Combustibility is a measure of how easily a substance will burn, through fire or combustion. This is an important property to consider when a substance is used for construction or is being stored. Special precautions are usually required for substances that are easily combustible. These measures may include installation of fire sprinklers or storage remote to possible sources of ignition.

Substances with low combustibility may be selected for construction where the fire risk needs to be reduced. Fire resistant substances are preferred for building materials and furnishings.

Code Definitions

For an Authority Having Jurisdiction, combustibility is defined by the local code. In the National Building Code of Canada, it is defined as follows:

• Combustible: A material which fails to meet acceptance criteria of CAN/ULC-S114, Standard Method of Test for Determination of Noncombustibility in Building Materials.

This leads to the definition of Noncombustible:

• ''Non-combustible: means that a material meets the acceptance criteria of CAN4-S114, "Standard Method of Test for Determination of Non-Combustibility in Building Materials".

Fire testing

Various countries have tests for determining noncombustibility of materials. Most involve the heating of a specified quantity of the test specimen for a set duration. Usually, the material cannot support combustion and must not undergo a certain loss of mass. As a rule of thumb, concrete, steel, ceramics, in other words inorganic substances pass these tests, which permits them to be mentioned in building codes as being suitable and sometimes even mandated for use in certain applications. In Canada, for instance, firewalls must be made of concrete.

Relevance in construction

In building construction, buildings are typically divided into combustible and noncombustible ones. The code provisions and safety measures that must be taken into account in the design and construction of a building depend to a significant extent upon whether the structure is made from noncombustible elements, such as concrete, brick and structural steel, or a combustible element such as wood. Combustible structures have greater limits in terms of maximum building height and area.

Related matters

The flammability article describes further the subcategorisations of combustible matters. Here, further fire tests are involved in quantifying the degree of flammability or combustibility.

The chemistry underlying the fire testing and resulting code classifications

The degree of flammability or combustibility depends largely upon the chemical composition of the subject material, as well as the ratio of mass versus surface area. As an example, paper is made from wood. A piece of paper catches on fire quite easily, whereas a heavy oak desk is much harder to ignite, although the wood fibre is the same in each substance, be it a piece of paper or a wooden board. Also, Antoine Lavoisier's law of conservation of mass, states that matter can be neither created nor destroyed, only altered. Therefore, the combustion or burning of a substance causes a chemical change, but does not decrease the mass of the original matter. The mass of the remains (ash, water, carbon dioxide, and other gases) is the same as it was prior to the burning of the matter. Whatever is not left behind in ashes and remains, literally went up in smoke, but it all went somewhere and the atoms of which the substance consisted before the fire still exist after the fire, even though they may be present in other phases and molecules.