Why does water keep running out of the lowest sprinkler head after the zone shuts off?

That is low-head drainage, and it is usually normal rather than a leak. When a valve closes, the water still sitting in that lateral has to go somewhere, and gravity sends it out of whichever head is lowest. Expect a short gush and a puddle that stops within a minute or two. If it runs for more than a few minutes, or the puddle keeps growing hours later, the valve is not sealing. The permanent fix for genuine drainage is check-valve sprinkler bodies on the low heads, which hold up to several feet of elevation.

How do I tell if a sprinkler valve is leaking?

Turn the controller off completely and watch the zone. A valve with a failed diaphragm or debris on the seat will let water past continuously, so heads on that zone weep or trickle with the system supposedly off, and the ground over the valve box goes soft. Confirm it at the water meter: with every fixture in the house off and the controller off, watch the low-flow indicator on the meter. If it turns, something on the pressurized side is passing water. Most valve failures are a torn diaphragm or a piece of grit on the seat, and both are repairable without digging up pipe.

Can a sprinkler leak raise my water bill a lot?

Yes, and it is one of the more expensive household leaks precisely because it is buried and silent. A valve stuck partly open passes water 24 hours a day, and irrigation flow rates are large — even 1 gallon per minute is 1,440 gallons a day. A mainline leak between the meter and the valves runs continuously regardless of the controller. If a summer bill jumps with no change in habits, do the meter test before you look anywhere else in the house.

Why is water coming out of my backflow preventer?

It depends which device you have and when it happens. A pressure vacuum breaker commonly spits a little air and water from its bonnet at the start of a cycle as the poppet seats — brief, at startup, is normal. Continuous discharge is not. A reduced pressure principle assembly that dumps water from its relief port is telling you a check is fouled or a pressure differential has failed, and that is a device fault. Because backflow assemblies protect the drinking water supply, repairs and the subsequent test generally must be done by a certified tester, and most utilities require the result to be filed. Do not disable or bypass one.

What causes a sprinkler pipe to crack?

Freezing is the biggest cause by a wide margin — water left in a lateral or valve over a hard winter expands and splits the pipe or the valve body, and the damage only reveals itself when the system is charged in spring. After that, ground movement and root pressure, mowers or aerators striking a shallow lateral, and water hammer from valves slamming shut on a zone piped too small for its flow. Systems in freezing regions must be drained or blown out each fall, which is why winterization is not optional north of the frost line.

Why Is My Sprinkler System Leaking?

Written & researched by Priya Anand DIY & Tools Editor

Updated October 2026

Wide view of a person watering a vegetable bed with a garden hose, water catching the light, in soft early-morning light

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Diagnose an irrigation leak by two things: where the water appears, and whether it appears while the zone is running, just after it shuts off, or all the time. Water at the lowest head for a minute after a cycle is normal low-head drainage. Water at a head while the zone runs is usually a worn wiper seal or a cracked body. Water with the controller off is a valve that is not sealing. And a wet patch that never dries regardless of the schedule is a mainline leak — the one to take seriously, because it runs 24 hours a day.

Start with the symptom

What you see	Most likely cause	Fix
Gush from the lowest head for 1–2 min after a cycle	Low-head drainage — normal	Check-valve sprinkler bodies on the low heads
Water bubbling around a head while the zone runs	Worn wiper seal or cracked body	Replace the head; keep the swing joint
A head weeping with the controller off	Valve diaphragm failing or grit on the seat	Rebuild or replace the valve
Soggy ground over the valve box	Leaking valve, union or solenoid seal	Open the box and inspect dry
Wet patch that never dries, controller off	Mainline leak	Isolate and repair — often a pro job
Geyser or fountain during a cycle	Broken head or sheared riser	Cap or replace immediately
Water from the backflow assembly	Brief at startup is normal; continuous is a fault	Certified tester — do not bypass
Whole zone weak plus a soft spot	Cracked lateral, often freeze damage	Excavate the soft spot and splice

Wide view of a person watering a vegetable bed with a garden hose, water catching the light, in bright midday sun

The meter test settles it in ten minutes

Before you dig anything, find out whether water is genuinely being lost. Turn the irrigation controller to off, and shut off every fixture in the house. Go to the water meter and look at the low-flow indicator — usually a small triangle, star or dial that spins on very low flow. If it is moving, something is passing water.

