Various stages of assault occur as a building is exposed to a wildland fire.
- Ashes are cast in front of a fire out of a smoke or convection column, which can result in secondary ignitions.
- Heavier embers that have more weight and may contain more heat, to serve as sources of ignition, arrive second.
- Finally, the actual intrusion of a flame front and the radiant heat flux can expose combustibles stored outside of a building and the exterior structure of a building to various levels of radiant heat.
A study has revealed the actual exposure of a building to the flame front of wildland fires is usually less than six minutes. However, the exposure time to other materials, such as embers, that can result in proliferation of ignitions can be much longer.
To enhance structural survivability, the self-defense mechanisms must, first, do everything possible to prevent the ignition of materials around your home from embers that are cast ahead of the fire and, second, they must withstand the assault of the flame front on the structure to prevent flames from penetrating into the interior of your home.
Many communities have homes that are not necessarily at direct risk from the main fire front. However, due to the close proximity of homes to one another and surrounding vegetation (cedars, junipers) or flammable building materials involved (cedar shake roofing and wooden siding), the community is at greater risk of structure ignition from embers. Once one or two homes have ignited, structure to structure ignition then becomes an imminent threat. Whole communities have been lost in wildland fires across the US in this manner. Many were not in the direct path of the main fire.
Statistically, embers are what burn down homes, not big flames. If you can harden your home from an ember storm, you can greatly increase the odds your home will survive without fire department intervention.