How Fire Spreads in a Building

Learn How Fire Spreads in a Building

How Fire Spreads in a Building

Once a fire starts in a building, it can spread quickly — even to areas without direct contact with the flames. All fire needs to start and spread is an ignition source, oxygen, and fuel. If it lacks any of these ingredients, the fire cannot start or will burn itself out fairly quickly. Building fires usually keep spreading until they use up all available fuel. Here is how fire usually spreads in a building.

Timeline for a House Fire

When a two-story home catches fire, it usually progresses as follows:

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  • Within 30 seconds, the fire will begin to spread and grow.
  • After 1 minute, the fire will spread from the initial flame and the room will start to fill with smoke.
  • After 90 seconds, the temperature will reach 190 degrees F as the smoke layer descends.
  • Within 2 minutes, the smoke detector will go off and there is time left to get out of the house safely.
  • After 2 minutes and 30 seconds, the temperature at the source room can reach 400 degrees F.
  • Within 3 minutes, the smoke will pour into other rooms and the temperature at the source room will reach 500 degrees — well above what a human can survive.
  • By 3 minutes and 30 seconds, escaping the home will be difficult and the upstairs halls of the home will be filled with smoke.
  • Before 4 minutes have passed, a flashover happens. Everything in the source room will ignite and the temperature will exceed 1,400 degrees.
  • By 4 minutes and 30 seconds, flames will have engulfed the exterior of the home.

Fire Blocks in Walls and Floors

In most homes, a fire will burn through construction materials quickly and spread from the ground floor to the top floor. Many condos and multi-family properties have fireproofing between levels to reduce this risk. These are known as fire blocks. Materials made from steel and concrete help stop the spread much better than a wood frame. Fireblocking between floors can also be done with fireproof caulking, fire-resistant joists, and other methods.

When properly installed, fire stops can stop the advancement of fire from one area of the building to another by cutting off oxygen and combustable materials that feed it.

Spreads Spreads Faster in Open Space
A building with wide, open space allows a fire to burn much faster than a building with limited interior walls and doors. This is because it will have more oxygen and, as it burns hotter, it creates an updraft in the open air. Fire will still burn through doors and walls, but these structures help slow down the fire and give emergency responders more time to douse the flames.

Spreads Through Ductwork
Buildings with central AC and heating have ductwork that can make it easy for smoke and flames to spread to another areas of the building. These forced air systems are very effective at spreading smoke damage in particular as the smoke is drawn into return air vents and through the ducts. The best way to keep ductwork from spreading a fire is keeping the ducts clean. A build-up of lint and debris only provides kindling!

Newer Homes and Furnishings Burn Faster
Research has shown that homes and furniture built 30 years ago were far slower to burn than those constructed today. This means less time to escape a fire. Three decades ago, you had an average of 17 minutes to escape a home fire, but this is down to only 3-4 minutes now. This is because today’s furnishings — including carpet, curtains, couches, and tables — have a lot of synthetic fibers versus the real wood and natural materials more common in the past. These materials burn faster and hotter than natural materials.

Window-to-Window Fire Spread
While fire can spread directly through ceilings and floors, the most common way for fire to spread vertically in high-rise buildings is from one window to the next. This happens when flames lap out a window and then flow upward. The heat melts or cracks the window above and the flames can spread into the floor above through the now broken window. As the flames spread to the next floor, combustible curtains and ceiling tile are ignited.

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