If you have a septic tank on your property, it is likely buried underground and out of sight. But how deep under the surface should a septic tank be installed? What factors determine the burial depth?
In this article, we’ll look at:
- Typical septic tank burial depths
- Recommended depth range
- How depth is calculated
- Effects of soil and climate
- Finding the depth of an existing tank
- Concerns with deep burial
- When to call a professional
Typical Septic Tank Burial Depths
In most cases, you’ll find the septic tank buried 2 to 4 feet below ground level. However, depth can range from just below the surface to 6 feet or occasionally more.
Some key points on septic tank burial depth:
- Minimum depth: 6-12 inches of soil cover
- Typical depth: 24-48 inches
- Maximum depth: 4-6 feet
Depth varies based on climate, soil conditions, and terrain during installation. Tanks must be buried below the frost line in cold climates. Sandy or wet soils may also dictate a deeper burial.
Recommended Depth Range
While specific depth depends on site factors, experts recommend a burial of:
- Minimum: 12-18 inches to top of tank
- Maximum: 4-6 feet to top of tank
A minimum of 12-18 inches allows for adequate soil cover to prevent damage from surface activity. Most tanks are installed 2-4 feet deep for protection.
Deeper than 4-6 feet is not recommended. Weight of soil can damage the tank, and access for maintenance becomes difficult.
How is Septic Tank Depth Calculated?
Proper burial depth balances the need for:
- Adequate cover and protection
- Proper sewer line slope and drainage
- Accessibility for pumping and repairs
To determine depth, installers consider:
- Sewer pipe slope from house to tank
- Terrain and surface slope
- Soil conditions and depth to groundwater
- Expected frost penetration depth
- Vehicle loading over the tank area
The septic tank must be deep enough for sewage to flow downhill and drain by gravity. But not so deep that groundwater enters the tank.
Soil Conditions and Climate Impact Depth
Some soil types and climates require deeper burial:
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Wet or sandy soils: High groundwater may dictate 4-6 feet depth or a raised tank.
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Clay soils: Can shift and damage tank if less than 2-3 feet deep.
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Frost areas: Depth below frost line prevents winter freezing. This can mean 4 feet or more.
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Heat: Keeps sewage cooler before entering drain field.
Conversely, drier soils or warmer climates can permit shallower tanks.
Finding the Depth of an Existing Septic Tank
If your septic tank was buried long ago, you may be unsure how deep it lies. Here are some methods to estimate depth:
- Review installation records if available.
- Measure from tank to house and calculate minimum depth based on pipe slope.
- Probe soil with a metal rod to feel for the tank edges.
- Excavate carefully with a shovel along tank perimeter until you reach the lid.
Tank access risers or markings on the lid can also provide clues once uncovered. If needed, a septic contractor can locate tanks with special equipment.
Concerns with Deep Septic Tank Burial
While sometimes necessary, deep septic tank burial has drawbacks:
- Weight and pressure can damage the tank.
- Harder to access for inspections and pumping.
- Can make locating tank or leaks difficult.
- Risk of soil cave-ins and riser collapse.
- Annoying for pump truck to reach and vacuum out.
If burial exceeds 4-6 feet, special design and precautions are advisable. Consult your local health department for requirements in your area.
When to Call a Septic Professional
Only a qualified septic contractor has the expertise to:
- Recommend proper septic tank depth for your specific property.
- Accurately locate an existing buried tank.
- Safely excavate down to the tank.
- Evaluate tank integrity and make repairs if needed.
- Coordinate with local code authorities if tank depth is unusual.
DIY excavation risks injury, tank damage, or unsafe trenches. Have a pro handle any work around your septic tank.
Key Points on Septic Tank Burial Depth
- Typical depth is 2-4 feet below ground level.
- Consider climate, terrain, soil type to determine ideal depth.
- 4-6 feet deep is maximum recommended in most cases.
- Finding a buried tank’s depth takes some investigating.
- Allow only qualified contractors to excavate septic tanks.
Knowing the basics on septic tank burial depth allows you to better maintain your system. But leave any excavations and depth adjustments to the experts. Contact a professional septic contractor if you need help locating, accessing, or servicing an existing buried tank on your property.