Check for Underground Obstacles Before Digging
You called 811 and got utility lines marked. Great. But there's a whole category of underground hazards that 811 does not cover: septic tanks, drain fields, wells, irrigation systems, and buried debris. Hit one of these, and you're looking at expensive repairs and possible code violations.
Quick Summary
Time needed
1-3 hours
Cost
$0-300
Difficulty
Research heavy
Why 811 Isn't Enough
The 811 service only marks public utility lines. Anything privately installed on your property is your responsibility to locate. This includes systems that could cost thousands to repair and may require permits to work near.
Common hidden obstacles: Septic tanks cost $3,000-$7,000 to replace. Drain field repairs run $5,000-$15,000. Old wells are hazardous and costly to properly abandon. Finding these after you've poured concrete is a nightmare.
Taking an hour or two now to research your property can save you from expensive mistakes later. Many of these obstacles also have code-required setbacks that affect where you can legally place your deck.
Underground Obstacles to Check For
1Septic Tank
A large underground tank that holds household wastewater. Usually concrete, fiberglass, or plastic. Typically 4-6 feet underground and 10-20 feet from the house.
Setback required: Most codes prohibit structures directly over tanks. Tanks need access for pumping every 3-5 years.
2Drain Field (Leach Field)
A network of perforated pipes that distribute septic effluent into the soil. Covers a large area (often 30x30 feet or more) and must remain uncovered.
Setback required: Typically 10+ feet. Never build on or compact soil over a drain field.
3Well (Active or Abandoned)
Drinking water wells and old abandoned wells can be anywhere on older properties. Abandoned wells are particularly dangerous and may need professional sealing.
Setback required: 50-100 feet from active wells in many jurisdictions to protect water quality.
4Irrigation Systems
Sprinkler lines, drip irrigation, and valve boxes. Usually 6-12 inches deep. Cutting these causes leaks, soggy soil, and water bills.
Solution: Map your sprinkler heads and trace likely pipe routes. Consider capping and rerouting lines.
5Buried Oil Tanks
Older homes (pre-1980s) may have buried heating oil tanks that are no longer in use. These can leak and create environmental hazards.
Warning: Leaking oil tanks can cost $10,000-$100,000+ to remediate. Check home records and look for fill pipes.
How to Locate Underground Obstacles
1. Review Your Property Survey
Your property survey (also called a plat) often shows the location of septic systems, wells, and easements. Find yours in:
- •Your closing documents from when you bought the house
- •Your county recorder's or assessor's office
- •The title company that handled your purchase
2. Contact Your Local Health Department
The health department maintains records of septic system installations, including detailed diagrams showing tank and drain field locations.
Pro tip: Ask for the "as-built" diagram of your septic system. This shows exactly where everything was installed, not just where it was planned.
3. Look for Visual Clues in Your Yard
Underground systems often leave visible evidence on the surface:
Septic Signs
- • Green grass in dry weather
- • Slight depression or mound
- • Cleanout pipes sticking up
- • Concrete or plastic lids
Well Signs
- • Capped pipe in the ground
- • Circular concrete cover
- • Old pump housing
- • Depression with different vegetation
4. Use a Probe Rod
A metal probe rod (or a long piece of rebar) can help locate shallow tanks and pipes. Push it into the ground and feel for resistance:
- •Solid resistance at 1-3 feet could indicate a septic tank lid
- •Hollow sound when tapped may indicate a tank cavity
- •Hard surface at 6-12 inches could be irrigation pipe
Caution: Don't probe aggressively near known utility markings. You could puncture gas or water lines.
5. Consider Hiring a Professional Locator
For complex sites or when you can't find records, professional utility locators use advanced equipment:
- Ground-penetrating radar (GPR) — Sees through soil to locate tanks, voids, and buried objects
- Electromagnetic locators — Traces metal pipes and cables even when not energized
- Cost: $150-$400 for residential property scan
Common Mistakes to Avoid
Assuming you know where the septic is
Previous owners may have given you incorrect information. Septic systems are sometimes relocated during repairs. Always verify with official records from the health department.
Ignoring old abandoned wells
Properties with newer wells often have old wells that were never properly abandoned. These can collapse, contaminate groundwater, and create liability issues. Check property records for well history.
Planning to build over the septic access
Even if code allows it, building over septic access means expensive deck removal every time the tank needs pumping. Design your deck layout to work around septic access points.
Forgetting about underground propane tanks
Some properties have buried propane tanks that aren't obvious. Check for fill pipes near the house and contact your propane company for tank locations and setback requirements.
Frequently Asked Questions
Can I build a deck over my septic tank?
Building directly over a septic tank is strongly discouraged and may violate local codes. Tanks need pumping every 3-5 years, which requires access. If your tank is in the desired location, consider designing around it or adding a removable deck section.
How close can I build a deck to a drain field?
Most codes require at least 10 feet clearance from drain fields. Deck footings can compact soil and crush pipes. Heavy loads on a drain field cause system failure. Check your local codes for specific requirements.
How do I find my septic tank location?
Contact your local health department for installation records. You can also follow the main drain line from your house, look for green grass patches, check for cleanout pipes, or use a metal detector to find tank lids.
What if I hit an irrigation line while digging?
Irrigation lines are less dangerous than gas or electric, but still problematic. Cut lines cause water damage and high bills. Before digging, map sprinkler heads and trace pipe routes. Turn off the irrigation system while working.
Ready for the Next Step?
Now that you know where underground obstacles are located, you can start laying out your deck footprint with confidence.