Technical Specifications

Fire-Rated CFS Assembly Matrix: Type IIB R-2 Residential Buildings ≤ 70 Feet

Type IIB R-2 buildings at or below Massachusetts's 70-foot high-rise threshold require different fire-rated assemblies depending on the occupancy below the residential floors: a parking garage (S-2) triggers a 3-hour horizontal separation (UL G602), while retail (M) triggers only a 2-hour separation (UL G557 or UL L541) — the structural CFS frame itself remains 0-hour throughout under Type IIB.
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Who this article is for: Architects, structural engineers, fire protection engineers, AHJ reviewers, and CM/GC project managers specifying fire-rated CFS assemblies for Type IIB R-2 mid-rise multifamily under 2021 IBC and Massachusetts 780 CMR.

Complete fire-rated CFS assembly reference for Type IIB R-2 multifamily buildings under 70 feet, covering garage and retail mixed-use scenarios, UL design details, exterior wall NFPA 285 requirements.

Side-by-Side Scenario Comparison

Both scenarios share the same construction type (IIB), sprinkler system (NFPA 13), and CFS superstructure above the separation floor. The critical difference is the occupancy below residential — parking garage (S-2) vs. retail (M) — which drives the required fire rating at the horizontal separation floor.

Building Element Code Ref Scenario A — Garage (S-2) Scenario B — Retail (M)
Horizontal Separation §406.6.1 / Table 508.4 3-HR — UL G602 2-HR — UL G557 or UL L541
Vertical Separation Table 508.4 2-HR — UL U425 2-HR — UL U425
Residential Floor/Ceiling §420.2 1-HR — UL H504 1-HR — UL H504
Corridor Walls Table 1020.1 1-HR — UL U424 1-HR — UL U424
Elevator/Stair Shafts §713.4 2-HR — UL U425 2-HR — UL U425
Structural Frame Table 601 0-HR — No SFRM 0-HR — No SFRM

Scenario A (garage below R-2) triggers IBC §406.6.1 requiring a 3-hour horizontal assembly at the garage ceiling — the single most impactful fire separation in either scenario. Scenario B (retail below R-2) requires only a 2-hour horizontal separation per IBC Table 508.4, saving one layer of gypsum in the ceiling assembly and reducing cost and schedule at the critical separation floor.

Scenario A: Garage Below (UL G602, 3-Hour)

Application: parking garage (S-2) ceiling below R-2 residential, per IBC §406.6.1. Substrate: STRUCTO-CRETE. Framing: CFS C-joists 10″–12″ at 24″ o.c. (14–16 ga).

Construction sequence, top to bottom: finish floor (carpet/LVP/tile over acoustic underlayment); 3/4″ USG STRUCTO-CRETE subfloor, T&G panels, #8 screws at 8″ perimeter / 12″ field (ICC-ES ESR-1792); CFS joists 10″–12″ deep, 54 mil (16 ga) minimum, 24″ o.c., ASTM A1003 Grade 50; 3-1/2″ fiberglass cavity insulation; resilient channels 7/8″ deep, 25 ga, 16″ o.c. perpendicular to joists; ceiling with three layers of 5/8″ Type X gypsum — layer 1 at 1″ screws 12″ o.c. to RC, layer 2 offset 24″ minimum from layer 1, layer 3 at 2″ screws 8″ perimeter / 12″ field, all joints taped and finished.

Performance data: STC 54–60; IIC 33 bare / 50+ with resilient mat; assembly weight approximately 45–50 psf.

At the level above the garage (transfer floor), the vertical occupancy separation is UL U425 (2-hour). Retail (Group M) does not trigger IBC §406.6.1 — the 3-hour “parking below occupancy” requirement is unique to enclosed parking garages.

Scenario B: Retail Below (UL G557/L541, 2-Hour)

Application: retail (M) to R-2 occupancy separation per IBC Table 508.4. Substrate: STRUCTO-CRETE. Framing: CFS C-joists 8″–12″ at 24″ o.c.

