On June 4, 2013, at about 0930 Pacific daylight time, a Piper PA-28-140, N5808U, was substantially damaged when it struck an airport sign following a loss of directional control during landing roll at Skagit Regional Airport, Burlington, Washington. The private pilot, the sole occupant, was not injured. The pilot/owner was operating the airplane under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the personal cross-country flight, which had originated from Friday Harbor, Washington, approximately 20 minutes before the accident. A flight plan had not been filed. The pilot said that while on landing roll, he applied left rudder to straighten the airplane, with no response. He said he attempted to correct the right veering tendency with left braking action, and that was ineffective as well. The airplane departed the right side of the runway and impacted an airport sign and a ditch, which damaged the right wing. Postexamination of the left rudder control bar found that it had failed. Examination of the rudder control bar at the NTSB Materials Laboratory found a fatigue crack, which originated from the edge of a weld. There was a rusted brown oxidized area near the middle of the crack, and the surface of this area had topography and crack patterns consistent with slow growth fatigue progressing from the exterior surface radially through the tube wall. The airplane was manufactured in 1970, and according to the owner, it had an airframe total time of 3,479 hours when the accident occurred. The most recent annual inspection was completed on May 25, 2013. A review of the Piper Aircraft's periodic inspection checklist, maintenance manual, and Service Letter No. 671 dated October 5, 1973, indicated that the rudder bar assembly should be inspected every 100 hours specifically for cracks in the area where the airplane's rudder bar failed. Further, the Service Letter stated that these periodic inspections could be discontinued if two reinforcement doublers were fabricated and welded to the rudder bar in the affected area. There were no reinforcement doublers on the airplane's rudder bar.