Class A fire-rated assemblies over steel framing.

Class A is the highest fire-resistance classification for roof and exterior assemblies, earned by the complete build-up of materials rather than any single product. A non-combustible light gauge steel frame is the ideal backbone for a Class A assembly because the structure itself adds no fuel, so the sheathing, gypsum, cladding, and roofing over it do the rating work on top of a frame that cannot ignite or carry fire.

What is a Class A fire-rated assembly?

Roof coverings are classified Class A, B, or C for fire resistance under ASTM E108 (equivalent to UL 790), with Class A representing the highest resistance to fire originating outside the building. The classification measures how the assembly performs against flame spread, burning brands, and intermittent flame, which are precisely the exposures a home faces in a wildfire. Because it is an assembly rating, Class A is a property of the whole build-up, not of the frame alone.

How is that different from an hourly fire-resistance rating?

Walls and floors also carry hourly fire-resistance ratings, such as one-hour or two-hour, established under ASTM E119. Those ratings describe how long an assembly resists fire and are typically achieved with gypsum board over the framing. Chapter 7A then adds ignition-resistance requirements for exterior walls, vents, and openings in wildfire zones. A fire-hardened home usually combines a Class A roof, ignition-resistant exterior walls, and appropriately rated interior separations.

How does light gauge steel support a Class A assembly?

Cold-formed light gauge steel is non-combustible: it does not ignite, burn, or add fuel. That means the assembly begins from a structural frame that is already out of the fuel chain, and the fire-rated layers over it are protecting a frame that will not itself carry flame. Steel is also dimensionally stable and does not shrink or warp, which helps the finished assembly stay tight and perform as designed over time.

What goes into a Class A wall and roof over steel?

A fire-hardened assembly over a light gauge steel frame is a layered system. The specifics are set by the designer and the tested assembly, but the components generally include:

• A Class A roof covering over the appropriate deck and underlayment.
• Non-combustible or ignition-resistant exterior cladding.
• Exterior sheathing suited to the wall's fire and structural requirements.
• Gypsum board where an hourly rating or interior protection is required.
• Ember- and flame-resistant vents, plus enclosed or protected eaves and soffits.

Are Class A assemblies required in California WUI zones?

In designated wildland-urban-interface areas governed by California Building Code Chapter 7A, roofs are generally required to be Class A, and exterior walls, vents, eaves, and openings must meet ignition-resistant standards. Building the structure in non-combustible steel aligns the frame with that intent from the inside out, so the assembly requirements are met over a frame that does not add fuel.

How does Cal Steel build to these assemblies?

Cal Steel engineers each home as a 3D digital twin in Vertex BD with AutoCAD and Revit BIM, then roll-forms and panelizes non-combustible light gauge steel in its Van Nuys facility, with additional manufacturing in Tecate. As a Los Angeles City Approved fabricator and erector certified to ICC-ES ESR-4905, Cal Steel delivers a non-combustible, dimensionally stable frame designed to carry the fire-rated wall and roof assemblies a California WUI project requires, and can extend the same approach into modular volumetric homes.

Frequently asked questions

For more answers across light gauge steel, structural steel, and modular, see the Cal Steel FAQ.