Under normal rules zombies can't go through doors.
A rule my group was tinkering with is that fatties and abominations can use their action to break down doors. Standard zombies are unable to bypasss doors but the tougher meaner boss zombies can manage to tear it down or find a way through. Think land of the dead's boss zombie with the jackhammer, or the tank zombie from L4D.
In scenarios where zombies pile up inside a building once a fatty is spawned they can break free and out into the streets. It alternately allows zombies to eventually go after suvivors who have holed up inside buildings by closing the doors. Sooner or later a fatty will spawn and be able to tear through the barricades.
We're designing a mission simular to the opening scene in dead rising, zombies are piled up against the gates of a mall. The zombies keep piling up against the door until one of more of the bosses show up and breach the gates.
Make-shift barricades are often contructed by survivors to help buy time and slow the zombie advance.
There are two types of barricades, light barricades and heavy barricades.
Light barricades consist of smaller semi-mobile objects like desks, tables and dumpsters. A survivor may spend two actions to construct a barricade within their zone. While a barricade is in place zombies may not move past that zone or attack any suvivors in that zone (as long as they are sepperated by the barricade). Zombies may spend an action to attack and destroy a barricade. Fatty and abomination models can destroy a light barricade as a free action. They are too strong and full of rage to be stuff by such simple barriers.
Heavy barriers are constructed by large and heavy objects, often over-turned cars, shipping containers, or security fences and gates. A heavy barricade requires two survivors and each surivor must spent 3 actions to create the barricade. (or 6 total actions spent by 2 or more survivors) While a heavy barricade is in place no zombie models may move past that zone or attack survivors in that zone (as long as they are sepperated by the barricade). Unlike light barriers heavy barriers may not be destroyed by normal zombies. Only fatties of abominations are capable of destroying a heavy barricade. A fatty or abomination may destroy a heavy barricade by spending their normal action.
It requires resources to construct a barricade and players must agree in advance if a tile has proper resources to do so. For example a tile has a building with an interior that displays a diner and shows multiple table. The players agree that it has proper material for a barrier. Another tile might depict a building with a single chair in it, that room would not have proper resources to construct a barricade.
Surviors wish to construct a heavy barricade, they move a pair of police cars into the same zone so that they can use them to construct a heavy barrier.
Security fences and gates are considered heavy barriers, but they cannot be built and must placed on the map prior to game play. Players may open and close a security gate by spending 2 actions.