Heat, fuel and an oxidizing agent, usually oxygen, must all be present for ignition to occur. The fire triangle graphic below defines the conditions needed for a fire to start but a fourth condition must be added to sustain the fire and allow it to grow. This is illustrated by the fire tetrahedron shown below, which shows how heat, fuel, oxidizer and an exothermic chain reaction together can trigger a building conflagration.
There are two types of methods of suppression used in building fire safety design: Passive and Active. Passive suppression uses materials, systems, building elements, and/or building layout to prevent or resist ignition, to limit its spread to other combustible contents in the room and to contain the fire within the room or zone to prevent its spread to other sections of the structure. Active suppression is the employment of mechanical devices such as sprinklers or extinguishers to extinguish the fire in its early stages and thus prevent its spread. Passive suppression uses the natural properties of materials and products that are part of the building design to suppress the migration of the fire. It is well known in the construction industry that the single most important characteristic of gypsum drywall is its fire resistance. This is provided by the principal raw material used in its manufacture, CaSO₄∙2H₂O (gypsum). Gypsum is noncombustible, which means that it contributes no fuel to a fire. As the chemical formula shows, gypsum contains 21% by weight chemically combined water, also called crystalline water that is part of the gypsum crystal itself. When gypsum drywall panels are exposed to fire, the heat of the fire converts the crystalline water to steam. The heat energy that converts water to steam is thus absorbed, keeping the opposite side of the gypsum panel cool as long as there is water left in the gypsum, or until the gypsum panel is breached.