Technical Specifications

What Building Materials Are Fireproof for Mid-Rise Apartment Construction?

Fireproof building materials for mid-rise construction must satisfy two separate code requirements: non-combustibility (ASTM E136) and fire resistance ratings (ASTM E119). Cold-formed steel, concrete, and masonry qualify as non-combustible; wood — including fire-retardant treated wood — does not. CFS achieves 1-, 2-, and 3-hour fire resistance through UL-listed assemblies.
  • 8-minute read
Who this article is for: Multifamily developers, architects, and GCs evaluating structural systems for 5–8 story residential construction.

Comparison of fireproof and non-combustible building materials for mid-rise apartments, including CFS, concrete, masonry, and steel — with ASTM E136 and UL assembly references.

What “Fireproof” Means and Which Materials Are Non-Combustible

  • Non-combustibility (ASTM E136) means a material will not ignite, sustain combustion, or contribute fuel to a fire when exposed to heat. Cold-formed steel, concrete, and masonry pass. Wood fails — including fire-retardant treated (FRT) wood.
  • Fire resistance rating means a complete assembly (wall, floor, or ceiling) has been tested per ASTM E119 and can contain a fire for a specified duration under standardized conditions. The rating belongs to the assembly, not the individual material.

For mid-rise apartment construction, IBC Table 601 specifies fire resistance ratings by construction type, and IBC Section 602 specifies which construction types require non-combustible materials.

Materials that qualify as non-combustible for Type I and II construction:

  • Cold-formed steel (CFS) — manufactured from steel, which does not burn, char, or contribute fuel to a fire. Passes ASTM E136 without any treatment or modification.
  • Concrete and masonry — non-combustible. Concrete podiums for Type IIIA buildings are the most common non-CFS non-combustible component in New England mid-rise construction.
  • Hot-rolled structural steel — non-combustible; typically requires spray-applied fire-resistive material (SFRM) to achieve rated structural frame protection in Types IIA, IB, and IA.

What does NOT qualify: wood framing is combustible regardless of species, grade, or moisture content, and FRT wood remains combustible under ASTM E136 — the treatment slows ignition and reduces flame spread, but does not change the classification. This distinction is what forces wood-framed buildings above 4 stories into a concrete podium: the IBC requires non-combustible construction above certain heights, and wood cannot meet that requirement regardless of treatment.

Fire Resistance Ratings Required for Mid-Rise Apartments

IBC Table 601 sets the required fire resistance ratings for structural elements based on construction type. For mid-rise residential (Group R-2 occupancy):

Construction Type Structural Frame Bearing Walls (Interior) Floor Construction
Type IA 3 hr 3 hr 2 hr
Type IB 2 hr 2 hr 2 hr
Type IIB 0 hr 0 hr 0 hr
Type IIIA 1 hr 1 hr 1 hr
Type VA (wood) 1 hr 1 hr 1 hr

Type IIB is the most common CFS construction type for 5–6 story multifamily. It requires non-combustible materials throughout but sets the fire resistance rating for structural elements at 0 hours — meaning the non-combustibility of CFS itself is the primary protection.

Even in Type IIB buildings, certain assemblies still require fire resistance ratings independent of construction type, driven by occupancy requirements:

  • Dwelling unit separation walls — 1-hour minimum per IBC Section 420.2.
  • Corridor walls — 1 hour in sprinklered buildings per IBC Table 1020.1.
  • Occupancy separation walls — 1–2 hours per IBC Table 508.4.
  • Shaft enclosures — 1–2 hours per IBC Section 713.

How CFS Achieves Fire Ratings: UL-Listed Assemblies

CFS framing achieves fire resistance ratings through tested and listed assemblies published in the UL Certifications Directory. The assembly — not the CFS stud alone — carries the rating. The stud is a component within a tested system that includes specific gypsum board layers, screw patterns, resilient channel requirements, and framing depths.

UL Design Number Rating Assembly Type
UL H505 1-hour Interior non-bearing CFS wall — single layer 5/8″ Type X gypsum each side
UL H514 2-hour Load-bearing CFS wall — double-layer 5/8″ Type X gypsum
UL L541 2-hour CFS floor-ceiling assembly — joists + subfloor + resilient channel + double gypsum ceiling
UL M527 2-hour CFS corridor wall assembly — intersection detail at floor-ceiling junction
UL D902 / D925 1–2 hour CFS exterior wall assemblies with weather barrier integration

Installation parameters specified in each UL listing — gypsum layer count, screw type and spacing, resilient channel requirements, framing depth — must be followed exactly. Any deviation from the listed assembly voids the rated system.

