land for new development
Nearly all county health departments or environmental agencies will be unable to issue a septic system permit without a valid perc test.
Perc tests are conducted by a licensed soil evaluator, engineer, or septic system designer. The process will generally include the following steps:
Test holes are dug (typically 6-12 inches in diameter, and 18-36 inches deep) in the anticipated drain field area.
Some counties may require the soil to be pre-soaked the day prior to testing, as this helps mimic real-world wet conditions.
Clean water will be poured into the holes, and the rate at which the water level drops is very carefully timed and recorded. If the drainage is slower than average, it may be determined the soil is less suitable for a conventional system.
The evaluator may also choose to take soil samples to better assess texture, color, and depth of the soil layers.
Reporting & Design Recommendations
Based on the final results of the perc test, a septic designer will recommend:
A standard gravity system
An alternative system (like a mound, aerobic, or drip system)
Or no system at all (if the site is unsuitable for septic)
A “failed” test does not necessarily mean you cannot build, it simply means you cannot use a conventional septic system in most cases.
You may still be able to successfully install:
Mound systems (elevated drain fields built with imported sand)
Aerobic treatment units (ATUs)
Low-pressure dosing systems
Drip irrigation systems
These systems may end up costing more, but they can make otherwise unusable land buildable.
Prior to hiring a soil evaluator, soil engineer, or septic designer, do this first:
Every jurisdiction has its own rules and regulations when it comes to testing and permits. Some may require you to enlist the help of their approved professionals.
You’ll want to have the Perc tests done in the designated area where the septic system will be installed.
Having a survey or site plan ready will help your evaluator best choose the proper testing locations and future drain field layouts.
A licensed septic designer or evaluator will be able to conduct the test and help design a compliant septic system based on the soil’s performance
Perc rate results are typically measured in minutes per inch (MPI)— which is how long it takes for water to drop 1 inch.
|
MPI |
Interpretation |
|
1–5 MPI |
Drains too fast — may require sand filter |
|
6–30 MPI |
Ideal for a conventional drain field |
|
31–60 MPI |
May still be usable with design adjustments |
|
60+ MPI |
Too slow — likely requires engineered system |
Note: Soil samples with perc rates under 1 MPI or over 120 MPI are often unsuitable for a traditional system.
How much does a perc test cost?
A perc test will usually cost around $300-$1,000 depending on the location, site prep, and testing depth.
How long are perc test results valid?
Most counties will deem perc tests valid for a period of around 3-5 years, yet some may require retesting if land conditions change due to development, weather events, etc…
Can I do a perc test myself?
No, as most jurisdictions will require testing by a licensed professional to ensure all data points are accurate and legally compliant.
Can I choose where to place my septic system?
Yes, you can certainly make a request, but the final placement will be determined based on the results of the perc test, soil suitability, and working around any obstacles which must be addressed.
If you’re going through the initial phases of a perc test, you should check out all we have to offer here at Wholesale Septic Supply when it comes to our septic system products. We’re the experts in all things septic systems, so if you have any questions, don’t hesitate to contact us. Feel free to browse through our extensive selection of products by clicking the button below. We look forward to working with you soon!
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