Irrigation Systems for the Outdoor Trades That Dig Around Them

Why this matters

A lawn sprinkler system is a pressurized water system buried a shovel's depth under the yard, with no marking on top and no record in the utility's files. Landscapers, fence crews, deck builders, concrete crews and anyone setting a sign post cut it all the time. Most cuts are a cheap fitting and a wet afternoon. Some are not: a mainline that sprays until someone finds a valve nobody knows the location of, a severed zone wire that kills half the system and takes a day to trace, or a damaged backflow assembly that leaves the house's drinking water connected to the yard.

Safety first

• A geyser or a steady flow from a hole you dug: you have probably hit the mainline, which is under pressure all the time. Close the irrigation isolation valve if you know where it is (often in the basement where the irrigation tees off, or right beside the backflow assembly outside). If you cannot find it within a few minutes, close the house's main shutoff; once the irrigation valve is found and closed, reopen the main and confirm the house has water and the hole stays dry. The irrigation stays off until it is repaired. Keep the flow away from the foundation and out of any open electrical box.
• You struck something that is not plastic water pipe: a cable in conduit, a gas line, a yellow or orange pipe, a metal line. Stop, keep people back, and treat it as a utility strike: for gas, everyone moves away, no engines started and no phones used beside it, call 911 and the gas utility from a distance. See related: The Utility Locate: Before You Dig.
• The backflow assembly is cracked, knocked over or spraying: close the isolation valve upstream of it. Do not cap, plug or bypass it to stop the leak; the house water can wait for the plumber or irrigation tech.

How a typical system is laid out

 PLAN VIEW, RESIDENTIAL SPRINKLER SYSTEM

  HOUSE      [CONTROLLER] garage wall
  +-------+        | wire bundle: 1 common
  | meter |        | + 1 wire per zone
  | -> tee|-->[ISOLATION VALVE]
  +-------+        |
        [BACKFLOW ASSEMBLY] above grade, by wall
             |
             | MAINLINE: always under pressure,
             | wire bundle usually in same trench
             |
     [VALVE BOX]--------------[VALVE BOX]
      zones 1-3                zones 4-6
      |    |    |               |
      |    |    +--o----o----o  LATERAL, zone 3:
      |    |       heads        only on when zone runs
      |    +--o----o----o
      +--o----o----o            laterals shallow,
                                often 6 to 12 in down

The tap and the isolation valve. The system tees off the house's water supply after the meter, through a shutoff valve that lets the irrigation be turned off without shutting off the house.

The backflow assembly. A brass or bronze device, usually above ground near the house wall, sometimes in a box below grade. It stops water that has sat in the yard's pipes from being pulled back into the drinking water if house pressure drops. See related: What a Backflow Assembly Is Actually Protecting Against.

The mainline. The pipe from the backflow assembly to the zone valves. It is pressurized whenever the isolation valve is open, so a cut mainline flows until someone shuts it off.

Valve boxes. Green or black plastic lids, round or rectangular, flush with the lawn. Under them sit the zone valves, each an electric solenoid valve that opens when the controller sends it power.

Laterals. The pipes from each valve to its heads. They carry water only while their zone is running, so a cut lateral is often invisible until the next scheduled watering, when it floods one spot. Laterals are commonly 6 to 12 inches down, and shallower where a bed was regraded or the lawn settled around them.

Heads and drip lines. Spray or rotor heads pop up out of the lawn; drip tubing runs at or just under the surface in beds, often under mulch.

The controller wire bundle. Low-voltage direct-burial wire, one common plus one conductor per zone, from the controller to the valves. It usually shares the mainline trench but not always. It carries about 24 volts AC, which is not a shock hazard in the way house wiring is, but a cut bundle can take out every zone at once.

Why the backflow assembly is a protected device

The backflow assembly is not a sprinkler part, it is a public health device. The water sitting in irrigation laterals is in contact with fertilizer, pesticide, animal waste and standing puddles at every head. Lose the protection and a pressure drop in the street (a main break, a hydrant opened by the fire department) can pull that water back into the house's taps.

So it is treated differently from the rest of the system:

• It is commonly installed under a plumbing permit and tested on a schedule. Many water utilities run a cross-connection control program that requires a certified tester to test the assembly, often annually, and send the result to the utility; whether your area does, and how often, is set by state and local rules.
• Its height and orientation are part of its rating. A pressure vacuum breaker, a common type, has to sit a set height above the highest head it serves, commonly 12 inches, per its listing and local code. Regrading a bed up around it, or lowering it to hide it, changes its protection.
• Its relief port must stay open. A reduced pressure zone assembly dumps water from a relief port on its underside when it detects a problem. Burying that port in mulch or rock, or boxing it in tight, blocks the discharge it is supposed to make.
• Testers need to reach it. A shrub planted tight against it, a fence post beside it or a decorative cover screwed down over it turns a routine test into a removal job.