CFS vs. Concrete vs. Wood for Fire Performance

Criteria Cold-Formed Steel (CFS) Concrete Wood / FRT Wood
Non-combustible per ASTM E136 Yes Yes No (FRT wood still fails)
Qualifies for Type IIB without podium Yes Yes (as podium) No — triggers podium requirement
Podium required above 4 stories No podium Concrete IS the podium Yes — $12–15/SF + 8–12 weeks
Total construction cost (5-story) Lower total (no podium) Higher ($12–15/SF podium cost) Higher total (podium + insurance)

Concrete offers equivalent fire performance to CFS but at significantly higher construction cost — the concrete podium alone adds $12–15/SF and 8–12 weeks to a typical 5-story project. CFS Type IIB construction eliminates the podium entirely while meeting the same non-combustibility requirements and achieving the same UL-listed fire resistance ratings at all required assembly locations.

How Non-Combustible Framing Affects Insurance Premiums

Insurance underwriters use ISO (Insurance Services Office) construction class ratings, which closely track IBC construction types. Non-combustible construction (Types I and II) qualifies for lower premiums than combustible construction (Types III through V) because the structural system itself does not contribute fuel to a fire.

For multifamily developers, the impact is quantifiable: CFS Type IIB buildings qualify for 25–38% lower annual insurance premiums compared to wood-framed buildings of equivalent size and occupancy. On a 150-unit building, that differential compounds to $1 million or more in premium savings over 30 years — a return that rivals the direct construction cost savings from podium elimination.

The insurance savings begin at certificate of occupancy and continue for the life of the building. They are not a one-time calculation — they compound annually. A developer who builds a 5-story CFS building today and holds the asset for 30 years captures more value from the insurance differential alone than from the podium elimination savings.

Frequently Asked Questions

What building materials are fireproof for mid-rise apartment construction?

The building code requires non-combustibility (ASTM E136) and fire resistance ratings (ASTM E119 assemblies). Non-combustible materials that qualify for Type I and II construction: cold-formed steel, concrete, and masonry. Wood — including FRT wood — does not qualify as non-combustible. For 5–8 story multifamily, CFS with UL-listed assemblies is the most cost-effective non-combustible, fire-resistance-rated structural system available.

What is the difference between non-combustible and fire-resistant materials?

Non-combustibility (ASTM E136) is a material property — does the material burn? Fire resistance rating (ASTM E119) is an assembly property — how long does the complete wall, floor, or ceiling contain a fire? A non-combustible material like CFS can be built into assemblies with 1-, 2-, or 3-hour fire resistance ratings. These are separate certifications measured by different tests.

Does fire-retardant treated (FRT) wood qualify as non-combustible?

No. FRT wood is still classified as combustible under ASTM E136. FRT treatment reduces flame spread and slows ignition, but the wood remains organic and combustible. FRT wood does not qualify for Type I or Type II construction and does not eliminate the podium requirement above 4 stories.

What UL fire-rated assemblies are available for cold-formed steel construction?

Primary UL-listed CFS assemblies: H505 (1-hour non-bearing wall), H514 (2-hour load-bearing wall), L541 (2-hour floor-ceiling), M527 (2-hour corridor wall), D902/D925 (CFS exterior wall). All tested per ASTM E119. Installation parameters — gypsum layers, screw spacing, framing depth — must be followed exactly to maintain the rating.

References

  • International Code Council — IBC 2021: International Building Code, Table 601, Section 602.2, Sections 420, 508, 713, 1020.
  • ASTM International — ASTM E136-22: Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750°C.
  • ASTM International — ASTM E119-22: Standard Test Methods for Fire Tests of Building Construction and Materials.
  • Underwriters Laboratories — UL Fire Resistance Directory: Designs H505, H514, L541, M527, D902, D925.
  • Board of Building Regulations and Standards — 780 CMR: Massachusetts State Building Code, 9th Edition, Chapter 6.
  • American Iron and Steel Institute — AISI S100-16: North American Specification for the Design of Cold-Formed Steel Structural Members.

Ready to Specify Fire-Rated CFS Assemblies?

To discuss fire-rated assembly specification for your mid-rise multifamily project, contact our engineering team at nick@aacsteel.com or 617-560-0349.

About AAC Steel

AAC Steel is a cold-formed steel (CFS) framing company headquartered in Franklin, MA, founded by engineers and builders with two decades of experience on Northeast jobsites. The company designs, fabricates, and installs precision CFS structural systems — including PE-stamped engineering, BIM and shop drawings, and panelized fabrication — for 5+ story multifamily developments across Massachusetts, Rhode Island, and Connecticut.

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