The wildland fire community is having a hard time wrapping their mind around one truth. Houses as commonly spaced in suburbia or towns will burn building to building.
In normal circumstances the fire department shows up and intervenes. Occasionally a second home is involved prior to arrival of that FD. They just call a second alarm. More of the brothers and sisters show up and put out the fire.
It’s reliable enough that we don’t worry about our neighborhoods burning down. Until there a dozen fires in a little town, with a big wildfire burning all over.
The fuel matrix most of us live in as it exists depends on active suppression to keep one fire from burning big chunks of it down each each unwanted ignition.
I have some friends who live in the mountains north of Santa Cruz. They were in the path of a huge wildfire this year and were told to evacuate. They live on what you might call a compound, with a main house and five or so outbuildings including a large woodshop, a music workshop, and a rental house. The fire came through as a ground fire, and there was a fire truck there spraying the main house. The main house was saved but the rest of the structures burned, including the rental with the belongings of the renters.
None of the trees burned. Very few shrubs burned, there were not many to begin with. Images that my friends took after the fire show a property with only subtle indications that a fire burned through, but the structures are all just piles of ash.
Rob, one of the friends, was a professional builder his whole life. These structures were all built to code, and that was the problem.
When a low intensity fire burns up next to a structure, airflow becomes restricted. Heating air increases the pressure. The higher pressure on the fire side begins to push out the cold air under the structure. when the flame reaches the structure, it essentially forms a flame nozzle under the structure, through the crawl space vent.
We know this now. There are technological solutions, but they were not implemented on decades of structures and are expensive to retrofit.
-------------- Between every two pines is a doorway to the new world. - John Muir
Some of this research work has been done by the insurance and building industries. The Nat’l Insitute of Standards has been been part of that, too. NIST has built full scale houses in wind tunnels, then studied how embers entered. short example:
Take away lesson - in high wind and a lot of ember producing fuel up-wind, 30’, 50’ or 100’ of clearing isn’t going to matter much.
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