A concrete septic tank does not fail because the concrete gives out. It fails because someone set it on a bad base, connected the pipes wrong, or backfilled in a hurry. The tank we pour will outlast the house it serves. Whether it works on day one comes down to the install.
We have delivered and set thousands of these tanks across Northwest Montana since 1989, and the pattern never changes: a good install is boring, and a bad one is expensive. This is how to install a concrete septic tank the right way, step by step, from the excavation to the final inspection.
Septic tank installation is the process of excavating a level hole, preparing a compacted bedding base, setting the tank, connecting the inlet and outlet piping, bringing the risers to grade, and backfilling before an inspection. Here is the short version, then the detail.
The 8 steps to install a concrete septic tank
- Pull the permit and mark the tank and drainfield location.
- Excavate a level hole sized to the tank plus working room.
- Prepare a compacted, level bedding base.
- Set the tank level onto the bedding with a crane or boom truck.
- Connect the building sewer to the inlet and the outlet to the drainfield.
- Install the effluent filter and bring risers and lids to grade.
- Backfill in lifts and water-test for watertightness.
- Pass the county inspection, then cover.
That sequence is the whole job. The difference between a system that runs for decades and one that gets dug up in year two lives inside those eight steps. Let’s take each one.
Step 1: Permit and layout
Nothing goes in the ground before the permit is in hand. In Montana, septic permits are issued at the county level by the local health department or sanitarian, under Montana DEQ standards. The permit follows the system design: tank size, drainfield location, and setbacks from wells, property lines, and water.
Mark the tank location and the drainfield before the excavator shows up. Check your setbacks against the approved design, not from memory. A setback means keeping the tank a set distance from things like your building and your property line, commonly at least 10 feet from each. Moving a tank after it is set is a bad day.
If you have not sized the system yet, start with the design and sizing work first. Our guide to septic system design covers how the tank and drainfield get sized together. We can help you lay the system out, and on a pumped system we run your pump calculations too, so the layout is right before you dig.
Step 2: Excavate a level hole
Dig the hole to the tank footprint plus about a foot of working room on each side. Depth is set by two things: the outlet invert has to match the grade running to your drainfield, and the lid has to end up reachable from the surface through risers. If you have a pumped system, the outlet elevation is more forgiving than on a simple gravity system, because the pump moves the effluent instead of relying on slope.
The floor of that hole matters more than the walls. It needs to be flat, undisturbed, and free of rock, roots, and soft spots. If you over-dig, you do not backfill loose soil and set on top of it. You build the base back up properly in the next step.
One Montana note: account for frost depth and the seasonal water table in your area. In high groundwater, a concrete tank can float before it is full, and the excavation has to account for it. Where that is a risk, we handle it, including heavier drive-over lids that add ballast against buoyancy.
Step 3: Prepare the bedding base
This is the step that ruins more installs than any other. A concrete septic tank is a rigid, heavy unit, and it needs uniform, level support across the entire bottom slab. Set it on high spots, buried rock, or uneven fill, and you build point loads into a tank that was engineered for even bearing.
Lay 4 to 6 inches of compacted sand or 3/8-inch minus gravel across the hole floor, screed it flat, and check it with a level in both directions. The base does the quiet work here: get it right and the tank sits true, drains as designed, and never sees the stress that starts a crack.
We build a base like this the same way every time, because it is the cheapest insurance on the job.
Step 4: Set the tank
Here is where a precaster earns its keep. Our tanks are big and heavy: a 1000-gallon tank runs 68 inches wide, 99 inches long, and 64 inches tall, and the 1500-gallon runs 68 by 144 by 64. That is thousands of pounds of concrete you cannot muscle into place.
We deliver and set our tanks with our own crane trucks and trained driver-operators. That crane placement is the difference between a supplier that drops a tank in your yard and one that places it, level, in the hole, in one controlled lift.
The tank comes off the truck, swings over the hole, and lowers straight down onto the bedding you prepared. No dragging, no dropping, no re-leveling. Set it slow, check it level front-to-back and side-to-side, and confirm the inlet and outlet are oriented to your piping before the crane lets go.
Step 5: Connect the inlet and outlet
The building sewer line ties into the tank inlet. The tank outlet ties to the drainfield or the next component in the system. Use the pipe size and material your design calls for, usually Schedule 40 PVC, and keep the pipe slope consistent so solids settle in the tank instead of running out to the field.
The inlet and outlet baffles are already cast or fitted into the tank. Their job is to keep the scum layer in and let the clarified middle layer move on. Do not knock them loose during connection.
Step 6: Effluent filter, risers, and lids
Install the effluent filter on the outlet. It catches solids before they reach the drainfield, and it is far cheaper to clean a filter than to replace a clogged field.
Then bring the access to the surface. Risers stack on the tank lids and extend up to grade so the tank can be pumped and inspected without digging. Set the lids so they are secure and, where kids or traffic are a factor, rated and fastened for it. A tank you cannot reach is a tank nobody maintains.
Step 7: Backfill and water-test
Backfill in even lifts around the tank, not all on one side. Uneven backfill can shove a tank off level after all the care you took setting it. Many installers fill the tank with water as they backfill to balance the pressure and, at the same time, check that the tank holds water.
A concrete septic tank should be watertight. If it holds water on the test, it keeps groundwater out and effluent in for decades.