Construction sequence, top to bottom: finish floor over acoustic underlayment; 3/4″ USG STRUCTO-CRETE subfloor; CFS joists 8″–12″ deep, 54 mil (16 ga) minimum, 24″ o.c.; 3-1/2″ fiberglass cavity insulation; resilient channels 7/8″ deep, 25 ga, 16″ o.c.; ceiling with two layers of 5/8″ Type X gypsum — layer 1 at 1″ screws 12″ o.c., layer 2 offset 24″ at 1-5/8″ screws 8″ perimeter / 12″ field.

Savings vs. G602: one fewer gypsum layer means reduced material and labor. Assembly weight approximately 35–40 psf.

UL H504 — 1-hour floor/ceiling (dwelling unit separation, IBC §420.2, applies to all R-2 floor levels regardless of scenario): substrate Plycem SX 20/20 fiber cement; 10″ CFS joists, 43 mil (16 ga), 1-5/8″ flanges, 16″ o.c.; 3-1/2″ fiberglass cavity insulation; 25 ga hat channels at 24″ o.c.; single layer 5/8″ Type X gypsum ceiling. Performance: STC 54–56, IIC 50–55 with acoustic mat, assembly weight approximately 8 psf.

Wall Assemblies and Exterior Wall / NFPA 285 Requirements

UL U424 — 1-hour load-bearing wall (corridors, unit separation, interior bearing per IBC Table 1020.1 and §420.2): 5/8″ Type X gypsum each side (8″ perimeter / 12″ field screws); 3-5/8″ (362S162) or 6″ (600S162) CFS studs, 20 ga minimum, 16″ o.c.; 3-1/2″ mineral wool cavity recommended for STC ≥50. Single-stud with mineral wool achieves STC 45–50; staggered- or double-stud configurations reach STC 55+ for party walls needing higher performance.

UL U425 — 2-hour wall (shafts per §713.4, stairwells per Table 1023.2, occupancy separations per Table 508.4): two layers of 5/8″ Type X gypsum each side (base layer 24″ o.c., face layer offset 16″ minimum), 3-5/8″ to 6″ CFS studs at 16–24″ o.c., mineral wool cavity — total 4 gypsum layers.

Exterior walls (ECOMAXci FR, NFPA 285): required for all exterior walls with foam plastic continuous insulation on buildings over 40 ft (IBC §2603.5.5). Assembly, interior to exterior: 5/8″ Type X gypsum interior; 3-5/8″ minimum CFS studs, 20 ga, 24″ o.c. maximum; DensGlass or gypsum exterior sheathing (or Plycem fiber cement); WRB/air barrier (Henry Blueskin SA or ECOMAXci FR integral facer); continuous insulation 2″–4.5″ (R-13.1 to R-31.0).

Fire Separation Distance Ext. Wall Rating Opening Limits CFS Assembly
0 to < 5 feet 1-HR (both sides) Not permitted UL U425 or U424
5 to < 10 feet 1-HR (exterior side) 15% max openings UL U424
10 to < 30 feet 0-HR 25% max openings Standard CFS wall + ci
≥ 30 feet 0-HR Unlimited Standard CFS wall + ci

NFPA 285 approval applies only to the exact tested configuration — substitution of any component (insulation type, thickness, cladding, sheathing) voids the certification.

Complete UL Assembly Matrix and Decision Table

UL Design Rating Substrate / Framing Application
H504 1-HR Plycem, 10″ CFS joists 16″ o.c. R-2 dwelling unit separation — preferred
L501 1-HR STRUCTO-CRETE, 6″ CFS joists 24″ o.c. Within same occupancy — cost-optimized
H505 2-HR STRUCTO-CRETE, 8″–10″ CFS joists Occupancy separations M/R-2
G557 2-HR STRUCTO-CRETE, 10″ min CFS joists M-to-R-2 separation — Scenario B
L541 2-HR STRUCTO-CRETE, 10″–12″ CFS joists Alternate 2-hour — fire barriers
G602 3-HR STRUCTO-CRETE, 10″–12″ CFS joists S-2 below R-2 — Scenario A (critical)

