The most vulnerable part of the house, or any structure, is the roof, because many embers being blown ahead of the fire are going to land on it. Unless the roof is in good repair and made of fire resistant material, the chance of serious damage or loss of your house is ten times what it would otherwise be.

Class A asphalt shingles will resist a substantial firebrand burning through to the wooden underside of the roof for a couple of hours--more than enough if someone is there to see--but they will burn through eventually. Better to use fireproof roofing materials if you have a choice.

The real improvement to survivability a fireproof roof can give is illustrated by the following graph. Based on studies of past major fires, it compares the likelihood that a given structure would survive, depending on whether the roof were of untreated wood shake or fire resistant. The fire resistant roofed structure is three times more likely to survive if flames can be kept at least 30 feet away.

State of repair is important, because gaps in coverage of places where the roofing may be torn or lifted up might allow embers to be blown in where the fire resistance may be degraded.

The eaves are especially vulnerable for either of two reasons. First, conventional roof construction leaves some space between the underside of the roof and the ceiling below. This space must be vented to allow air to circulate through to prevent moisture buildup, otherwise the excess moisture can lead to rotting in the roof structure. The winds in a wildfire can blow hard enough to blow burning embers through the vents into this space where fire could break out, unseen, hours after the flame-front has moved on. This is actually a common cause of the loss of houses.
The best preventive is to put metal screen over the vents to keep embers out. The screen should be 1/8-inch mesh. Vents needing to be screened may be found in several places, not only under the eaves. Other vents are located at the roof ends. Vents are also found near laundry dryers and utility areas, especially where gas-fired water heaters are installed. Vents may be found wherever spaces are closed to free-air circulation. Search them out and screen them.

Unusually deep eaves may also trap heat, preheating parts of the house and risking becoming hot enough to ignite. The problem is especially bad at the inside corners of "L's" around the house. Sun shades, arbors, awnings, etc., can cause the same problems.

Leaf and small branch litter can build up on flat or low-angled roofs. Even if the roof surface itself is fire resistant, if there is enough litter to support a fire, that heat alone could ignite the underlying structure. Gutters pose a similar problem if they are allowed to accumulate litter. Fire in the gutter could cause nearby structures to ignite directly or by the build-up of sufficient heat.

A prime source of this kind of litter is overhanging branches. The dangers of the litter apart, there is also the possibility that substantial branches falling onto the roof could break a skylight or damage the fire resistant roofing enough to expose the interior of the structure to embers and super-heated air. Branches too near the structure could also be blown against it hard enough to break windows or otherwise expose the interior to the fire.