Fire-rated assemblies in multifamily and commercial construction define the structural and life-safety backbone of the building. Interior finishes installation that damages, penetrates, or compromises a fire-rated assembly creates a code violation that must be corrected before the certificate of occupancy is issued. In buildings where the fire-rated assembly is also the acoustic assembly, a finishes installation error can simultaneously create a fire code deficiency and an acoustic performance gap.
Understanding how fire-rated assemblies interact with interior finishes installation, and what finishes subs must know before working in fire-rated construction, protects the project from the late-stage corrections that compromise fire-rated assemblies generate.
UL assembly numbers and what they mean
Underwriters Laboratories (UL) publishes tested fire-rated assembly designs identified by assembly number. A wall assembly designated UL U305, for example, specifies the exact construction: stud type and size, stud spacing, drywall layer count, and thickness, fastener type and spacing, and any other components that are part of the tested assembly. The fire rating, expressed in hours, applies only to the assembly as tested, not to variations that deviate from the specification.
Interior finishes that alter the tested assembly configuration can potentially affect the fire rating, though in practice most finishes installations do not affect fire rating. The concern arises when finishes installation involves penetrating the assembly, adding mass to the assembly in a way that changes its thermal performance, or removing components of the tested assembly.
Flooring installation over fire-rated floor-ceiling assemblies
The floor-ceiling assembly in multifamily construction typically carries a one-hour fire rating between dwelling units. This assembly is also the primary acoustic assembly. In concrete construction, the floor-ceiling assembly is the concrete slab plus any ceiling treatment below. In wood-frame construction, the floor-ceiling assembly is the wood joist or truss system, the subfloor above, and the ceiling below, with specific insulation and air gap requirements defined by the UL assembly number.
LVP installation over a fire-rated floor-ceiling assembly generally does not affect the fire rating because floating LVP is a non-combustible surface layer that does not penetrate or alter the assembly. However, glue-down installation that involves scraping the existing flooring from a concrete slab should be reviewed to confirm that the scraping does not damage the concrete surface in a way that affects the slab’s fire resistance.
For wood-frame fire-rated assemblies, confirm that the acoustic underlayment specified beneath the LVP is the same product used in the UL assembly or is an approved substitute. Substituting a different underlayment in a tested assembly can potentially affect both the acoustic rating and the fire rating.
Penetration firestopping requirements
Any penetration through a fire-rated assembly, including through fire-rated walls and floor-ceiling assemblies, must be firestopped to maintain the fire rating at the penetration. Interior finishes installations that penetrate fire-rated assemblies include wire shelving anchors that penetrate into wall cavities, recessed fire extinguisher cabinets that penetrate fire-rated walls, and electrical conduit for motorized window treatment power supplies that penetrates fire-rated partitions.
For recessed fire extinguisher cabinets installed in fire-rated walls, the cabinet must be listed by an approved listing organization for installation in fire-rated assemblies, or the penetration must be firestopped to maintain the wall’s fire rating. Standard recessed fire extinguisher cabinets are not always listed for fire-rated wall installation. Confirm the cabinet listing against the wall’s fire rating before specifying a recessed cabinet in a fire-rated location.
Wire shelving anchors that penetrate through gypsum board into the wall cavity do not typically affect fire rating because they do not create a through-penetration. However, confirm with the project’s fire protection engineer if any concern exists about specific anchor locations in fire-rated walls.
Corridor construction and finishes installation
Multifamily building corridors typically have one-hour fire-rated construction separating the corridor from the dwelling units. This fire-rated separation includes the corridor walls and the corridor floor-ceiling assembly. Finishes installation in corridors, including corridor flooring, corridor wall base, and corridor light fixtures, must not compromise the fire-rated separation.
Corridor flooring transition strips at unit entry doors are a common location for penetration concerns. The door frame at a unit entry is itself a fire-rated assembly component. A transition strip that is screwed through the floor at the threshold should be confirmed not to penetrate the floor-ceiling assembly at a location that affects the fire-rated separation.
How finishes subs should approach fire-rated construction
The interior finishes sub working in fire-rated construction should understand two things: which walls and floor-ceiling assemblies in the building are fire-rated, and whether any element of their installation scope creates a penetration through a fire-rated assembly. On a project where the fire-rated assemblies are not clearly communicated in the construction documents, the finishes sub should request a fire-rated assembly location plan from the GC before beginning installation.
Any penetration through a confirmed fire-rated assembly requires either a listed penetration firestop product applied according to the manufacturer’s instructions or confirmation from the project’s fire protection engineer that the penetration does not require firestopping. The finishes sub should not make this determination independently.
How Innergy approaches fire-rated assemblies
On Innergy projects, we request fire-rated assembly location information before beginning installation in any area where our scope creates wall or floor penetrations. For recessed fire extinguisher cabinets in fire-rated walls, we confirm the cabinet listing against the wall’s fire rating before ordering. For Division 9 and Division 10 scope in fire-rated construction in TX, WA, OR, CO, UT, NM, or AZ , contact us and we respond within one business day.
Smoke compartment boundaries and finishes installation
In addition to fire-rated assemblies, commercial buildings and larger multifamily projects have smoke compartment boundaries defined by smoke barriers and smoke partitions. Smoke barriers are fire-rated assemblies that also resist the passage of smoke. Smoke partitions may not be fire-rated but must resist smoke passage. Interior finishes subs working in corridors and common areas of commercial buildings should understand which walls are smoke barriers and confirm that their installation scope does not create penetrations through smoke barriers that are not properly firestopped with listed smoke barrier penetration products.
Innergy covers Division 9-Flooring and Division 10-Specialties for multifamily construction and commercial construction under a single subcontract.
The distinction between fire-rated assemblies and smoke compartment boundaries is important because firestopping products are rated for both fire and smoke resistance separately, and a product rated for fire penetration may not be rated for smoke penetration through a smoke barrier.