Field decision matrix:

Condition Rating Assembly Code Reference
Enclosed parking garage below residential? 3-HR Horizontal UL G602 + UL U425 wall IBC §406.6.1
Retail (M) or business (B) below residential? 2-HR Horizontal + Vertical UL G557/L541 + UL U425 wall IBC Table 508.4
Residential floor between dwelling units? 1-HR Horizontal UL H504 + UL U424 wall IBC §420.2
Elevator shaft or stairwell, 4+ stories? 2-HR Vertical UL U425 IBC §713.4 / Table 1023.2
Structural CFS frame member (Type IIB)? 0-HR No SFRM required IBC Table 601

Frequently Asked Questions

What is the most critical fire-rated assembly in a Type IIB R-2 building with a basement garage?

The 3-hour horizontal floor/ceiling assembly separating the parking garage (Group S-2) from the residential occupancy above (Group R-2) — typically UL G602 with STRUCTO-CRETE substrate and three layers of 5/8″ Type X gypsum on the garage ceiling side. IBC §406.6.1 explicitly requires the 3-hour rating whenever an enclosed parking garage is located beneath any other occupancy.

What is the difference between garage-below and retail-below scenarios?

Parking garage (S-2) below R-2 requires a 3-hour rated floor/ceiling per IBC §406.6.1 (UL G602, three gypsum ceiling layers). Retail (M) below R-2 requires only a 2-hour rated floor/ceiling per IBC Table 508.4 (UL G557 or UL L541, two gypsum ceiling layers). Both scenarios share the same 2-hour vertical occupancy separation (UL U425) and the same 1-hour residential floor/ceiling assemblies (UL H504) on the floors above.

Why is Type IIB construction the standard for 5–6 story CFS multifamily?

Type IIB allows non-combustible CFS framing with zero hours of structural fire-resistance rating required — no SFRM needed on any CFS column, beam, or load-bearing wall stud. The structural members satisfy the IBC requirement through non-combustibility under ASTM E136, not through encasement, eliminating the cost, schedule, and inspection complexity that Type IIA or Type IB would impose while still allowing 6 stories / 75 ft / 36,000 SF per floor for sprinklered R-2 occupancy.

When is NFPA 285 testing required for exterior walls?

NFPA 285 is required when foam plastic continuous insulation is used in an exterior wall assembly on a building taller than 40 ft (IBC §2603.5.5). Almost all 5–6 story CFS multifamily buildings in Massachusetts trigger this requirement because the Stretch Energy Code requires continuous insulation on exterior walls. Any component substitution — cladding, insulation thickness, sheathing type — voids the test approval.

References

  • International Code Council — IBC 2021: International Building Code, Sections 406.6.1, 420.2, 506.2, 506.3, 508.4, 602.2, 713.4, 716.5, 1020.1, 1023.2, 2603.5.5; Tables 504.3, 504.4, 506.2, 508.4, 601, 602, 1020.1.
  • Board of Building Regulations and Standards — 780 CMR: Massachusetts State Building Code, 10th Edition, Sections 403.1 and 2603.5.5.
  • Underwriters Laboratories — UL Fire Resistance Directory: Designs U424, U425, U419, U348, U356, U465, U497, W456, W459, V471; H504, H505, H511, L501, L541, G556, G557, G602.
  • ASTM International — ASTM E119, ASTM E136.
  • National Fire Protection Association — NFPA 285, NFPA 13.
  • American Iron and Steel Institute — AISI S100-16, AISI S240-20.
  • ICC-ES — ESR-1792 (USG STRUCTO-CRETE Structural Cement Panels).

Need Help Specifying the Right Assembly?

To discuss fire-rated CFS assembly selection, UL listing coordination, or floor-by-floor assembly scheduling for your Type IIB R-2 multifamily project in Massachusetts, contact our engineering team at nick@aacsteel.com or 617-560-0349. AAC Steel will produce a project-specific assembly recommendation based on your occupancy mix, height, and architectural intent.

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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