Do not move, lift, lower, reroute or enclose it. If the design needs it somewhere else, that is a plumbing or licensed irrigation job.

Why 811 never marks it

The 811 locate service sends the utilities to mark what they own: water up to the meter, gas, power, communications. Everything after the meter is the property owner's, and nobody marks it. The irrigation system, landscape lighting, a gas line to a grill, power to a shed and a downspout drain line all fall in that gap. See related: How to Find the Private Lines a Utility Locate Will Not Mark.

The practical consequence is that the one system most likely to be under your shovel is the one with no paint on the grass. The marks you do get are no evidence the ground is clear.

Reading the yard before you dig

• Ask for the zone map. Many controllers have a sheet inside the cover listing zones by area. Some installers leave an as-built drawing with the homeowner.
• Run the zones. With the homeowner's permission, run each zone for a minute or two from the controller and flag every head that pops. Laterals run from the valve to the heads, often in straight lines between heads in a row, but they can jog.
• Find the valve boxes and note which direction the mainline enters and leaves.
• Look for the patterns. A straight line of greener or browner grass, a narrow sunken strip, or a sod seam often marks an old trench.
• Hand dig inside the zone of doubt. Within about 2 feet of a flagged line, expose it with a hand shovel rather than an auger or a machine bucket. Irrigation pipe is thin-wall plastic or polyethylene tubing and an auger shreds it.

Worked example: four trees along a bed edge

A landscape crew is planting four trees along the edge of a lawn bed. The plan puts the tree centers 36 inches inside the bed edge, 6 feet apart. Each tree comes with a 30 inch root ball, and the crew digs planting holes twice the ball's width, so each hole is 60 inches across: 30 inches from the center to the edge of the hole.

Run the zones. The controller shows six zones. Zone 4 covers this bed. When it runs, a row of spray heads pops up in a straight line 18 inches inside the bed edge, parallel to it, the full length of the bed. That row is a lateral.

Check the conflict. The distance from each tree center to the lateral is 36 - 18 = 18 inches. The hole reaches 30 inches from center. 18 is less than 30, so every one of the four holes cuts through the lateral, four cuts in one zone.

Move the trees, not the pipe. The crew moves the tree centers to 54 inches inside the bed edge. Center to lateral is now 54 - 18 = 36 inches, and the hole edge sits 36 - 30 = 6 inches short of the pipe. Six inches of margin is thin for a line found by its heads rather than by digging, so before augering the crew hand-digs a short test trench at one tree to find the lateral's real position and depth. They find it 7 inches down, 2 inches further into the bed than the heads suggested, so the margin is 4 inches. The holes are dug with a flat spade on the lateral side, not with an auger.

The surprise. At tree 3 the crew finds a 1/2 inch drip line feeding a shrub, not on any map, crossing the hole. They cut nothing yet. They stop, show the homeowner, and on the homeowner's instruction loop the drip line around the hole with a fitting, which is a landscape repair the crew does routinely. Had it been the mainline or the wire bundle, the crew would have backfilled by hand and called the irrigation company instead.

The backflow on the way out. The bed design also put a boxwood tight against the backflow assembly by the house corner. The crew moves it 3 feet away so the tester can reach the assembly from both sides, and keeps the mulch below its relief port.

The tally. Four planned cuts to one lateral avoided by moving the trees 18 inches deeper into the bed; one unmapped drip line found by hand digging and rerouted; one shrub moved off the backflow assembly. Zone 4 run again at the end: every head pops and no water bubbles up in the new beds.

What changes the answer

Freeze climates. Systems are blown out with compressed air in the fall and recharged in spring. A line cut in the off-season shows nothing until spring start-up, so photograph and tell the homeowner about any hit at the time, even a dry one.

Commercial and HOA properties. Systems are bigger, mainlines are larger and deeper, and the backflow is often a large assembly in an enclosure. Cutting the mainline on a commercial site can drain an irrigation system serving many buildings; ask for the property's irrigation contractor before digging.

Well-fed systems. Some systems run from a private well or a pond pump instead of the city supply. The pump starts on pressure drop, so a cut line runs the pump continuously. Switch the pump off at its control or breaker and call the well or irrigation contractor. See related: Private Wells and Septic for Non-Specialists.

The words to use when you call

Had the spade found the zone 4 lateral after all, the call goes like this: "I'm with the landscape crew at [address]. Planting a tree in the back bed, I cut the zone 4 lateral, about 7 inches down and 20 inches inside the bed edge. The isolation valve is closed and the zone is off at the controller. Nothing else was hit. Can you come repair it before the next watering?"

References

• Your water utility's cross-connection control program, for backflow testing requirements in your area.
• The backflow assembly manufacturer's installation instructions, for height and relief-port clearance.
• See related: Irrigation System Reference
• See related: What a Backflow Assembly Is Actually Protecting Against
• See related: How to Find the Private Lines a Utility Locate Will Not Mark