Technical Specifications

Fire-Rated CFS Wall Assemblies: UL Design Guide for Multifamily Mid-Rise

Cold-formed steel offers a comprehensive library of UL-listed fire-rated assemblies from 1-hour to 3-hour ratings, allowing mid-rise multifamily designs to meet IBC fire resistance requirements without the cost and structural weight of concrete or masonry fire barriers — CFS area separation walls weigh 8–12 psf compared to 55–75 psf for 8" CMU.
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Who this article is for: Architects, specifiers, and structural engineers selecting UL-listed fire-rated CFS assemblies for mid-rise multifamily projects under IBC 2021 and Massachusetts 780 CMR.

Complete UL design guide for fire-rated CFS wall, floor-ceiling, shaft, and area separation assemblies in mid-rise multifamily construction, covering IBC requirements and acoustic performance.

IBC Fire Resistance Requirements for Mid-Rise Construction

IBC Table 601 establishes minimum fire-resistance ratings for structural frames, bearing walls, floor construction, and roof construction. Three construction types dominate CFS mid-rise design: Type IIA requires 1-hour fire-resistance-rated structural frame, bearing walls, floor construction, and roof construction, with all materials non-combustible per IBC §602.2 — the most common type for 4-to-5-story CFS multifamily. Type IIB permits 0-hour structural frame, bearing walls, and floor construction when fully sprinklered per NFPA 13, though corridor and demising walls remain required per IBC §708 and §1020. Type IIIA requires 1-hour non-combustible exterior bearing walls but permits combustible wood interior framing — CFS competes against wood here, and the non-combustible advantage is decisive: the structural frame contributes zero fire fuel load, unlike wood.

IBC Table 602 sets exterior wall ratings by fire separation distance (FSD): less than 5 ft requires 1-hour rating with 0% openings; 5–10 ft requires 1-hour with 25% max openings; 10–30 ft drops to 0-hour for non-combustible Types I/II construction; beyond 30 ft, 0-hour with no opening limitation for all types. CFS exterior wall assemblies (UL U400 series) readily achieve 1-hour and 2-hour ratings with standard gypsum and continuous insulation — without masonry backup at any FSD.

IBC §706 requires fire walls for Group R occupancies to carry a minimum 2-hour rating (3-hour for Type I). IBC §711 requires horizontal assemblies to be continuous except at listed shaft and exit enclosure openings. IBC §714 requires through-penetration firestop systems tested per ASTM E814 — generic caulk is not a listed firestop system; the product and configuration must be tested and listed for the specific penetrant type and assembly configuration.

Understanding UL Fire-Rated Assembly Designations

Fire resistance ratings are determined per ASTM E119 or equivalent UL 263, exposing the assembly to a standard time-temperature curve (1,000°F at 5 min, 1,700°F at 1 hour, 2,000°F at 4 hours). The assembly must simultaneously satisfy structural integrity (load-bearing assemblies maintain capacity under applied load), flame passage prevention, and temperature limitation (unexposed side rise ≤250°F average, ≤325°F single point). An assembly that meets criteria for 55 minutes receives a 0-hour rating — not 55 minutes; the designer cannot interpolate between rated configurations.

UL Design Numbers specify every component of the tested assembly — stud size, gauge, spacing, gypsum type and layers, fastener pattern, insulation. Any deviation voids the listing. Relevant series: U-series (walls, U400–U499, key listings U411/U419/U423/U425/U466), L-series (floor-ceilings, L500–L599, key listings L514/L524/L541/L547), G-series (roof-ceilings), and D-series (shaft walls, D902/D925).

A critical distinction: bearing-rated assemblies are tested under load per ASTM E119 §8, while non-bearing assemblies are tested without load. A non-bearing 1-hour assembly cannot be used for a load-bearing wall without separate structural justification. IBC §703.3 permits engineering judgment to extend ratings beyond tested assemblies under specific conditions, but most AHJs strongly prefer directly listed UL assemblies for unambiguous compliance.

