straight to the heli."

First things first, once you're up on the roof, make sure everyone grabs all the ammo, health kits, and any throwable items like Molotovs they can find. Don't skip this! Then, interact with the radio to call for extraction.

Now, you need to head up to the very top of the building you're on. You know, the one with the minigun. Keep climbing until you're on the highest rooftop.

At the back of this rooftop, look over the edge. You'll see a really small ledge down below. It might take a few tries to land on it, so don't get discouraged. Get your teammates over there too.

Once everyone's on the ledge, crouch down. This spot is pretty safe from most regular infected, but you'll still need a couple of people to keep an eye out for Special Infected.

When the Tank shows up, don't worry, he can't climb up to your ledge. You've got a couple of options here: you can either have one person unload on him with an auto-shotgun while the others watch for Specials, or you can grab a propane tank or two, place them at the bottom of the building, and blow them up when the Tank arrives to deal some serious damage.

Just hang tight on the ledge until the second Tank is dealt with. Then, you'll hear the chopper is on its way.

When you hear the chopper's arrived, it's time to move. Jump down from the ledge, toss a pipe bomb away from your group to distract any immediate threats, and then make your way to the chopper.