Sprinkler Leak or Slab Leak? Finding Where the Water Goes

In much of Texas, lawn irrigation is the single biggest use of water at a home during the summer. It is also one of the most common sources of hidden leaks. A cracked lateral line, a stuck valve or a broken head can waste thousands of gallons before anyone notices a soggy patch in the yard. Because the first sign is usually a high water bill, homeowners often can't tell whether the sprinklers or the pipes under the house are to blame. This guide shows how to separate the two in an afternoon.

How sprinkler systems leak

An automatic sprinkler system has two parts that matter here. The mainline runs from the home's water supply, through a backflow prevention device, to a group of zone valves. The mainline is pressurized all the time. Lateral lines run from each valve to the sprinkler heads, and they only hold pressure when that zone is running.

That difference shapes how leaks behave:

• A mainline leak runs around the clock, like a slab leak, because the line is always under pressure. It shows on the meter even when the system is off.
• A lateral line or head leak only leaks while the zone runs. It wastes water during each watering cycle but won't show on the meter when the system is off.
• A valve that doesn't fully close lets water trickle to the lowest heads in that zone all the time. You often see a puddle or a constantly wet spot around one or two heads.
• A leaking backflow device may drip or spray at the device itself, usually near the house.

Step 1: Look and listen in the yard

Walk the yard a day or two after the last watering cycle. Look for spots that stay green when everything else is dry, soggy turf, sunken areas, water running into the street, or algae on the sidewalk or curb. Check around each valve box and the backflow device. Listen at the valve boxes for a hiss. A head that sprays while the system is off is a sure sign of a valve that isn't closing.

Step 2: Shut off the irrigation supply and test the meter

Most systems have a shut-off valve before the backflow device, often in a small box near the house or the meter. Close it. Then turn off every fixture and appliance in the house and do a meter test: read the meter, wait one to two hours using no water, and read it again. Watch the small leak indicator on the meter face too. Our water meter leak test guide explains how to read the meter.

• Meter stops with the irrigation shut off: the water was going to the sprinkler system. A mainline, valve or backflow leak is likely.
• Meter keeps moving with the irrigation shut off: the leak is somewhere else in the home's plumbing.

Step 3: Separate the yard line from the house

If the meter is still moving, close the main shut-off where the water line enters the house. If the meter still moves, the leak is in the service line between the meter and the house. If it stops, the leak is inside the house, and when nothing above ground is dripping, a line under the slab is a strong possibility.

Comparing the clues

Drip irrigation and soaker lines

Many Texas homeowners have switched flower beds to drip irrigation to save water. Drip systems run at low pressure through thin tubing, and the tubing is easy to cut with a shovel or chew through if you have pets or wildlife. A cut drip line can run for weeks under mulch without anyone noticing. Like lateral lines, most drip zones only hold pressure while the zone is running, so they rarely explain a meter that moves with the system off. But if a drip zone's valve doesn't seal, the tubing stays pressurized and can leak around the clock. Check the beds for areas that stay wet, sinking mulch or plants that look unusually lush next to plants that look dry.

Foundation watering systems

Some homes on expansive clay have soaker hoses or a dedicated drip line running around the foundation to keep the soil moisture even. These systems are often tied into the irrigation controller or a hose bib on a timer. A soaker hose left running, or a timer that failed open, can use a surprising amount of water and keep the soil next to the slab soggy, which can look like a slab leak from the outside. Turn it off before you run the meter test.

When it's the sprinklers

In Texas, irrigation work is regulated. Installing or repairing an irrigation system generally requires a licensed irrigator, and backflow devices are tested by licensed backflow testers. Your irrigator can find a mainline break, rebuild a valve that won't close, or replace a damaged head. Many cities also offer free irrigation checkups during the summer as part of their water conservation programs, so check your city's website.

While you wait for a repair, you can usually leave the irrigation shut-off closed and water by hand. That stops the leak and keeps the rest of the house running normally.

When it's under the slab

If the meter keeps moving with the irrigation off and every fixture off, and the house shut-off test points inside, the next step is slab leak detection. A licensed plumber will confirm the leak with a pressure test, separate the hot and cold lines at the water heater, trace the leaking line's route, and pinpoint the leak through the concrete with acoustic listening and thermal imaging. Our step-by-step process page explains each part of the visit in plain English.

Telling the plumber what you already ruled out saves time. Mention that the irrigation was off, the house shut-off test results, and anything you noticed inside: a warm floor, a running water sound, damp flooring. That information helps the plumber focus on the right lines from the start.

Don't assume one leak means no other

It's possible to have both an irrigation leak and a slab leak. Clay soil movement in North and Central Texas stresses both the irrigation lines in the yard and the pipes under the house, and both can fail in the same dry season. If you fix a sprinkler leak but your bill doesn't drop back to normal, repeat the meter test with the irrigation shut off. A meter that still moves means a second leak.

Seasonal patterns worth knowing

A summer bill spike is normal in Texas because of watering. A good rule of thumb is to compare against the same month last year. If July's usage is far above last July's, with similar weather and watering schedules, something has changed. A winter bill that's as high as a summer bill is a strong sign of a constant leak, because irrigation use is usually low in winter.

Some smart irrigation controllers and smart water monitors show daily usage. If your usage never drops to zero overnight when nothing is running, you have a constant leak, and the irrigation shut-off test tells you whether it's the sprinklers.

Getting a water bill credit

Many Texas utilities offer partial credits for water lost to a hidden leak once it's repaired, but their rules vary. Some exclude irrigation leaks, some treat irrigation and slab leaks the same, and many limit credits to one per year. Keep your repair invoice, note the dates, and contact your utility soon after the repair. Our FAQs cover common questions about leak credits and insurance.

Sprinkler leaks are a frequent false alarm on the large irrigated lots of Allen, Keller and Mansfield.

The bottom line

Shut off the irrigation, then test the meter. If it stops, the leak is in the sprinkler system and an irrigator is the right call. If it keeps moving, test with the house shut-off closed to find out whether the leak is in the yard line or inside the house. If it's inside and nothing is dripping, have the lines under your slab tested. One afternoon of simple checks can save you from paying the wrong specialist, and from leaving the real leak running.