Key CFS Wall Assemblies for Multifamily

UL Design Rating Configuration STC Application
UL U425 1-hour bearing 20-ga, 16″ o.c.; 1 layer 5/8″ Type X each face 45–50 Exterior load-bearing walls
UL U425 2-hour bearing 20-ga, 16″ o.c.; 2 layers 5/8″ Type X each face, staggered 50–55 2-hr exterior bearing, occupancy separations
UL U419 1-hour bearing 20-ga, 24″ o.c.; 1 layer 5/8″ Type X each side, glass fiber cavity 45–48 Dwelling unit demising walls (IBC §708)
UL U411 2-hour bearing 3-5/8″ studs, 16″ o.c.; 2 layers 5/8″ Type X each side 55–58 2-hr corridor walls, exit access corridors
UL U466 2-hour Double-stud, 24″ o.c., 1″ air gap; 2 layers 5/8″ Type X each face 60+ Area separation walls (IBC §706)

Continuous insulation can be added outboard of the exterior gypsum sheathing without affecting the fire rating, since the tested assembly addresses the interior gypsum configuration and the outboard CI is exterior to the rated membrane — enabling Stretch Energy Code compliance without assembly re-testing.

Shaft walls (elevator, stairwell, MEP per IBC §713) use a C-runner (C-H stud) system permitting construction from one side. UL D902 (1-hour: 2-1/2″ C-H studs at 24″ o.c., 1″ gypsum liner panels, one layer 5/8″ Type X on the exposed face) covers shafts ≤3 stories; UL D925 (2-hour, two layers 5/8″ Type X) covers shafts ≥4 stories per IBC §713.4. Penetrations at shaft walls are among the most common code violations found during inspections — proper UL firestop system product numbers in the construction documents are essential.

CFS Floor-Ceiling Fire-Rated Assemblies

Floor-ceiling assemblies must simultaneously meet the fire-resistance rating of IBC Table 601 and the acoustic separation requirements of IBC §1207 (STC 50 / IIC 50 minimum between dwelling units) — a single assembly must satisfy both requirements at once.

UL Design Rating Configuration STC / IIC Application
UL L524 1-hour 18-ga (43 mil) joists, 16″ o.c.; 3/4″ subfloor + 1-1/2″ LW concrete + 1 layer 5/8″ Type X ceiling on resilient channels 52–55 / 52–55 R-2 dwelling unit separation (§708); Type IIA floor construction
UL L547 2-hour 16-ga (54 mil) joists, 16″ o.c.; 2 layers 5/8″ Type X staggered; 1-1/2″ LW concrete min. 55–60 / 55–58 Occupancy separations: retail-to-residential (Table 508.4)
UL G602 3-hour Per listing; 3 layers 5/8″ Type X; STRUCTO-CRETE topping Per listing Parking garage to residential (§406.6.1)

The critical acoustic variables: concrete or gypsum-concrete topping weight (mass law — 1-1/2″ lightweight concrete is the standard minimum for STC 50 in CFS joist assemblies); resilient channel isolation at 24″ o.c. (double-fastening through both flanges short-circuits decoupling and degrades IIC); cavity insulation density (mineral wool outperforms glass fiber at equivalent thickness); and ceiling gypsum board mass. Assemblies without concrete topping typically require additional acoustic mat layers to meet IIC 50 — verify both fire and acoustic data from the same UL listing before specifying.

CFS floor-ceiling assemblies offer a decisive post-membrane-failure advantage over wood joist assemblies: if the ceiling gypsum membrane fails in a fire exposure, CFS joists lose strength gradually as steel temperatures rise but do not ignite and do not contribute fuel to the fire — wood joists ignite and accelerate structural collapse once the membrane fails.

Firestopping at CFS Penetrations

The fire-resistance rating of an assembly is only as effective as its weakest point — consistently at penetrations. IBC §714 requires through-penetration firestop systems tested for the specific combination of assembly type, penetrant type and size, and annular space dimensions. ASTM E814 (UL 1479) measures both F-rating (time until flame passage) and T-rating (time until unexposed-side temperature exceeds 325°F above ambient); IBC §714.4.1.2 requires the T-rating to equal or exceed the required assembly fire-resistance rating for penetrations through fire-rated floor assemblies — a requirement frequently overlooked when firestop products emphasize F-rating without prominently displaying T-rating.

Common penetrant approaches: steel pipe (intumescent wrap products or collars, listed for the specific diameter and assembly configuration); PVC/CPVC pipe (intumescent collars that collapse the pipe as it melts, requiring T-rated systems per §714.4.1.2); electrical conduit and cable (intumescent caulk or mortar fill, or caulk with wrap for cable bundles); and duct penetrations (fire and smoke dampers per IBC §717, listed per UL 555 and UL 555S — ductwork without dampers at rated locations is a code violation).

