Fire resistance ratings and penetrations

Buildings are divided into compartments so that a fire starting in one place is held there long enough for people to leave and firefighters to arrive. The walls and floors that form those compartments are fire-resistance-rated assemblies, built to a rating measured in hours. Every cable that passes through one makes a hole in it. If the hole is left open, or filled with the wrong material, fire and smoke pass through in minutes and the rating means nothing. Cabling trades make more penetrations than almost any other trade, often hundreds per floor, and in renovations they find the holes left by every trade before them. This lesson is about recognizing rated construction and understanding what the building code asks of a penetration. Lesson 9 is about the firestop systems that seal them.

Fire-resistance ratings

A fire-resistance rating is the time a wall, floor or other assembly withstands the standard fire test, ASTM E119 or UL 263, while limiting the passage of flame and hot gas and the temperature rise on the unexposed side, and, for loadbearing assemblies, continuing to carry its load. Ratings are given in hours: 1, 2, 3 and 4 hours are common, with some half-hour assemblies. The building code decides which walls and floors need which rating: floors in most multistory buildings, shafts, exit stairs, corridors in some occupancies, walls between dwelling units in apartments and hotels, and walls around certain rooms.

The building code uses several related terms. Fire barriers and fire partitions are rated walls with different continuity and protection rules; fire walls divide a building into separate buildings for code purposes; horizontal assemblies are rated floors and roofs; shaft enclosures surround vertical openings such as stairs, elevators and risers. Smoke barriers and smoke partitions limit smoke movement and may have their own rules for penetrations. Openings in rated walls, such as doors and dampers, have fire-protection ratings that can be lower than the wall's fire-resistance rating.

Finding rated walls and floors

Rated construction is shown on the architect's life safety plans, usually with colored or patterned lines around the rated walls and a legend giving the rating. The IBC, in section 703, requires fire walls, fire barriers, fire partitions, smoke barriers and smoke partitions to be identified above ceilings and in other concealed spaces with stenciled signs or labels repeated along the wall, such as FIRE AND/OR SMOKE BARRIER: PROTECT ALL OPENINGS. Cobalt Line's crews treat every wall that runs to the underside of the deck above as rated until the drawings say otherwise, and every floor slab as rated.

What the building code asks of a penetration

Section 714 of the International Building Code governs penetrations of rated walls and floors. Through-penetrations of rated walls must be sealed with a through-penetration firestop system tested to ASTM E814 or UL 1479, with a positive pressure difference of at least 0.01 inch of water, and an F rating of at least 1 hour but not less than the rating of the wall. Through-penetrations of rated floors need both an F rating and a T rating of at least 1 hour but not less than the rating of the floor, with exceptions, for example where the penetration is inside the cavity of a wall above or below. NEC 300.21 and 800.26 point the same way from the electrical side: openings for wiring through rated walls and floors must be firestopped using approved methods so the rating is maintained.

The F rating keeps flame from passing; the temperature rating matters where combustibles on the unexposed side could ignite from heat conducted along the penetrating items, which is a real concern with metal conduits and large cable bundles. Air leakage ratings matter in smoke barriers, in hospitals and wherever the design depends on keeping smoke out of an area.

Through-penetrations and membrane penetrations

A through-penetration passes completely through the assembly, as a cable bundle through a slab or a sleeve through a wall does. A membrane penetration passes through only one face of a hollow assembly, as an outlet box in one side of a rated stud wall does. Both are regulated.

Outlet boxes for data jacks in rated walls are allowed as membrane penetrations under IBC 714.4.2 when they are listed steel boxes of no more than 16 square inches, with an aggregate area of no more than 100 square inches in any 100 square feet of wall, and when boxes on opposite sides of the wall are separated horizontally by at least 24 inches, or by other listed means such as putty pads or a tested system. Non-metallic boxes and larger boxes must be tested for use in rated walls. Boxes back to back in the same stud cavity, a common shortcut, defeat the wall unless such protection is installed. The architect's details for rated walls usually show the accepted method.

Sleeves, cable trays and openings for future cable

Cabling penetrations change over the life of a building, as cables are added and removed. Designers therefore provide sleeves through walls and slabs, short lengths of conduit set in the assembly, which are firestopped with a re-enterable product, or listed fire-rated pathway devices that seal themselves around however many cables pass through them. A cable tray passing through a rated wall is a special case: the tray is usually interrupted at the wall and the cables pass through sleeves or a tested tray penetration system, because the open tray itself cannot be sealed easily.

