Residential sprinkler systems require the supply lines that carry water to the sprinkler heads to be installed underground. The subsurface installation insulates pipes and helps prevent damage from frozen pipes. There is no standard depth for sprinkler lines, given the wide variety of terrain and uses. Deciding how deep to dig the trenches for your sprinkler lines requires consideration of several factors, including the sizes of various components that make up the underground system.
Supply Line Depth
One of the most basic requirements in burying a sprinkler line is related to the size of the supply pipe. Sprinkler supply lines typically are 3/4 to 1 inch in diameter. The depth of the lines needs to be at least equal to the diameter of the pipe plus 4 inches. This will ensure the pipe sits far enough underground to insulate the pipe and make sure that it isn't easily disturbed by surface activity. This is the minimum depth for pipes that run over long distances and do not connect to sprinkler heads.
For underground pipes that connect to sprinkler heads, you must account for the depth of the sprinkler riser. The riser is a piece of plastic that connects the sprinkler to the supply line. Risers are available in almost any size. If you want to bury your line in a more shallow trench, purchase risers that sit flush between the sprinkler and the supply line. To bury sprinkler lines deeper for better insulation from the cold, purchase risers that are between 6 and 12 inches long.
The length of the sprinkler body will also help determine how deep a sprinkler line needs to be buried. Most sprinklers come in sizes of 2, 4 and 6 inches. Whatever size sprinkler you choose to use, it must sit underground at least the length of the sprinkler plus the length of the riser plus the diameter of the supply line. For example, a 2-inch sprinkler on a 3-inch riser with a 1-inch supply line needs to be buried at a total depth of 6 inches.
Depth for Grass Surfaces
Surfaces that are covered with grass require special consideration when installing underground sprinkler lines. Grass has roots that extend into the ground to a depth of several inches. To prevent sprinkler lines from becoming tangled in grass roots, pipes must be installed at least 4 inches below ground. This depth will put the lines out of reach of grass. To make sure your lines are installed at a proper depth in grassy surfaces, use sprinkler bodies that have a total depth of 4 inches. This depth, plus the riser and the supply line, will ensure the sprinklers don't interfere with the grass.