The Best Pipe to Use in Rock Soil for an Underground Sprinkler System

Kinds of Pipe

• For mainline pipes or pipes that connect the irrigation system to the main water supply, metal pipes and fittings may be required by local plumbing codes regardless of soil type. For the lateral pipes that go from the main pipe to the sprinkler heads, two kinds of plastic pipes are most commonly used. Metal and other piping materials are too expensive or too hard to work with in landscaping jobs. Polyvinyl chloride, called PVC, plastic piping is semi-rigid and usually white or gray. Polyethylene tubing, also known as poly or as black roll pipe, is black and flexible and most tolerant of rocky soil. Both types of plastic pipes come in different diameters and thicknesses.

Polyethylene Tubing

• Poly's flexibility makes it the best choice for rocky soils, since it can bend around big rocks, doesn't crack as readily if laid beside large rocks, and can move somewhat rather than crack as rocks shift. It is favored in cold-winter climates because the flexible pipe walls allow some expansion of water in freezing weather without bursting the pipe. Clemson University extension specialists note that poly tubing may be somewhat more difficult to install than PVC.

Polyvinyl Chloride

• Two types of PVC pipes are suitable for sprinkler systems, Schedule 40 and PR160, which is 160 psi pressure-rated piping. Don't choose less expensive PVC pipes designed for drains, vents or waste systems because they aren't able to handle the higher water pressure of sprinkler systems. Schedule 40 PVC is better able to withstand rocky soils than PR160 because of its thicker wall. Even though poly is better suited for rocky areas, PVC is sometimes chosen because it's easy to work with. It is commonly used in warm-winter areas. If you do choose Schedule 40 PVC, you may wish to buffer the pipe with non-rocky fill.

Installation

• Digging trenches that are deep enough in rocky soil can be a monumental task. Renting a trencher is advisable for do-it-yourselfers for digging pipelines to get the needed depth of 8 to 12 inches. You may wish to over-excavate by a couple inches so you can add a layer of sand or rock-free fill to cushion the pipe. Add more sand or rock-free fill around and on top of the pipe after it's laid. Professional installers are often required to use rock-free fill dirt. Some companies use a chain trencher, which pulverizes rocks so that they're no longer a problem within the trench.