Siding (cladding) is an important aesthetic attribute for houses, but it also has a key role as part of a protective enclosure to help shed rain, while permitting excessive vapor to move through and out of the house.
If ignited, vertical flame spread on combustible siding will threaten other vulnerable components of a house, such as windows or the under-eave area. In the photograph shown below, flame has spread over the entire surface of the combustible siding product. As expected, the noncombustible product did not ignite when a burning wood crib was placed at the base of the wall. A similar burning woood crib was placed at the base of the combutsible siding product.
A common way for combustible siding to ignite by flame contact from burning vegetation, combustible mulch, or other combustible materials (firewood, lumber, etc.) stored near the home. All of these adjacent combustible materials very likely would have been ignited by embers.
A potential contributing problem with wood and wood-based siding products is fungal decay, particularly when present along the bottom edge. When dry, decayed wood is more easily ignited.
Vinyl siding deforms when exposed to heat from the wildfire, or other heat sources such as nearby burning vegetation or a burning building. Deformed vinyl siding will expose the underlying sheathing or wall cavity. At this point, a house will be relying on these materials for protection.
Vinyl siding deforms with heat.
Tests conducted by the University of California Forest Products Laboratory have shown that siding without sheathing (both combustible and non-combustible) ultimately fails by burning through laps or conduction of heat to the studs. However, the presence of sheathing (plywood or OSB) largely prevents failure. The type of lap is also very important; the plain bevel siding (on the left below) failed in just over one minute, but the rabbet bevel siding (on the right) lasted over 21 minutes.
The relationship observed with horizontal lap siding would also apply to vertically lapped products, such as T1-11-type panel products and board-and-batten siding. A more complicated joint is preferable from a flame entry perspective. Panel products can have a butt-joint covered with a sealant, or with ‘batten’ cover, or have a more complicated joint. Board-and-batten consist of a number of wood-to-wood joints similar to plain bevel, and so flame penetration into the stud cavity would occur more easily In these cases.
All lapped wood siding should have an interlocking type of lap (such as the rabbet bevel shown above) to protect against flame penetration. If you have combustible siding, carefully inspect it annually for gaps and make sure that they are filled with a high-quality caulk.