What to Expect During a Septic System Installation in Thurston County: A Step-by-Step Homeowner Guide
Whether you are building your dream home on a rural parcel or replacing a failing system at your current residence, installing a septic system is one of the most critical steps in property development. Because households not served by public sewers depend on septic systems to treat and dispose of wastewater, a well-designed and properly installed system is essential for your family's health and your property value.1
For many homeowners in Thurston County, the septic installation process can feel overwhelming. It involves soil testing, engineering designs, county permits, and heavy excavation. Knowing what to expect can remove the stress and help you plan your construction timeline effectively.
Here is a step-by-step guide to how a septic system goes from an idea to a fully functioning wastewater treatment system in Thurston County.
Step 1: The site evaluation and soil test
Before any digging begins, you must determine what type of system your property can support. There are different types of septic systems that fit a wide range of soil and site conditions.1
To find out what your property needs, a licensed septic designer will perform an on-site evaluation. This involves digging test holes (often called "perc holes") to examine the soil profile. The designer looks at the soil type, the depth to groundwater, and the presence of restrictive layers like hardpan clay.
Because the soil below the drain field provides the final treatment and disposal of the septic tank effluent, the soil type dictates the system design.1 For example, gravelly soil may drain too fast to provide treatment, while clay soils may be too tight to allow wastewater to pass through.1
Step 2: System design and county permitting
Once the soil is evaluated, the designer will create a customized septic system plan. This plan will specify the size of the tank, the type of drain field (such as a conventional gravity system, a pump system, or a mound system), and the exact location of the components on your property.
Every new home or building served by a septic system is also required to have a designated replacement or reserve area.1 This is a designated area suitable for a new drain field if the primary one ever fails.1
The completed design is then submitted to Thurston County Environmental Health for approval. The county reviews the plan to ensure it meets all local and state health codes. You cannot begin installation until the county issues an approved permit.
Step 3: Site preparation and excavation
Once the permit is in hand, the physical work begins. A licensed septic installer and excavation contractor will arrive on-site with heavy equipment.
First, the area is cleared of brush and debris. Then, the excavation begins. The contractor will dig a large hole for the septic tank, ensuring it is set at the correct depth and perfectly level. Next, they will excavate the trenches or bed for the drain field.
This is a delicate process. The contractor must carefully follow the approved design, ensuring the trenches are dug to the exact depth and slope required. They must also protect the surrounding soil from compaction, as heavy equipment should not park or drive over the drain field area.1
Step 4: Installing the components
With the excavation complete, the system components are installed.
- The Tank: A concrete, fiberglass, or heavy-duty plastic septic tank is lowered into the hole. The sewer pipe from the house is connected to the inlet of the tank.
- The Drain Field: A network of perforated pipes (or "laterals") is laid in the gravel-filled trenches or beds.1 If the system requires a pump, the pump chamber and electrical controls are also installed and wired.
- Distribution: In most gravity systems, the wastewater first flows into a distribution box (d-box) or tee, which then disburses the effluent equally among the trenches in the drain field.1
Step 5: The county inspection (The "As-Built")
Before the system is covered with dirt, it must be inspected. A Thurston County health inspector (or the licensed designer, depending on the permit conditions) will visit the site to verify that the system was installed exactly as designed.
Once approved, the designer or installer will create a "record drawing" (previously referred to as an "as-built").1 This is a diagram showing exactly where your septic system components are located on the property.1 This document is filed with the county and is crucial for future maintenance, pumping, or property sales.
Step 6: Backfilling and final grading
After the inspection is passed, the contractor will backfill the trenches and cover the tank. The soil is carefully graded to ensure that rainwater will flow away from the septic system. Diverting water away from the drain field is critical, as water runoff can saturate the soil and prevent the system from functioning efficiently.1
Finally, the area over the drain field should be seeded. You should plant only grass or shallow-rooted plants over the drain field to prevent soil erosion.1 Never pave over the drain field, build structures on it, or plant trees nearby, as roots can clog and damage the drain lines.1
Choosing the right contractor
The success of your septic system depends heavily on the skill of the installer. A poor installation can lead to premature failure, property damage, and costly repairs.
South Bay Septic and Excavation LLC is a licensed and experienced septic installation contractor serving Thurston, Mason, Pierce, and Lewis Counties. Because we handle both the excavation and the septic installation, we manage the entire physical process seamlessly—from the first scoop of dirt to the final grading.
If you are planning to build a new home or need to replace a failing system, contact South Bay Septic and Excavation LLC at 360-233-2783. We can guide you through the process, work with your designer, and ensure your new septic system is installed to last.