Then isolate. Close the shut-off valve that feeds the irrigation system (typically just upstream of the backflow assembly). If the meter indicator stops, the leak is in the irrigation system; if it keeps turning, the problem is inside the house. That single test saves an enormous amount of speculative digging, and it is the same test your utility will ask you to run.

Valves are the most common real leak

An irrigation control valve is a diaphragm held closed by water pressure above it. When the solenoid energizes, that pressure is bled off and the diaphragm lifts. Two things go wrong. A torn diaphragm means the valve can no longer hold itself shut, so the zone weeps continuously. A piece of grit on the seat means it almost closes, which produces the same symptom intermittently — often after a mainline repair or a spell of dirty supply water.

Both are repairable without touching buried pipe. Shut off the water, unscrew the valve bonnet, lift out the diaphragm and spring, rinse everything, inspect the rubber for splits and reassemble — or fit a rebuild kit for the specific valve model. Keep the parts in order as they come out; the spring seats one way only.

A related fault worth knowing: a zone that will not turn on is usually electrical (solenoid or wire splice), while a zone that will not turn off is usually mechanical (diaphragm or debris). That distinction alone narrows most call-outs.

Low-angle view of a person watering a vegetable bed with a garden hose, water catching the light, in soft overcast light

Heads, seals and swing joints

Water welling up around a head while the zone runs points at the head, not the pipe. The wiper seal that scrapes the riser clean on retraction eventually wears, usually accelerated by grit — which is why flushing every lateral with the nozzles off at install matters so much. Replacement is a five-minute job: dig a small cone around the head, unscrew it from the swing joint, screw on the new one, backfill.

If the head itself is sheared off, the culprit is nearly always a mower or a vehicle, and the underlying fault is a head set proud of grade or mounted on a rigid riser. Swing joints flex and absorb the blow; a rigid PVC nipple transmits it straight to the tee and cracks the lateral, turning a $6 repair into an excavation.

Head choice and seal quality are covered in best pop-up sprinkler heads, and setting arcs after a replacement is in how to adjust sprinkler heads.

Mainline leaks and why they matter most

The mainline is the section between the supply tap and the valves. It is pressurized every hour of every day, so a leak there runs continuously whether or not the controller is on — which is how buried irrigation leaks produce four-figure water bills. Signs are a persistently soggy area unrelated to any zone, a drop in household pressure, or a meter indicator that never stops.

Repair is straightforward in principle — excavate, cut out the damaged section, splice with a repair coupling — but locating the leak precisely can be difficult, and the excavation is often near the water service. If the leak is on the supply side of the backflow assembly, or if you are not certain where the service line runs, this is the point to call a licensed irrigation contractor or plumber. Call 811 before digging in any case.

Wide environmental view of a person watering a vegetable bed with a garden hose, water catching the light, in warm late-afternoon light

Never bypass the backflow assembly

If your backflow preventer is discharging, the temptation is to plug it or remove it. Do not. That device is the only thing stopping water from your lawn — carrying fertilizer, herbicide, pesticide and whatever is standing in the valve boxes — from being siphoned back into the drinking water when street pressure drops. The EPA's Cross-Connection Control Manual documents repeated real contamination incidents traced to exactly this path, and its cross-connection control fact sheet names lawn irrigation among the most common residential hazards.

Most US jurisdictions require an approved assembly on any irrigation system connected to potable supply, tested at installation and annually thereafter by a certified tester, with results filed with the utility. Repairs to the assembly usually have to be made by a certified person and the device retested afterward. Requirements vary by city and state — call your water utility's cross-connection control program.

Preventing the next one

Winterize. Drain the system or have it blown out with compressed air before the first hard freeze. Water left in laterals and valve bodies is the single largest cause of cracked irrigation components. See how to winterize an irrigation system.
Use swing joints on every head so an impact bends a joint rather than cracking a tee.
Keep heads flush with grade as the lawn builds thatch, so mowers pass over them.
Do not overload zones. Water hammer from valves closing on an undersized zone fatigues fittings over years. Zone sizing is in how many sprinkler heads per zone.
Walk each zone once a month in season with the water on. Two minutes per zone catches most faults while they are still cheap.

Where to go next

For building or rebuilding, see how to install a sprinkler system and best sprinkler system. For scheduling once it is watertight, how often to water your lawn and the irrigation water cost calculator will tell you what a leak was costing you. Everything sits under the watering and irrigation hub.

We never buy or physically handle the products we rank — recommendations come from aggregated owner reviews and manufacturer specifications. Full method: how we rank.