[Piotrsko]

What I am hearing from possibly accurate sources is: you get this constant hot mass of embers hitting burnables. Duh. Raises the point of attack to burnable temperatures. Typically eaves and vent holes, but siding and also decoration. 20 year old wood has a lower ignition point, maybe only 250 degrees. lots of these places had oil based paint and varnishes. Add a resin bearing bush of any type and there is an attack point. Pretty much all the surviving homes were covered in stucco or concrete block construction and had airgaps with no vegetations against the house.

Just so you know: if you do the roof sprinkler thing, you need a potload more than just wet, think monster rain storm. Swimming pool source, and fuel powered genset. All the typical attack points are still dry because the roof is designed to do just THAT. Knew a crazy enginneer in Laurel canyon that survived a fire with a commercial roof fire sprinkler system outside, but his pool was dry, the genset cooked (literally)