The pilot reported that he made a traffic pattern approach to the airport and observed the wind direction. The surface wind was from 300 degrees at 5 knots. He made a normal three-point landing on runway 36. During rollout, the pilot lost directional control of the airplane, which had conventional landing gear. The airplane veered off the runway and came to rest in a drainage ditch. The pilot reported that no mechanical malfunction or failure with the airplane precipitated the accident. A subsequent examination of the airplane's structure revealed that two outboard wing ribs and a right elevator tube were bent. Also, a fuselage frame component (yoke fitting) was bent.