No material is “fire proof” however, proper use and assembly of fire-rated building materials can reduce a fire’s spread and extend the amount of time it takes for a home to ignite and burn. (Structural assembly is the process of layering materials when building exterior walls and roof.) Your roof is vulnerable to wildfire because it is the largest surface area of your home. The exposed, uneven surface of a roof can easily trap hot, wind-blown embers. Simple roof forms are easier to protect than complex ones due to less surface area and intersections, which may create heat traps. Use class A or B roofing materials to reduce risk.
Your roof can have one of four fire classifications: Class A, Class B, Class C, or unrated. A Class A roof is ideal for fire protection and may be required by your building codes depending on your place of residence. Whether you’re constructing a new home or outfitting an addition, it’s important to understand and carefully consider the fire rating as you’re choosing roofing materials.
Class A Roofing
Class A roofing is the preferred choice for any home, but this type of roofing is particularly important if you live in an area that is prone to wildfires. To achieve a Class A rating, the roof must be effective against severe fire exposure. This is proven if it can:
• Experience maximum flame spread of 6 feet
• Withstand a burning brand measuring 12″ x 12″ and weighing 2,000 grams
• Last 2 to 4 hours before ignition
• Resist 15 cycles of a gas flame turned on and off
Common stand-alone Class A roof coverings include clay tiles, slate, asphalt glass fiber composition shingles, and concrete tiles. Assembly-rated Class A roof coverings are those that meet Class A standards when combined with other elements. For example, shake roofing with a fire-retardant treatment rates Class B on its own, but achieves a Class A rating when combined with specified underlying materials such as Type 72 roll roofing material. If you’re using an assembly-rated roofing material, it’s crucial that you read the manufacturer’s specifications carefully. These will detail exactly what materials must be combined for your roof to achieve a Class A rating.
Class B Roofing
Class B roofing is effective against moderate fire exposures. This is proven when the roofing can:
• Experience maximum flame spread of 8 feet
• Withstand a burning brand measuring 6″ by 6″ and weighing 500 grams
• Last 1 hour before ignition
• Resist eight cycles of a gas flame turned on and off
Pressure-treated shakes and shingles are the most common roofing materials to fall under the Class B rating.
Class C Roofing
Class C roofing provides only light fire protection. Roofing with a Class C rating is able to:
• Experience maximum flame spread of 13 feet
• Withstand a burning brand measuring 1.5″ x 1.5″ and weighing 1/4 gram
• Last 20 minutes before ignition
• Resist three cycles of a gas flame turned on and off
Examples of common Class C building materials include untreated wood shakes and shingles, plywood, and particleboard. This is not a recommended roof covering.
Common Roofing Materials
Commonly used roofing materials include wood shakes, wood shingles, fiber-cement, clay, concrete and slate shingles, and metal roofing
Wood shakes and shingles
The thin physical make-up and surface structure of wood shakes and shingles are readily combus-tible and conducive to fire spread. Asphalt shinglesAsphalt shingles are the most economical in terms of cost and life expectancy. Mineral reinforced asphalt shingles have a Class C rating and are gradually being replaced by fiberglass reinforced asphalt shingles, which are Class A or B materials.
These synthetic cement shingles are manufactured with either a fiberglass or wood mixture and are less brittle then solid cement shingles. They are a non-combustible material, but require an underlayment for a Class A assembly rating.Membrane roofsThese hard or semi-solid materials (i.e. hot tar and rubber) are applied to flat roofs and are slightly combustible. However, they are often used in conjunction with other materials, such as cement, and can be applied over a gypsum underlayment for a Class A assembly rating.
Tile, clay tile, concrete and slate shingles
These thick noncombustible materials can be manufactured to look like wood shingles. They have a Class A rating and provide the best protection against fire.
Metal: sheets and shingles
Metal roofing is sturdy, lightweight, and non-combustible. However, it requires a gypsum underlayment for a class A assembly rating. Metal roofing comes in the form of galvanized steel with paint; aluminum with paint; stainless steel; and, copper. It is also manufactured in the form of imitation wood shingles.