Using plastic pipe for outdoor plumbing projects can save money and eliminate the risk of pipe corrosion. Locating a buried plastic pipe, however, can be a challenge. Unlike metal piping, a traditional metal detector will not locate buried plastic pipes. Instead, in order to locate buried plastic pipes you will need a specialised locator device or you will have to locate the pipe manually using a probe. Before digging, you are legally obligated to call 811 to ensure you will not hit any underground utility lines.
Rent or purchase a specialised plastic pipe locator device that uses radar. These units can be fairly expensive, but are often available at equipment rental locations. The radar locators look similar to a push lawnmower and are equipped with a display screen.
Guide the locator over the area with buried pipes. If possible, walk the locator perpendicular to the flow of the pipes. As you cover the area, slightly overlap your tracks to ensure you get a radar reading of the entire area.
Watch the radar display for any detected underground objects. When the locator detects an object, walk along it with the locator to determine if it runs similar to a pipe. Following the pipe pattern will help you avoid digging up a rock or other underground item.
Read the building blueprints to determine the estimated location of the pipes. Without blueprints, or a rough estimate of the location, probing for pipes will be an extremely time-consuming method.
Press the probe into the ground, starting at one corner of the estimated location. The probe should go into the soil 12 to 24 inches depending on the depth of the pipes. If you do not feel the probe encounter any object, pull it out of the ground and probe again 2 to 3 inches to the side. Continue this process in rows until you encounter an object.
Insert the probe into the ground on all four sides of any encountered object. If you encounter another object, move a few inches down the line and probe again. If you continue to encounter an object along the line, continue probing until you find the end of the pipe. When one end has been found, continue probing in the opposite direction until the full length of the pipe is determined.
Locate the nearest indoor connection to the pipe. This could be a drain, faucet or other connection. Turn the water supply to this connection off and remove the drain or faucet.
Insert a steel cable into the drain and begin feeding it into the pipe. The steel cable should be thin enough that it moves and bends within the pipe, but thick enough to generate a reading from the metal detector. Depending on the length of the pipe, you may need to run more than 40 feet of cable through the drain.
Survey the estimated outdoor location of the pipe with a metal detector. Walk slowly, in overlapping lines swaying the metal detector gently from side-to-side. When the metal detector garners a reading, run the detector over the surrounding area. If the reading continues, follow it until it stops. A long reading most likely indicates your pipe. Use a probe to test the area for pipe, or digs a small hole to confirm the pipes location. This method requires more work and supplies than other detection methods, while producing varying results. It should only be used as a last alternative.