ok, your problem may be what my problem was. my septic pipe went up to the roof like yours. what the plumber found out was that the septic pipe was too close to the air conditioning unit and the odors were getting sucked back into our home. the plumber installed an extended pipe to the existing septic pipe that exceeded the height of the air conditioning unit by five feet. we have not had a single smell since. it cost us 430.00 to have this done and the pipe is noticeable from the street but it sure beats a smelly house. we actually got so sick we had flu-like symptoms.