Every penetration Cobalt Line makes is recorded. The specification for St. Brendan's requires a log of every penetration of rated construction, with its location, the assembly and its rating, the penetrating items, the tested system used, the installer, the date and a photo. Hospitals inspect firestops closely because patients cannot always leave, and the hospital's own fire safety staff check the log against the walls before the building is occupied.

# Penetration log check: does each seal's rating meet what the assembly needs? (simplified rules)
def needed(assembly_hr, is_floor):
    f = max(1, assembly_hr)                  # IBC 714: at least 1 h, not less than the assembly
    t = max(1, assembly_hr) if is_floor else 0  # floors need a T rating too, unless an exception applies
    return f, t
log = [  # (id, location, assembly rating h, floor?, system F h, system T h)
    ("P-201", "2nd fl slab at riser",      2, True,  2, 2),
    ("P-202", "corridor wall C-2",         1, False, 1, 0),
    ("P-203", "shaft wall S-1",            2, False, 1, 0),
    ("P-204", "2nd fl slab at nurse stn",  2, True,  2, 0.5),
    ("P-205", "smoke compartment wall",    1, False, 1, 0),
]
for pid, where, hr, floor, f, t in log:
    need_f, need_t = needed(hr, floor)
    ok = f >= need_f and t >= need_t
    print(f"{pid} {where:26s} needs F{need_f:g}{'/T' + format(need_t, 'g') if need_t else '':5s} has F{f:g}/T{t:g}  {'OK' if ok else 'REJECT'}")

P-201 2nd fl slab at riser       needs F2/T2   has F2/T2  OK
P-202 corridor wall C-2          needs F1      has F1/T0  OK
P-203 shaft wall S-1             needs F2      has F1/T0  REJECT
P-204 2nd fl slab at nurse stn   needs F2/T2   has F2/T0.5  REJECT
P-205 smoke compartment wall     needs F1      has F1/T0  OK

The US version applies the IBC's basic rules. A real job would also check the exceptions, such as floor penetrations within a wall cavity that do not need a T rating, and the L rating where a smoke barrier is involved.

Existing buildings

Renovations bring their own problems. Old penetrations may be open, packed with fiberglass or paper, or sealed with products whose system is unknown. When Cobalt Line's crew opens a ceiling at Kellerman and finds an open hole around old cables, the hole is now their concern too, because new cable added to it makes the penetration their work. The specification requires the crew to report existing deficiencies in rated construction to the owner's representative, with photos, and to seal any penetration their cables pass through with a tested system, even if the hole was there before. Greta keeps those reports separate from Cobalt Line's own work in the log, so that responsibility is clear.

Smoke barriers and smoke compartments

Hospitals, care homes and some other occupancies are divided into smoke compartments, so that patients who cannot leave the building can be moved from a compartment with smoke into a clean one on the same floor. The walls between compartments are smoke barriers, often also rated for fire, and their penetrations must limit smoke as well as flame. That is where L ratings matter: a firestop system with a low air leakage rating keeps smoke from passing through the seal under the pressure differences the building's smoke control system creates. St. Brendan's specification requires an L rating at every smoke barrier penetration, and Gil checks the system sheets for it.

Smoke barriers usually run from the floor to the underside of the floor above, and they are often hidden above corridor ceilings, so they are easy to cross without noticing. The drawings show them with a distinct line type and the walls are labeled above the ceiling. When a cable tray crosses a smoke barrier, the tray is interrupted and the cables pass through sleeves or a tested tray penetration system, sealed for both fire and smoke. The same applies to the many small penetrations of hospital walls for nurse call, data, security and audio cabling, which over the life of the building are the most frequently disturbed seals.

Coordinating with other trades

Every trade that penetrates rated construction is responsible for its own penetrations, but the trades share walls and sleeves. Electrical, mechanical, plumbing, fire protection and low-voltage trades often pass through the same opening, and a single opening with mixed penetrants needs a system tested for that mix, usually from the 8000 series. Cobalt Line's coordination meetings agree which trade seals each shared opening, so that no hole is left on the assumption that someone else will seal it. The general contractor's firestop coordinator, where there is one, keeps the master log, and each trade's entries are reconciled with it before the inspection.

Coordination continues after construction. The penetration log is handed to the owner, and the owner's facility staff use it to control later work: anyone who wants to pass cable through a rated wall must say which penetration they will use and how they will restore its seal.