What Is a Perc Test in AL? Complete Guide for Mobile-Area Properties
A percolation test - universally called a "perc test" - is the foundational soil evaluation that determines what type of septic system can be installed on a given property. In Mobile and throughout Mobile and Baldwin County, where clay-heavy soils create significant variation in drainage characteristics, the perc test result is often the single most consequential factor in septic system design and cost.
What a Perc Test Measures
A perc test measures the rate at which water moves through soil - specifically, how many minutes it takes for one inch of water to drop in a test hole of defined dimensions. This "minutes per inch" (MPI) result directly determines the minimum required drainfield area for a given daily flow rate, and whether a conventional system is even permissible. AL ADEM uses perc test results along with soil profile evaluation to classify sites and specify appropriate system types. Sites with perc rates between 1 and 60 minutes per inch are generally suitable for conventional systems, with slower rates requiring larger drainfield areas. Sites slower than 60 MPI typically require alternative systems - mound, ATU, or sand filter.
Perc Test Results in Mobile's Clay
In Mobile's clay-dominant soils, perc test results commonly run 30 to 90+ minutes per inch. Results above 60 MPI - common in the heavier clay areas of Mobile County - disqualify properties from conventional drainfield installation under AL code. This is why mound systems are so common in the Gulf Coast - they are frequently the only compliant option for sites where native soil simply cannot perc fast enough for a conventional system.
When Perc Tests Are Required
In AL, perc testing is required for all new septic installations and for major repair or replacement projects. The test must be conducted by or under the supervision of a licensed professional and the results submitted to the county health department as part of the permit application. Perc test results remain on file with the county and are available to future owners as part of the permit documentation.
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✆ Call 251-351-8091Perc Test FAQs
What does a perc test measure?
A perc test measures the rate at which water moves through soil — specifically, how many minutes it takes for one inch of water to drop in a test hole. This “minutes per inch” (MPI) result determines the minimum required drainfield area and whether a conventional system is even permissible.
What perc rate is required for a conventional septic system in Alabama?
Sites with perc rates between 1 and 60 minutes per inch are generally suitable for conventional systems, with slower rates requiring larger drainfield areas. Sites slower than 60 MPI typically require an alternative system like a mound, ATU, or sand filter.
Why do so many Mobile-area properties end up needing mound systems?
Mobile's clay-dominant soils commonly perc at 30 to 90+ minutes per inch, and results above 60 MPI disqualify a property from conventional drainfield installation under AL code — mound systems are often the only compliant option on those sites.
When is a perc test required in Alabama?
Perc testing is required for all new septic installations and for major repair or replacement projects. It must be conducted by or under the supervision of a licensed professional, with results submitted to the county health department as part of the permit application.
Who uses perc test results and how long are they kept on file?
AL ADEM uses perc test results along with soil profile evaluation to classify sites and specify system types. Results remain on file with the county and are available to future property owners as part of the permit documentation.