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.