How DEXcell Roof Boards deliver Class A fire ratings for sloped roofs

While commercial roofs are designed to protect buildings from the weather, they also provide an essential fire protection system. There are two roof/ceiling assembly ratings provided by UL Solutions to consider when designing a roof assembly. Class ratings indicate resistance to fires that start on top of the roof or inside the building, while hourly ratings show how a roof assembly will perform in situations where a fire starts inside the building.

This article will explore how DEXcell^® Roof Boards, manufactured by Gold Bond Building Products, LLC and PermaBASE Building Products, LLC, affiliates of National Gypsum Company, provide a thermal barrier to help achieve both rating types.

What causes rooftop fires?

Architects often specify a Class A roof on commercial buildings to ensure the highest level of protection, even if local building codes don’t require it.

Rooftop systems and maintenance activities can combine to create fire threats. Common sources that contribute to rooftop fires include:

• Mechanical equipment malfunctions such as overheating motors and electrical shorts
• Electrical issues from faulty wiring in HVAC units, lighting or solar panels
• Maintenance or installation work, including welding, cutting or torching
• Old roofing materials that have become dry and damaged
• Lightning strikes that can ignite materials such as organic debris or insulation
• Organic debris such as leaves and tree branches
• Buildup of grease in poorly maintained restaurant kitchen exhaust systems

Another reason for specifying this level of fire safety — particularly in the Western U.S. — is the toll of wildfires on developed areas near wildland that’s prone to fires. These transition areas are known as the wildland urban interface. Consider these statistics on the WUI from the U.S. Fire Administration:

• Growth of WUI acreage: WUI acreage grows by 2 million acres per year.
• Structures lost in WUI areas: On average, more than 3,000 structures burned annually in WUI areas between 2002 and 2016.
• More communities affected: Over 60,000 communities face WUI risk.

Airborne firebrands, also known as embers, are one of the main causes of wildfire spread, particularly fires that start on rooftops. Wind fuels wildfires, which can generate additional air currents that carry burning embers aloft. When embers land on roofs, they can start fires that result in the wildfire “jumping” ahead of the fire front.

In addition, roofing material manufacturers specify the ideal slope for their product so the roof adequately sheds rainwater. However, sloped roofs burn faster. So, the question is, How can you achieve a Class A rating and maximize slope?

DEXcell Roof Boards can help roof/ceiling assemblies achieve a Class A fire rating for unlimited slope

When used as a component in a roof/ceiling system, DEXcell Roof Boards can help provide a Class A or Class 1 fire rating with the manufacturer-specified slope — and sometimes they make it possible to increase the slope.

Any of the following DEXcell products can be used to meet unlimited roof slope and Class A fire rating requirements while also meeting specific environmental needs:

• DEXcell^® Glass Mat Roof Board: Use with mechanically attached single-ply membranes and metal roofing. Available in ¼", ½" and ⅝" thicknesses and a 4-by-8-foot size. Use the ¼" thickness to maximize your budget.
• DEXcell FA® Glass Mat Roof Board: Use in fully adhered single-ply membranes, cold-applied modified bitumen, fluid applied, spray foam and metal. Available in ¼", ½" and ⅝" thicknesses and two sizes: 4-by-4-feet and 4-by-8-feet.
• DEXcell FA VSH® Glass Mat Roof Board: Use in areas that experience severe hail. Available in ⅝" thickness and two sizes: 4-by-4-feet and 4-by-8-feet.
• DEXcell^® Cement Roof Board: Use in high-moisture areas. Available in 7/16" thickness and two sizes: 4-by-4-feet and 4-by-8-feet.

How are roof boards tested for fire resistance?

Roof boards are tested for fire resistance as individual materials and as a component in a roof/ceiling system.

The individual material test, ASTM E84 Standard Test Method for Surface Burning Characteristics of Building Materials, uses a Steiner tunnel furnace to evaluate surface-burning characteristics such as flame spread and smoke development. Created by engineer Albert J. Steiner, the Steiner furnace is a 1.5-by-25-foot horizontal tunnel that provides a controlled environment to test a wide variety of building materials.

During ASTM E84 roof testing, the roof board is positioned horizontally in the furnace and exposed to airflow and flames. Results include measurement of flame spread distance, ignition time and smoke development. Materials receive a Class 1, Class 2 or Class 3 classification based on how quickly flames spread on their surface.

• Class 1 must have flame spread between 0 and 25.
• Class 2 includes materials with flame spread between 26 and 75.
• Class 3 materials have flame spread between 76 and 200.

DEXcell Roof Boards have a flame spread and smoke development of zero based on ASTM E84 testing.

Roofing material manufacturers also test roof/ceiling systems as a whole with the ASTM E108/UL 790 Standard Test Methods for Fire Tests of Roof Coverings to determine a Class A, B or C rating. They select some components of the assembly based on their ASTM E84 test results.

The UL 790 evaluation includes three tests: spread of flame, intermittent flame and burning brand; however, if the roof/ceiling system includes a noncombustible deck, such as gypsum roof boards, only the spread of flame test applies. To represent a burning brand during the test, wood slats are used to construct a “crib” by placing the slats in a lattice structure. The burning crib is larger and heavier (12" × 12" × 2.25") for the Class A test, to test a significantly bigger fire threat. Smaller cribs (6" × 6" × 2.25" and 2.25" × 1.25" × 25/32" respectively) are used for the Class B and Class C tests.

Here’s how the spread of flame test works:

• The roof assembly is subjected to a 1400 F flame carried by a 12 mph wind for 10 minutes.
• It earns a Class A rating if the flame spread is 6 feet or less.
• It earns a Class B rating if the flame spread is 8 feet or less.
• For Class C ratings, the flame is decreased to 1300 F and the exposure time to 4 minutes. The roof assembly earns a Class C rating if the flame spread is no more than 13 feet.

DEXcell roof boards provide thermal barrier protection against fires inside the building for UL 1-hour and 2-hour P assemblies

UL assigns a letter prefix to each type of building assembly. Roof/ceiling assemblies carry a “P” prefix, while walls and partitions carry a “U” prefix. DEXcell Roof Boards provide thermal protection as a component of 1- and 2-hour UL fire-rated roof/ceiling P assemblies.

Roof/ceiling P assemblies are evaluated using the ASTM E119 Standard Test Methods for Fire Tests of Building Construction and Materials. The E119 test exposes the roof/ceiling assembly to fire on the ceiling side and measures:

• Heat transmission
• Hot gas transmission through the assembly
• Effectiveness of load-bearing elements during test conditions

View a ​​full list of roof/ceiling P assemblies that include DEXcell Roof Board.

Beyond fire resistance

DEXcell Roof Boards provide additional benefits, including a thermal heat sink, protection against maintenance crew foot traffic and improved acoustics. Contact 1-800-NATIONAL^® Construction Services for technical assistance and documentation on optimizing roof assemblies with DEXcell Roof Boards.