Wildfire activity is on the rise and with this comes an increased impact on buildings, including homes and businesses, in the wildland-urban interface. Residents and business owners can take steps to reduce the risk to their property located in wildfire-prone areas. Eliminating fuels from the immediate area surrounding the building, as referred to as defensible space, can decrease its ignition potential by reducing the possibility of a direct flame contact and radiant heat exposure to the building.
Historically there have only been two defensible space zones, 0–30 ft and 30–100 ft (or to the property line) [California Public Resources Code, 2010], which are referenced in building codes related to wildfire. As far back as 2011, educational organizations such as the National Fire Protection Association (NFPA) and IBHS have been advocating for dividing the 0–30 ft zone into two separate zones (0–5 ft and 5–30 ft), acknowledging that ember ignition of combustible materials immediately near the building poses a threat to the building from direct flame contact and radiation. IBHS wildfire preparedness guidance currently recommends creating and maintaining a 0–5 ft noncombustible zone around a building, including the entire footprint of an attached deck. This zone is designed to protect the building from ignition that can result from wind-blown embers that can accumulate at the base of the exterior wall, and from exposure to radiant heat or direct flame contact that would occur due to the ignition of combustible materials located near the building or under an attached deck. Landscaping and vegetation management in the 5-30 ft zone should prevent the fire from climbing into the upper portions of trees or shrubs and to stop any fire from burning directly to the building. Wind-blown embers may still be able to ignite individual islands of plants in the 5–30 ft zone, which is why the near-building noncombustible zone is critical.
Current guidance is to create and maintain a 5-ft near-building noncombustible zone, based on expert judgement. The objective of this research project was to provide scientific data to evaluate the effectiveness of the 0–5 ft noncombustible zone as currently defined and determine whether a larger or smaller distance for this zone is warranted.