In low-density communities, a structure ignition will increase local ember exposures; observations have shown these ember exposures to be significant in the first 300 ft downwind. Locally there will also be increased exposures from fire (radiation and/or convection). These exposures may impact parcel-level combustible features; however, there should not be any direct fire exposures to adjacent properties due to larger SSD. The impact to the adjacent properties and community will be indirect in terms of fire, and possibly low for ember exposures (because of the large SSDs). Therefore, in low-density communities with large SSDs the impact of structure ignition on the surrounding structures is low.
In moderate-density communities, a structure ignition can ignite an adjacent property under a “favorable” wind direction and other local conditions. The embers generated from a burning structure will generate significantly higher exposures to the downwind structures relative to the low-density case due to separation distance. Compared to low density, the relative impact of a structure ignition to the surrounding structures can be seen as medium.
However, high-density communities are in a very different exposure category. Here the ignition of a structure will almost invariably result in the ignition of one or more adjacent properties and will likely result in the loss of a significant fraction of the community, as evidenced by several large loss WUI fires in the U.S. This is not only because fire spread occurs very easily between tightly spaced structures, but also because it is very difficult to contain a fully involved structure fire, even when only moderate winds of 10 mi/h to 15 mi/h are present. Fire spread is difficult to stop as it is very challenging to remove/block the heat between residential buildings when they are constructed 6 ft to 10 ft apart. In high-density construction, a single ignition can have a disproportionate impact on the overall community losses.