Mainline vs. Lateral Line Leaks: How to Tell

Not all irrigation leaks are the same. Knowing whether a leak is on the mainline or a lateral line tells you how urgent it is and how it's fixed. Here's how to tell the difference and handle each.

Mainline vs. lateral line

The mainline is the pipe that's always under pressure, carrying water from the supply to the valves. The lateral lines run from each valve to its sprinkler heads and are only pressurized when that zone is running. This difference is the key to diagnosis.

How to tell which is leaking

• Mainline leak: water flows constantly, even when no zone is running and the controller is off — because the mainline is always pressurized. These are urgent and waste water nonstop.
• Lateral line leak: the wet spot or bubbling only appears when a specific zone is running. When that zone is off, the leak stops.

Why it matters

A mainline leak is a priority — it wastes water 24/7 and can undermine soil and hardscape. Shut off the irrigation main supply immediately to stop it until repaired. A lateral leak only wastes water during that zone's run time, so it's less urgent but still worth fixing promptly.

Repairing the leak

Both are repaired by exposing the damaged pipe and splicing in a new section with proper fittings, but mainline repairs must handle constant pressure and are best done correctly the first time. Locating the exact leak point is often the hardest part.

When to call a pro

Mainline leaks, deep or hard-to-find leaks, and leaks under hardscape are best left to professionals. Our irrigation team can locate and repair both types.

Frequently asked questions

How do I know if it's a mainline leak?

If water flows or the meter moves even with the controller off and no zone running, the always-pressurized mainline is leaking.

Is a mainline leak an emergency?

It's urgent — it wastes water continuously. Shut off the irrigation main supply right away and repair it promptly.

Arbol Roble has cared for Inland Empire landscapes since 1997, serving Rancho Cucamonga, Ontario, Fontana, Eastvale, Corona and Riverside. Request a free quote or browse our residential and commercial services.