The distance between structures and the density of housing in communities significantly impact how fires spread in areas where the wildland and urban environments meet (WUI). In high density neighborhoods during a severe wildfire, extreme burning conditions lead to a higher number of structures to burn. Once a few buildings ignite, a domino effect occurs, with fire spreading rapidly until the extreme burning conditions, usually driven by strong winds, diminish. This type of fire spread in the built environment is associated with costly wildfires in the WUI.

Defending homes during non-extreme fire conditions is easier when they are closer together, as it requires fewer firefighting resources. However, under extreme wildfire conditions, defending structures becomes harder due to safety concerns for both firefighters and the public. During these conditions, the main way fires spread is through wind-driven flames jumping from building to building. Most buildings are not designed to withstand intense exposure to flames, so once one catches fire, it adds more fuel to the flames, making it even more intense. To prevent this “domino effect”, it is vital to maintain proper spacing between buildings and ensure their resilience in the face of extreme fires. The proper distance depends on factors like the buildings’ locations, geometry, and wind speed.

What is the proper building-to-building separation? To investigate this question, IBHS is conducting an extensive research project called the Wind-Driven Building-to-Building Fire Spread Research. This project is divided into multiple phases and spans over several years. The first phase, which started in 2020 involves studying how fire spreads from different-sized burning sheds to a specific “target building”. Collaborating on this phase include the National Institute of Standards and Technology (NIST), the United States Forest Service (USFS), and the California Department of Forestry and Fire Protection (CALFIRE).

The findings from the first phase shed light on the subsequent phases, which investigates the fire spread from burning auxiliary dwelling units to the target building. For this second phase, the primary collaborators will be IBHS, CALFIRE, and the University of California, Berkeley.

The ultimate goal of this research is to determine critical pathways of building-to-building fire spread and how that can be applied to help home and business owners reduce their wildfire risk.