Wildland-Urban Interface Demonstration Building - Siding

Different siding materials were used on each side of the building. A composite “hardboard” siding was used on this exposure. It is a wood-based siding product, but fire tests have shown that it doesn't perform as well as solid wood siding.

See a video demonstration of some issues to keep in mind about siding and wildfire.

In this photograph we are looking at the back of the hardboard composite siding product. "Fire safe" siding should do two things:

• minimize the upward spread of flame on the exterior of the building, and
• prevent, or resist, the entry of the fire through the horizontal (as shown here) or vertical lap joints into the underlying stud cavity.

A combustible siding product will be vulnerable to both upward flame spread and flame penetration at lap joints. Noncombustible or "ignition resistant" materials have demonstrated their ability (via a standard test) to limit flame spread. Testing has shown that these products have durable lap joints. Incorporating an underlying sheathing product (such as plywood or oriented strandboard) will improve the "penetration resistance" performance of combustible siding products. Incorporating an additional fire-rated gypsum wall board product can also be used for more protection. The sheathing products won't impact the upward flame spread on the combustible siding product.

This is a photograph of fire testing at the University of California Fire Research Laboratory. A propane gas burner is providing a flame impingement exposure on this fiber cement lap siding product. The output of the burner is intended to provide an exposure similar to that of a burning intermediate-sized plant. The exposure time was longer than the time it would take for a plant to burn.

The fiber cement siding didn't ignite, but cracks did develop during the course of the test. Fire didn't spread into the stud cavity.

Even though fiber cement siding is noncombustible, it does transfer heat via conduction. In this case, the propane gas burner was left on until “failure” occurred (in this case, ignition of the edge of the stud facing the siding). In this case, ignition occurred after a 22-minute exposure to the flame. This kind of exposure isn't likely, even in severe wildfires, but could be developed if a wood pile was located immediately adjacent to the siding.

This is an example of an interior (or “re-entrant”) corner. If ignited, the close proximity of the siding making of the intersecting corner results in more rapid fire and flame development.

There are a few types of vents used to provide air exchange in attics. Vents help maintain moderate moisture levels in the attic (and crawl space). This is a photograph of a gable-end vent. Ember entry through vents into the attic, with possible subsequent ignition of combustible materials, is of concern during wildfires. Given the distance between the ground and most gable end vents, flame spread up the wall to these vents isn't as likely unless a combustible siding product is used, and it ignites.

A neighboring house that ignited during a wildfire and was completely destroyed provided the radiant exposure to the vinyl siding on this house. The destroyed house was about 50 feet from the vinyl sided home. The vinyl siding had been installed over wood siding (plywood T1-11 and shiplap). Some window glass was broken, but fire did not spread into the living space of the home.