Rooftop Finale

Upon reaching the roof, the Survivors find a makeshift defensive position in one of the main structures, and must use a radio to alert the helicopter to their presence. The pilot tells them to prepare themselves, and call again once they are ready. Once the second call is made, the helicopter will begin an approach, but the pilot warns that it will take some time to reach the hospital.

The Survivors must then hold out against waves of Infected which swarm their position, including several Special Infected and at least two Tanks. After fifteen minutes (with the pilot giving periodic updates), the helicopter finally arrives at the partly destroyed helipad, and the Survivors board it to make their escape. The Survivors may trigger an additional wave of zombies by shooting at the rotors of the helicopter as it lands.

With all the Survivors aboard, the helicopter flies off, going behind the hospital before coming around and passing above the camera. The camera fades to black, and the credits roll.

The Original Ending
The original ending of No Mercy was that after they got on the helicopter, the pilot was complaining about how he was attacked while saving another group of people. After he died from the infection, the Helicopter crashed on the highway at the beginning of Death Toll. Valve thought that this left players with an unsatisfactory ending to the campaign.

1

 * When you climb up the ladder and first emerge on the roof, do not leave the building that you are on. Instead, select one person to go down.
 * Do not call the radio yet. Instead, the person who jumped down should grab a first aid kit, and bring it back to the building. One at a time, each person stand on the very edge, and let the person heal them. Repeat this until the 3 left up top are healed.
 * The person who dropped down can now call the helicopter. The other three should stay on top, watching out for Smokers and Hunters.

2

 * Once you are on the rooftop and healed, get into the closet near the ammo and weapons cache.
 * Two people should be near the door crouched and meleeing, while the other two are behind them shooting, preferably with dual pistols to conserve ammo.
 * Once the helicopter is called, all four survivors should stay in the closet, until the first Tank appears. When the Tank appears, all four survivors should leave the closet, and defeat the Tank like normal.
 * When the Tank is dead, return to the closet, and repeat with the melee and shooting formation until the helicopter comes.