If the masses of dead bodies pile up high enough most materials give way. In certain numbers, Zombies can break through nearly anything in their way: Barricades, Fences, Doors, Gates, etc. Each Zombie behind an obstacle counts toward a score to break the obstacle between the zombies and their targets. A Walker counts one point for the determination of the Pile-Up Score, a Runner or Fatty counts as two points and an Abomination counts as three points.
Obstacle Pile-Up Score to break Barricade 7 Door 4 Fence 3