How does fire spread?

Government statistics show in the year ending September 2022 there were 67,389 primary fires within the UK, with 27,202 of these being home or dwelling fires. Our safety campaign aims to crack down on the prevention of fires – so we’ve answered ‘How does fire spread’ and all your other burning questions around fire safety.

To understand the underlying factors behind the question ‘How does a fire spread?’ we should start with the basics.

What is fire, and how does it start?

Fire is a chemical reaction created or caused by rapid oxidisation of fuel, this is also known as combustion. The oxidisation process is an exothermic reaction, which happens rapidly and produces energy such as heat and light.

Fire can either be:

Friendly– purposefully ignited with the intension to heat a room or cook etc.

or

Unfriendly– when a purposeful fire escapes its confines or is accidentally created.

The combustion or fire triangle

Fire requires three main components to combust: oxygen, heat and fuel. If fire is starved of one of these three components, it will cease to burn.

Oxygen makes up 21% of the air we breathe, meaning most climates on earth possess the right conditions to produce fire.

Fuel can be any sort of combustible material or organic matter, which has the burning qualities needed to facilitate the oxidisation process when exposed to heat. The heat source must reach the correct temperature threshold to ignite, which then produces a chain reaction.

Oxygen is the oxidising agent that reacts with the other components during complete combustion- this produces water vapour, a by-product of hydrocarbon combustion, as well as various other oxides. Solids, wood, paper, rubber and liquids, petrol and turpentine are all combustible when heated to “vapour stage” if oxygen is present.

The density and form of solid makes a difference to how the matter burns. When heat is applied, the decomposition of a solid mass must also be substantial enough to reach vapour stage and ignite.

For example, a small flame to a large log may result in the log absorbing the heat. Thin wood shavings would be more likely to ignite as they have a greater surface area to mass ratio, and therefore more contact with oxygen molecules to fuel a fire.

Soot and smoke

Smoke is a form of gas emitted during this process, which is coloured by carbon particles and residues. Soot is the name given to any residual substance of organic matter which is a product of incomplete combustion.

In layman’s terms, soot is where materials have burned but not disintegrated. There are different types of smoke and fire residue that can be produced, depending on three factors: the nature of the material burning, amount of oxygen available and rate of combustion.

Sooty carbon deposits can often be carried by wind or air flow and found far outside the confines of the primary fire. This causes further consequential damage to the surrounding areas.

Combustion is ‘incomplete’ when there is a limited supply of oxygen, this is also when carbon monoxide, a poisonous gas is formed.

An odorous residue is left from the burning gases. This is the unpleasant burning smell often encountered during and following a fire.

VOC’S

The odorous compounds emitted into the air as vapour are known as volatile organic compounds (VOC’s). These can be absorbed onto surfaces and even in dust particles, which can then become a health risk to humans as they enter the body through inhalation or skin contact.

VOCs can be found in a variety of materials such as wood, paints, and adhesives which release when they burn. They also release energy which can increase the rate of a fire, and even cause rapid fire spread from causing the simultaneous combustion of the contents of a room.

VOCs cause poor air quality following a fire, so it is essential the property is properly ventilated and cleaned to avoid negative health risks.

What are the different stages of a fire?

There are 4 main stages when considering how does a fire spread; ignition, growth, fully developed and decay. Combustion starts at the ignition stage, and the fire goes through a life cycle whilst it builds until it is starved of oxygen, heat or fuel.

Three ways a fire can spread

Once a fire is ignited, it can spread quickly through different ways of heat transfer.

How does fire spread: Convection

When combustion occurs, the surrounding air is heated. As it heats, the gas particles become less dense and lighter, which causes the hot air to rise to the ceiling.

This will then travel across the ceiling of its confines to the coldest part of the room where it eventually drops in temperature and returns to floor level.

The cold air then replaces the hot air near the fire, and from this cycle convection currents are created. This process feeds the fire with more oxygen which encourages it to grow.

How does fire spread: Conduction

Solids, liquids and gases can all be thermal conductors which transfer energy and aid the spread of fire. The combustible materials must be touching or very close to facilitate conduction.

Certain properties will affect how efficiently the heat is transferred, and how fast the fire spreads.

How does fire spread: Radiation

Heat travels through space as heat rays, which is why the closer you are to a fire the warmer you get. This principle also applies to how close a combustible item is to a heat source, and the greater chance it has in igniting.

With radiation, materials do not need to touch to transfer heat. However, when combustible items are closer in proximity, it is likely the fire will spread quicker and become more intense.

The three methods of heat transfer can be observed similarly during cooking in the diagram below.

Methods of Heat Transfer Vector by Vecteezy

How does fire spread through a building?

This can depend on the materials of the building and how well it is ventilated. The presence of flammable materials such as insulation or curtains will feed a fire.

Air movement from an open window or door will reduce the temperature of a room, amount of smoke and potentially diminish the fire. However, think critically about which are left open as doors and windows can create a barrier for the fire, opening these may allow the smoke and fire to spread.

Now equipped with the knowledge to confidently answer ‘How does fire spread,’ we can take a look at preventative measures aimed at averting and extinguishing fires.

How to prevent fires and fire damage

The first step in preventing fire damage is to always practice precaution.

• Follow all safety instructions provided by manufacturers of white goods and other electrical items.
• Be cautious with any open flames such as candles, matches, stove and ovens, as well as cigarettes. Never leave these unattended.
• Install smoke detectors and keep fire safety equipment such as blankets or extinguishers in a place that is easy to access.
• Closing doors and windows will starve the fire of oxygen, and is the easiest way to prevent fire spread and mitigate any damage.
• Installing sprinkler systems and appropriate fire extinguishers in commercial buildings is essential.
• You can implement fire proof doors and walls which will contain a fire within a certain room.
• Being prepared with an evacuation plan will ensure all individuals have the ability to vacate the building safely and efficiently.
• Having insurance cover ensures you will mitigate the financial cost of fire damage.

In the event of a fire, you can follow our 5 steps in How To Reduce Stress In The Event Of A Fire Or Flood. If the damage has already been done, we have resources on How To Clean Smoke Damage On Walls And Ceilings and advice for those with commercial properties.

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If your property has suffered extensive damage that is outside of your capabilities to manage, its time to call the professionals. Rainbow Restoration has decades of experience in fire restoration – to find your nearest branch on our website, or give us a call on our 24/5 helpline 01623 422 488, 365 days of the year.