Training Night, February, 2006. Training was meant to familiarize our neighboring department with our Aerial as we respond to their town for chimney and structure fires. The aerial is a 1985 Seagrave, 100′ rear mount. The apparatus was setup and the operator extended the ladder off the side, approximately 75 degrees (?). Two firefighters began to accend the ladder (which as it turned out was extended approximately 90-95′). The tip of the ladder extended over the back of our station roof approximately 10-12′. The operator was explaining things to the neighboring Chief. As the first F.F. approached the end of the ladder (second F.F. approximately 20′ behind) …someone indicated that the ladder was going down. The ladder very slowly tipped down as the opposite side of the truck lifted up. Fortunately, the ladder was over the roof, as is was stopped when it contacted the roof edge. It was a real delicate balancing act, there was no severe impact. Most importantly, no F.F.’s were injured. After all F.F’s exited the apparatus, we were able to gently lower the truck. We were then able to lower the ladder and rebed. We have since had the truck inspected for any damage (mechanical, structural (frame) and the turntable/ladder unit. Incredibly, no damage was done to the unit. This could have been a disastrous incident – definitely a CLOSE CALL avoided. (We have instituted changes).