Cross bores: when a gas line runs through a sewer pipe
Updated October 30, 2026 · 3-minute read · Old House Sewer Lines
A cross bore is a natural gas line that was accidentally drilled through a sewer or septic pipe when it was installed. It stays hidden until the sewer clogs, and then a cutting tool sent to clear the blockage can slice the gas line open.
NYSEG calls a cross bore a rare situation in which a gas line has been inadvertently installed through a sewer or septic pipe (NYSEG, Cross Boring). The federal pipeline regulator, PHMSA, defines it more broadly as any new underground utility line drilled through or intersecting an existing facility such as a sewer (PHMSA).
How it happens
NYSEG traces cross bores to directional boring, which pushes a new line underground without opening a trench. With no trench, it says, the bore can pass through another buried line. The owner's sewer lateral is a private line, and the one-call center UDig NY is explicit that its members leave private facilities unmarked. A boring crew working from an 811 markout will not see the lateral on the ground. See the 811 rule.
National Grid says most sewer blockages come from roots, grease or other routine clogs, but in rare cases a gas, electric, water or telecom line has been bored through the pipe (National Grid cross bore brochure). If a cross-bored gas line is damaged, it warns, gas can travel through the sewer main to neighboring homes as well.
Inside clog or outside clog
Location decides the risk, by NYSEG's account. A blockage inside the building is not a gas-system issue and can be cleared. A blockage outside the building could be a cross bore. That is the lateral, the part that runs under the yard.
Before anyone clears an outside clog, the utilities' guidance runs in this order:
- Camera first. NYSEG calls a camera inspection of the line a good practice. A sewer camera inspection lets the crew see what the obstruction is before anything cuts into it.
- Ask if unsure. When nobody can say for certain that the blockage is not a gas pipe, NYSEG's advice is to call the local gas company, which for most homes covered here means Con Edison. No Con Edison cross-bore program page was found.
- No camera, no cutter. Where no camera is on hand, National Grid tells plumbers to clear the line with a hydro-jetter and keep cutting heads out of it; a camera first is still the safer order. The difference is explained in hydro-jetting vs a cable machine.
- Stop at strange resistance. If the tool meets resistance that does not feel like a root or a normal clog, do not force it. Stop.
- Check the blades. Gas lines are usually yellow plastic, National Grid says, and rarely orange, green or black. Colored plastic on the cutter, or bubbles at the toilet or the clearing equipment's entry point, are warning signs.
Clearing the lateral with machines is licensed contractor work. A homeowner's part is to stop using water, keep the clog from being forced, and make sure whoever comes knows the clog is outside.
National Grid says that if its own line caused the blockage, it will remove the line and repair the sewer at no charge; that is National Grid's stated policy, not Con Edison's. The Cross Bore Safety Association finalized its Leading Practices for Cross Bore Risk Reduction in 2019, and PHMSA encourages excavators to follow them.
Video from Southwest Gas, an outside channel: A homeowner about to rent a power auger learns that a clog outside the house could be a gas line bored through the sewer. Nothing loads from YouTube until you press play.
Key facts
- A cross bore is a gas line inadvertently installed through a sewer or septic pipe (NYSEG).
- Indoor clogs are not a cross-bore issue; outdoor clogs could be (NYSEG).
- Camera first; without one, National Grid says use a hydro-jetter and not a cutting tool (National Grid).
- Gas service lines are typically yellow plastic (National Grid).
- One-call utilities do not mark private lines like the sewer lateral (UDig NY).