Fireboard, also known as gypsum board, is a type of fire-retardant drywall that is used in a variety of applications in the construction of homes and buildings. Fireboard slows the spread of a fire in two different ways. Fireboard contains a higher concentration of water in it's composition, so that in the event of a fire, the higher concentration of water in the material makeup of the board works, in effect, as a miniature sprinkler system. Too, fireboard has special fibers in it's material makeup that helps to maintain the barrier longer in a fire, keeping the fire from spreading from one area of a home or building to another. The length of time that the fireboard can retard the fire will vary depending on thickness, but ultimately, the overall construction and materials of the building is what determines overall fire rating.
Fireboard is most commonly found in one-half (12mm) and five-eighths (15mm) inch thicknesses. There are companies in other countries (such as the UK and China) that offer additional thicknesses such as three-eighths inch (9.5mm). There are many types of gypsum board, but the only type that is rated and approved as a fire barrier is the "X" type gypsum board or fireboard. This designation means that the board prevents the spread of fire as opposed to being consumed. There is a direct correlation between the fireboard thicknesses and the amount of them that they will retard the fire; the thicker the board the more time the fireboard will hold up and not deteriorate in the event of a fire. Five-eighths inch fireboard will last for up to one hour, one-half inch will last forty-five minutes. Three-eighths will last one half an hour, and so on.
While different thicknesses offer different values in the times that they will hold up in a fire, the overall rating is done on the entire structure it's self, as opposed to the board. For example, there may be five-eighths board up on a garage wall, but if there is a hole in the board to allow for wiring that is not also covered by the board, it will compromise the integrity of the fire suppressing abilities of the fireboard, and essentially eliminate the protection that the fireboard provides.
This is just one example and it is important to remember that the board it's self is not capable of preventing a fire spreading; a fireboard that is affixed to studs, but surrounded by two feed on all sides of bare studs will not retard a fire. Fireboard is not necessary on all walls, but is certainly required on certain ones, such as the ceiling of a garage that has a bedroom or other living space above it. However, due to the increased safety it provides, it could be used throughout the house. If there is any question on whether a particular wall should have fireboard, err on the side of caution. It is also wise to consult a code inspector to ensure that the building or home is not in violation of any ordinances or safety code.