The pilot reported that the brakes were not effective during landing rollout. He allowed the airplane to depart the paved runway into the adjacent grass area to slow down. The airplane encountered a shallow ditch which damaged the fuselage immediately aft of the firewall, resulting in substantial damage to the airplane. The firewall and nose landing gear were also damaged. A mechanic completed a postaccident examination and was unable to identify any anomalies with respect to the brake system. The mechanic commented that the brake system can be affected if the parking brake is partially engaged at takeoff; however, the pilot reported that the parking brake was not used during the flight. The mechanic noted the airplane departed the pavement about 800 ft from the end of the 4,400 ft runway and encountered a drainage ditch. The airplane traveled about 600 ft past the ditch before it stopped. Based upon this distance, the pilot likely landed long and with excessive airspeed which resulted in an inability to slow the airplane sufficiently before encountering the end of the runway.