Step 8: Inspection and cover
The county inspects the installation before it gets covered. That is the point of the permit: an independent set of eyes confirms the tank, the connections, and the setbacks match the approved design. On a pumped system, the county also confirms the squirt height, so it knows the pump and piping deliver even pressure to the field. Once it passes, backfill to grade, leaving the risers accessible, and the system is ready.
Build it once, build it right, and this is the last time anyone thinks about the tank for a very long time.
Installing a pumped septic system: what changes
Not every site drains by gravity. When the drainfield sits uphill from the tank, or too far away for a gravity line to hold its slope, the system needs a pump. We do a number of these across Northwest Montana.
A pumped septic system uses an effluent pump and control floats, usually in a separate pump tank after the septic tank, to move clarified effluent to the drainfield under pressure instead of by gravity. That changes a few things at install. The tank outlet elevation is more forgiving, because the pump does the lifting. The drainfield is often a pressure-distribution field with laterals, and the county inspection checks the squirt height, the height effluent rises from the lateral orifices, to confirm even pressure.
Here is where we do more than sell you the tanks. We run the pump calculations and lay the system out with you, so the pump, the floats, and the field are sized to work together before anyone digs. Get the pump math wrong and the field never doses evenly. Get it right up front and the system runs quietly for decades.
How deep is a concrete septic tank buried?
Most concrete septic tanks are set so the lid sits somewhere between about 6 and 24 inches below the surface, with risers extending the access up to grade or above grade. The exact depth is driven by the outlet invert, which has to match the elevation feeding the drainfield, and by local frost depth. Deeper burial needs taller risers and, for traffic areas, a traffic-rated lid.
Can you install your own septic tank?
In most Montana counties, the tank itself must be set as part of a permitted system, and many jurisdictions require a licensed installer for the connections and the inspection sign-off. That is true across the counties we serve. Even where a homeowner can do some of the work, the tank is too heavy to set safely without a crane or boom truck. The realistic split: a licensed installer or excavator handles the dig, the set, and the connections, and the precaster delivers and places the tank.
Precast vs. site-poured: why installation is faster
A precast concrete tank shows up cured, tested, and ready to set. There is no forming, pouring, and waiting on site. That is most of why precast installs in an afternoon instead of a week, and why the quality is consistent: the tank was made in a controlled plant, not in a hole in variable weather. If you are still weighing materials, our concrete vs. plastic septic tank comparison breaks down the tradeoffs.
Frequently asked questions
How long does it take to install a concrete septic tank?
Setting the tank itself takes a few hours once the hole is ready. A full system install, including excavation, the tank, connections, the drainfield, and backfill, is commonly completed in one to a few days depending on site conditions and access. Precast tanks save time because they arrive cured and ready to set, with no on-site pour or cure delay.
How much does concrete septic tank installation cost?
Total cost depends on the tank size, excavation and site access, the drainfield, permit fees, and local labor rates. The tank and its delivery are one line item; the excavation, connections, and drainfield are usually handled by the installing contractor. Because those variables swing widely by site, an itemized quote is the only accurate number.
Do you need a permit to install a septic tank in Montana?
Yes. Septic systems in Montana are permitted at the county level through the local health department or sanitarian, under state DEQ standards. The permit covers the system design, tank size, drainfield, and setbacks, and an inspection is required before the installation is covered.
What kind of base does a concrete septic tank need?
A concrete septic tank needs a level, compacted bedding base, typically 4 to 6 inches of sand or 3/8-inch minus gravel screeded flat across the excavation floor. Uniform support across the entire bottom slab prevents point loading. Setting a tank on rock, roots, or uneven fill is the most common cause of installation problems.
When do you need a pump for a septic system?
A septic system needs a pump when the drainfield is uphill from the tank or too far away for a gravity line to hold its slope. A pumped system uses an effluent pump and control floats to move clarified effluent to the drainfield under pressure. Pumped systems also allow more flexibility in tank and outlet placement, since the pump does the lifting instead of gravity.
How is a concrete septic tank set into the hole?
A concrete septic tank is set with a crane or boom truck that lifts the tank off the delivery truck and lowers it in one controlled motion onto the prepared bedding. The tank is checked for level in both directions and oriented so the inlet and outlet line up with the site piping before it is released.
Get the base right and the tank does the rest
The concrete is the easy part. We have been pouring watertight tanks that last for decades since 1989.
Knowing how to install a septic tank that lasts comes down to three unglamorous things: a level compacted base, a tank set true, and connections that respect the baffles. On a pumped system, add a fourth: pump math that doses the field evenly. Get those right and you will not think about your septic tank again for a generation.
If you are planning a system in Northwest Montana, North Idaho, or Southern Alberta, we can help you install a concrete septic tank before the excavator shows up, not after.
Talk to the plant that delivers and sets your tank
When you install a concrete septic tank from our plant, your installer does not start from scratch. We build the tank in our NPCA-certified plant, deliver it on our own crane trucks, and set it level in the hole in one lift, so the job starts from a tank that is already placed right. On pumped systems we go further, running the pump calculations and laying the system out with you before anyone digs.
Every tank ships with one more thing built in that most homeowners never think to ask about, and it is the reason our tanks pass the water-test decades later.
Request a septic tank quote and tell us about your site. If you want the product specs first, the concrete septic systems page has sizes and drawings, and the complete guide to concrete septic tanks covers sizing, lifespan, and cost.