I hired a lawn irrigation contractor to install a lawn irrigation system in our yard. He buried the lawn irrigation pipes about 7 - 8 inches below the surface. He maintains that the pipes are "plenty deep" to avoid winter freezes. He claims it does not get very cold in Portland. I know in the Midwest lawn irrigation pipes are routinely buried 36 or more inches underground. What is the appropriate depth to bury lawn irrigation pipes in Washington County?
My recommendation is to bury sprinkler lines 8-12 inches. I usually try to keep a standard depth of 10" because it provides more safety if I need to do some type of ground work so I don't have to worry about hitting lines. A walk-behind rototiller generally won't go deeper than 8". From a freezing perspective, 7-8 should be fine since it's not frequent to have temperatures below freezing for extended durations. I would still winterize your lines by blowing them out with compressed air.