Vaults are among the structures having the strongest doors. They are designed to protect valuables such as money, documents, jewelries and records. Vaults may differ in size according to the amount of valuables to be stored. The strongest vault doors are designed for bank vaults where huge amounts of valuables are safely kept.

Designing a door for a bank vault is difficult. It should have a maximum strength that can resist strong heat during fire and cannot easily be damaged during attempted theft or robbery. Its durability and strength is dependent mostly on its thickness and not on the type of material used. However, since bank vaults have doors made of either concrete cased with alloyed metals or purely metal, the hardest substances are chosen.

In making a vault door, most manufacturers use a regular mold to form the steel before screwing the metal clad on it. This allows the concrete to reach its ultimate strength before the actual metal cladding. However, some manufacturers pour the concrete directly on a special metal clad that altogether forms the vault door. In this way, the time of assembling will be minimized.

In other words, the most common vault doors are not pure steel or iron doors but concrete reinforced doors. Concrete is used to resist compression forces that might be applied on the door. Unlike normal concrete structures wherein the steel bars are the ones covered with concrete, vault doors are made with concrete clad with the steel. The reinforcement results in a strong and massive closing.

The most important part of a vault door is the lock system. It consists of different small sized metals held in different significant arrangements to form a strong hold along the side of the impenetrable door. Lock system can be of various types such as combination lock, high security key lock, dual control combination lock, time lock and safe-cracking lock. As long as vaults are made with iron doors, any of these lock systems can be applied.

The same principle used in making vault doors are applied in making special prison doors, submarine doors and doors of vehicles used in space. The iron doors used are strong enough to resist extreme pressure at a certain depth underwater or extreme lack of pressure within a vacuum and are perfectly tightened that even air cannot penetrate along the side.