Construction flags and spray-painted lines follow a national color code that tells you exactly what is buried below. Under the American Public Works Association (APWA) Uniform Color Code, construction flag colors have these meanings: red marks electric power, yellow marks gas and other hazardous materials, orange marks communication lines, blue marks drinking water, green marks sewer and drain lines, purple marks reclaimed water and irrigation, pink marks temporary survey points, and white outlines the area someone plans to dig. The same colors mean the same things in every state, whether they appear on a flag, a stake, or spray paint on the ground.1American Public Works Association. APWA Guide Uniform Temporary Marking of Underground Facilities
The Full APWA Color Code
The APWA developed the code through its Utilities and Public Rights-of-Way Committee to give excavators, utility owners, and surveyors a shared visual language. Every state’s “Call Before You Dig” program uses the same colors, so a yellow flag in Maine means the same thing as a yellow flag in Arizona.2American Public Works Association. Uniform Color Code
- Red: electric power lines, cables, conduit, and lighting cables
- Yellow: gas, oil, steam, petroleum, or other hazardous materials
- Orange: communication, alarm, or signal lines and cables
- Blue: potable (drinking) water
- Green: sewer and drain lines
- Purple: reclaimed water, irrigation, and slurry lines
- Pink: temporary survey markings
- White: proposed excavation route or boundary
Red Flags: Electric Power
Red marks everything carrying electricity: high-voltage transmission lines, residential service drops, traffic signal wiring, and street lighting cables.1American Public Works Association. APWA Guide Uniform Temporary Marking of Underground Facilities Striking an energized line can electrocute the equipment operator instantly or arc through the ground to anyone standing nearby. Even a near-miss that nicks insulation can cause a delayed fault that starts a fire or triggers a widespread outage hours later.
Red-marked lines deserve the widest berth you can manage. If a dig plan puts equipment close to red markings, hand-dig or use vacuum excavation to expose the line before bringing in mechanical equipment.
Yellow Flags: Gas and Hazardous Materials
Yellow covers natural gas, oil, steam, petroleum, and any other flammable or toxic material moving through a pipeline. A ruptured gas line can leak or explode, and even a small nick that does not breach the pipe wall immediately can weaken it enough to fail later under pressure. Yellow is the color that most often turns a routine dig into an emergency, and it demands the same hand-exposure discipline as red.
Orange Flags: Communication Lines
Orange marks telephone lines, fiber optic cables, cable television, alarm systems, and signal circuits. Telecommunications and cable TV are the most frequently damaged facility types, accounting for roughly half of all reported utility strikes nationwide.3Common Ground Alliance. Spotlight on 2024 Data – DIRT Report The damage often looks minor at the surface, but a severed fiber optic trunk line can knock out internet, phone service, and 911 connectivity for thousands of people at once. Repair costs for a major fiber cable can run $15,000 to $20,000, and the responsible party typically pays for both the physical repair and any resulting service-interruption claims.
Blue Flags: Drinking Water
Blue marks potable water supply lines. Breaching a water main creates an immediate mess of flooding, road damage, and lost pressure, but contamination is the bigger concern. A broken water line operating under pressure can suck soil, bacteria, and groundwater into the pipe through the breach when pressure drops, which is how an entire neighborhood ends up under a boil-water advisory. The party that caused the break generally pays for the lost water volume, emergency repair labor, and any contamination cleanup.
Green Flags: Sewer and Drain Lines
Green marks sanitary sewers and storm drains. These lines run deeper than most other utilities and rely on gravity to move their contents, which makes them sensitive to shifts in the surrounding soil. A breach that releases raw sewage into the ground or nearby waterways can trigger environmental enforcement in addition to repair liability.
Purple Flags: Reclaimed Water
Purple identifies reclaimed water, irrigation, and slurry lines. Reclaimed water is treated wastewater used to irrigate parks, golf courses, and landscaping, and it is not safe to drink. The critical distinction is purple versus blue. If a contractor confuses the two and creates a cross-connection between a reclaimed water line and a potable water system, non-potable water enters the drinking supply. That kind of contamination event triggers emergency public health responses and can expose the responsible party to serious liability.
Pink Flags: Temporary Survey Markings
Pink marks are placed by surveyors to indicate temporary reference points, property boundaries, or construction layout positions. They do not warn of anything underground. But disturbing or removing them is one of those mistakes that does not look expensive until it is. A misplaced foundation wall or a fence built six inches over the property line can trigger disputes, tear-down orders, and resurvey costs.
White Flags: Proposed Excavation
White marks outline where the excavator plans to dig. When a locate request goes in through 811, utility locators only mark lines within the white-bordered area identified on the ground. This is often a legal requirement, and it has a practical consequence most people do not think about. Digging outside the white-marked boundaries forfeits the legal protections that come with having a valid locate ticket. Any line hit outside that zone falls on the excavator, even if a ticket covered the adjacent area.
How To Read More Than Just the Color
Color tells you what type of utility is below, but the markings usually carry more information than that. Utility locators paint a line following the path of the buried facility, with marks spaced closer together where the line changes direction or where laterals branch off. When a line cannot be marked directly over its location because of pavement, a wall, or some other obstruction, locators use offset marks: an arrow pointing toward the actual line, with a distance in feet written alongside it.
You will also see letter abbreviations near the painted lines identifying which company owns the facility. These codes vary by region, but common examples include short forms for the local electric utility, gas company, or telecom provider. If markings appear that you cannot interpret, call the 811 center that issued the ticket. They can clarify what each mark means and which utility placed it.
The Tolerance Zone Around Each Mark
Utility markings are not GPS-precise. Locating equipment has a margin of error, and the buried pipe or cable may not sit exactly under the painted line. Every state defines a “tolerance zone,” a buffer on each side of the mark where excavation has to proceed carefully. States are roughly split between requiring 18 inches and 24 inches on each side of the marked line. The total protected width includes that buffer on both sides plus the diameter of the utility itself, so an 18-inch tolerance zone around a two-inch pipe produces roughly 38 inches of ground where mechanical equipment cannot be used freely.
Inside the tolerance zone, hand-digging, vacuum excavation, or other careful exposure methods are required. Backhoes, trenchers, and augers stay outside the zone. Most utility strikes happen right here: someone sees the paint, decides they are close enough with a machine, and clips the line. The tolerance zone exists precisely because “close enough” does not work when you cannot see what is underground.
How Long the Marks Stay Valid
Locate tickets do not last forever. Depending on the state, markings remain valid for roughly 10 to 30 calendar days. Projects that run longer need a renewal request before the ticket expires. Weather, traffic, and additional construction can also degrade surface markings over time. If the paint or flags become unclear, request a remark rather than guessing at faded color.
What the Flags Do Not Show: Private Lines
The 811 system has a blind spot that catches many homeowners off guard. Public utility companies only mark the lines they own and maintain, and they typically stop at the meter, main service connection, or property boundary. Anything past that point running across a property to the house, garage, pool heater, or backyard workshop is a private line, and 811 will not mark it.
Common private lines include irrigation systems, underground wiring to outbuildings, private propane or gas lines, landscape lighting circuits, invisible pet fences, and septic system piping. When private utilities may exist on a property and digging is planned, a private utility locator can find them using ground-penetrating radar and electromagnetic equipment. The cost typically ranges from a few hundred dollars to a couple thousand depending on the property size and complexity, which is far less than repairing a severed private gas line or a flooded septic field.
Read the flags for what they say, and remember what they cannot say. The colors are a shared language, but they only speak for the utilities that put them there.