I don't think a simple spray on a roof is going to save a home from this type of fire. Non-flammable roofs are not that uncommon, particularly tile roofs, but the houses with those are not immune to brush fires. Now, if you are talking about an old adobe building with walls a foot or more thick and a tile roof, then spraying wooden frames for doors and windows might do some good. Thick metal shutters with internal insulation would probably do much more good, though. And, if there is enough heat transferred through the roof tiles to ignite the wooden rafters, all is lost, anyway. So, maybe that old adobe house needs asbestos subroofing and steel rafters? Remember, the house surrounded by fire becomes a very hot oven for everything inside it. If it is not insulated well enough against that, the interior can catch fire even if the shell does not. There was a piece on the news earlier this week showing how a guy saved his house by rigging sprinklers on the roof that ran off his garden hoses. That should work quite well, so long as there is an uninterruptable supply of water. But, if he has a well and needs electricity, then he needs some sort of off-grid supply of electricity. And, he needs to protect that electrical source (generator and its fuel or whatever) from the fire, too. On the other hand, if everybody with city water did that, then the city water supply might be depleted, or at least lose the pressure needed to get water to the roof with enough force to spray it, at the very time that it was most needed and everybody turned on their sprinklers.