The head-of-wall joint — where a fire-rated wall meets the floor-ceiling assembly above — requires a linear gap firestop system tested per ASTM E1966 (UL 2079), accommodating structural deflection and thermal movement while maintaining the wall’s fire-resistance rating. This condition is frequently detailed incorrectly, with the wall assembly specified accurately but the head-of-wall joint left to field interpretation.

Massachusetts 780 CMR note: the 70-foot high-rise threshold (five feet lower than IBC’s 75-foot) triggers $600,000–$1,300,000 in fire command center, emergency voice/alarm, emergency power, and smoke control requirements — making floor-to-floor height management critical in schematic design. Massachusetts building departments require the specific UL design number called out on drawings; “1-hour fire-rated wall” without the UL number is the most common plan review comment causing resubmission delay.

Frequently Asked Questions

What UL design numbers should I specify for a 5-story Type IIB CFS multifamily building with ground-floor parking?

UL G602 at the garage-to-residential floor-ceiling (3-hour per IBC §406.6.1); UL U424 or equivalent at corridor walls (1-hour per §1020 and §708); UL U425 at exterior bearing walls where FSD requires 1-hour per Table 602; UL D925 at elevator and stair shaft enclosures connecting all 5 stories (2-hour per §713.4); and UL L524 at floor-ceiling assemblies between stacked R-2 units (1-hour per §708 with STC/IIC 50 per §1207). Type IIB’s 0-hour structural frame does not mean zero fire-rated assemblies — occupancy separation, corridor, shaft, and FSD requirements still mandate rated assemblies at specific locations.

Can I add continuous insulation outboard of a UL-listed CFS exterior wall assembly without voiding the listing?

Yes, for most UL U400-series assemblies — the fire test is conducted with fire exposure from the interior face, and outboard CI does not affect the interior membrane’s ability to achieve the rated duration. However, the exterior wall assembly as a whole must still satisfy NFPA 285 for buildings over 40 feet using foam plastic CI per IBC §2603.5.5, which is a separate test from ASTM E119.

What is the difference between the F-rating and T-rating in firestop system specifications?

The F-rating is the time during which the firestop system prevents flame passage. The T-rating is the time during which it prevents temperature rise on the unexposed side from exceeding 325°F above ambient — limiting the risk of igniting nearby combustibles. IBC §714.4.1.2 requires T-rating to equal or exceed the required floor assembly fire-resistance rating for floor penetrations, meaning a firestop in a 2-hour floor assembly must achieve both F-2hr and T-2hr.

What are the STC and IIC requirements for CFS floor-ceiling assemblies under IBC Section 1207?

IBC §1207.2 requires STC 50 minimum for airborne sound between dwelling units; §1207.3 requires IIC 50 minimum for impact sound. Both apply to the same floor-ceiling assembly, which must simultaneously satisfy fire rating, STC ≥50, and IIC ≥50. Achieving IIC 50 reliably requires either 1-1/2″ lightweight concrete topping or a tested acoustic mat layer — assemblies without topping or mat typically achieve only IIC 44–48, failing §1207.3 regardless of fire rating.

References

  • International Code Council — IBC 2021: Sections 602.2, 706.2, 706.4, 708, 711, 713.4, 714.4.1.2, 1020, 1207.2, 1207.3, 2603.5.5; Tables 504.3, 504.4, 506, 601, 602.
  • Board of Building Regulations and Standards — 780 CMR: Massachusetts State Building Code, 10th Edition, Sections 403.1, 1705.11.
  • ASTM International — ASTM E119-22, ASTM E136, ASTM E814 (UL 1479), ASTM E1966 (UL 2079), ASTM C1396, ASTM A1003.
  • Underwriters Laboratories — UL Fire Resistance Directory: Designs U411, U419, U423, U425, U466, D902, D925, L514, L524, L541, L547, G602.
  • American Iron and Steel Institute — AISI S100-16 Appendix B (Fire); AISI S240-20 Chapter E.
  • National Fire Protection Association — NFPA 13, NFPA 285.

Planning a CFS Mid-Rise with Code-Compliant Fire-Rated Assemblies?

AAC Steel’s engineering team provides design-assist services including construction type analysis, UL-listed fire-rated assembly selection, acoustic compliance verification, and Massachusetts 780 CMR permit submission documentation. Contact us at nick@aacsteel.com or 617-560-0349 to discuss the fire-rated assembly schedule for your project.

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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