The pilot reported that during the takeoff roll on a wet dirt airstrip, the airplane reached rotation speed quickly but indicated no further speed. The pilot felt the main gear impact softer areas on either side while keeping the airplane under control just prior to lifting off. The airplane was airborne briefly then settled back down. As the airplane was approaching the departure end of the runway where the surface slopes down it became airborne. The pilot reported that he turned toward lower terrain and attempted to follow the airport access road but failed to maintain aircraft control. The airplane did not attain sufficient altitude and collided with a fence and nosed over. The wings, vertical stabilizer and rudder were substantially damaged. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.