My question involves real estate located in the State of: Washington

My water main has sprung a leak and the waterline runs through a portion of my neighbors property. The tree that is near my line on their property caused the leak. Are the liable at all?
My home was built in the 1950's and was in place before the other home was built. Also what are my responsibilities if they are not liable in regards to the tree that is causing the damage? can I cut roots to get to leak and trim them back to make sure it doesn't happen again?

Thanks M