Fallout Shelter: Best 3-3-2 Defensive Vault Layout Blueprint

To build the most secure and efficient base in Fallout Shelter, configure each floor using a 3-3-2 room layout bordered by exterior elevator shafts, combined with a dead-end top floor kill zone. Encasing your production rooms with outer elevators shields your main workforce from Molerat infestations, while a dead-end entrance forces invading Raiders, Feral Ghouls, and Deathclaws to traverse your strongest defenders twice.

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The 3-3-2 Grid and Outer Elevator Placement

Every subterranean floor in Fallout Shelter contains space for eight room segments and two elevator shafts. Merging these segments into two three-wide rooms and one two-wide room maximizes dweller capacity and production efficiency.

To protect rooms from burrowing hazards, place elevator shafts along the far left and right edges:

[Elevator] — [3-Wide Room] — [3-Wide Room] — [2-Wide Room] — [Elevator]

Why Exterior Elevators Stop Molerats

Dirt Contact Trigger: Molerats only spawn in rooms that directly touch unexcavated dirt or rock walls along the vault perimeter.
Elevator Buffer: Elevators do not count as standard rooms and cannot spawn Molerats. Placing continuous elevator shafts along the left and right rock faces completely isolates production rooms from the vault's side borders.
Controlled Vulnerability: With outer walls buffered, Molerat spawns are restricted strictly to the lowest unexcavated floor of the vault.

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Designing the First Floor Kill Zone

Invaders always enter through the Vault Door and move horizontally toward the first accessible elevator to descend into the vault. Setting up a dead end on Floor 1 traps enemies in combat and forces them to double back through your heaviest weapons before they can reach other dwellers.

Floor 1 Build Order

Vault Door: Station two guards armed with high-damage weapons, such as Plasma Rifles or Combat Shotguns.
First Elevator Shaft: Place an elevator immediately to the right of the Vault Door to serve as the sole descent point to lower floors.
Three-Wide Power Room: Build a fully merged three-wide Power Generator or Nuclear Reactor directly to the right of the elevator, staffed by six dwellers with high Strength.
Dead-End Wall: Leave the far-right edge of Floor 1 empty without a second elevator.

When enemies breach the Vault Door and enter the Power Room, they engage all six workers. Because there is no elevator on the far side, surviving enemies must turn around and run back across the entire three-wide room to reach the entry elevator, taking a second full rotation of weapon damage from your workers.

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Managing Room Tiers and Incident Scaling

Internal incident difficulty—such as Radroach infestations and fires—scales directly with the upgrade tier of the room, not dweller character level.

Room Merging: Merging three base-level rooms into a three-wide complex increases storage and output without increasing incident danger.
Tier Upgrades: Upgrading a merged room to Tier 2 or Tier 3 multiplies enemy health and damage dealt during an incident.
Equipment Check: Keep all production facilities at Level 1 until assigned dwellers have outfits boosting relevant stats and weapons dealing at least 7–10 damage. High-tier rooms without heavily armed dwellers will quickly result in casualties.

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Isolating Unmanned Rooms with Firebreaks

Living Quarters and Storage Rooms are essential for population and inventory capacity, but dwellers rarely occupy them. If an incident triggers in an empty room, it spreads horizontally and vertically into adjacent spaces.

Placement: Build Living Quarters and Storage Rooms at the bottom of the vault.
Firebreak Gaps: Separate unmanned facilities from the rest of the vault using a buffer floor of unexcavated dirt or empty space.
Self-Extinguishing Incidents: When an isolated room catches fire or spawns pests, you can leave it unattended. Without adjacent occupied rooms to infect, the incident burns itself out after a short time without harming your dwellers or draining vault resources.