The under-eave area is vulnerable if embers enter the attic area through any gaps that may exist in this area, or if flames from ignited vegetation, siding, or other near-home combustible materials reach the area. Attic vents are commonly found in the under-eave area, and embers can also enter the attic through these openings (for more about vents, click here).
In an open-eave design, the roof rafters or joists and roof sheathing are visible. With a soffited-eave design, the roof rafters and sheathing are hidden through the use of a panel or boards that extend horizontally from the edge of the roof (typically being attached to the bottom of the fascia) back to the exterior wall. “Boxing-in” the eave when sheathing or boards are attached to the bottom side of the roof rafters in the exterior portion of the eave.
The following photograph is a soffited eave, with a strip vent included.
The following two photographs show an open-eave design. The second shows a gap between the roof rafter and the edge of the blocking. This gap would be vulnerable, potentially trapping embers, or allowing them to enter into the attic.
Many wildfire retrofit guides suggest replacing open-eave framing with either a soffited or boxed-in eave design. Research recently conducted at the University of California does support this finding. The open-eave design tends to trap heat in the under-eave area. If ignition occurs, the fire spreads laterally more quickly than when a soffited-eave construction is used.