Fire separations, fire resistance and spatial separation

Codes do not try to prevent every fire. They aim to keep a fire small and local long enough for people to escape and for fire fighters to act. Two strategies do most of the work. Compartmentation divides the inside of a building into fire compartments with rated walls and floors, so that a fire in one apartment, store or corridor is held back from the rest of the building. Spatial separation limits the openings and combustible materials in exterior walls close to property lines, so that a fire in one building does not spread to the next. This lesson explains both, together with fire-resistance ratings, the testing behind them, the protection of openings and penetrations, and the site practices that keep ratings intact.

Construction managers have a large influence on whether a building's fire compartmentation works in reality. Most failures are not in the design but in installation: a missing firestop around a pipe, a fire damper not installed, a rated door with the wrong hardware, a gap at the top of a rated wall above the ceiling. These items are hidden after construction, so getting them right during the work is essential.

Fire-resistance ratings and testing

A fire-resistance rating is the time, in minutes or hours, that a building element or assembly withstands a standard fire test while continuing to perform its function. Walls, floors, roofs, columns and beams are tested in large furnaces following a standard time-temperature curve that rises to about 840 °C (1,550 °F) at one hour and about 1,010 °C (1,850 °F) at two hours. During the test the assembly must carry its design load if it is loadbearing, must not let flames or hot gases through, and must keep the temperature rise on the unexposed side below a set limit, typically an average of 140 °C (250 °F). Some tests also include a hose stream applied after the fire exposure to check that the assembly holds together.

In the US, fire-resistance tests follow ASTM E119 or the equivalent UL 263. Ratings for common assemblies are taken from listings such as the UL Product iQ directory (for example UL Design No. U305 for a wood stud wall), from the prescriptive assemblies in IBC Section 721, from calculated fire resistance under IBC Section 722, or from the Gypsum Association's Fire Resistance and Sound Control Design Manual.

A rated assembly is a complete system, not a list of materials. A one-hour wall listing specifies the stud size and spacing, the type and thickness of gypsum board, fastener type and spacing, joint treatment and any insulation. Using Type X gypsum board without the rest of the system does not produce a rated wall. Construction managers should require submittals to identify the listed assembly for every rated element and should check that site work matches it.

Fire barriers, fire partitions and smoke barriers

The IBC uses several kinds of rated element. Fire walls divide buildings, as Lesson 5 explained. Fire barriers are rated walls that separate occupancies, enclose exit stairs and shafts and create fire areas, and they must be continuous from the floor to the floor or roof deck above. Fire partitions are rated walls, usually 1 hour, separating dwelling units, sleeping units and corridors, and they may terminate at a rated ceiling in some cases. Horizontal assemblies are rated floors and roofs. Smoke barriers and smoke partitions resist the passage of smoke and are used mainly in institutional occupancies, where occupants are protected in place.

Openings, closures and penetrations

A rated wall is only as good as its weakest opening. Doors, windows, ducts, pipes, cables and joints all create paths for fire and smoke, so codes require each to be protected to maintain the separation.

• Doors in rated walls must be labeled fire door assemblies, including frame, hardware and glazing, with a rating usually less than the wall itself: for example a 45 min or ¾ h door in a 1 h wall and a 1.5 h door in a 2 h wall. They must be self-closing and latching, and many must close automatically on fire alarm.
• Glazing in rated walls and doors must be a listed fire-protection or fire-resistance-rated glazing, limited in size and marked with its rating.
• Ducts passing through rated assemblies need fire dampers, and some also need smoke dampers or combination fire and smoke dampers, installed according to their listing with access for inspection.
• Pipes, conduits and cables passing through rated assemblies need listed firestop systems, chosen for the specific penetrating item, the size of the opening and the type of wall or floor.
• Joints between rated walls and floors, and at the top of walls, need listed joint systems that accommodate movement while maintaining the rating.

Firestop systems in the US are tested to ASTM E814 or UL 1479 for penetrations and ASTM E1966 or UL 2079 for joints, giving F ratings (flame) and T ratings (temperature rise). The IBC generally requires an F rating at least equal to the assembly's rating, and a T rating for penetrations of rated floors in many cases. Fire dampers follow UL 555, smoke dampers UL 555S, and fire doors are tested to UL 10C and installed under NFPA 80.

Firestopping is one of the most frequently failed inspections. Common problems include using a sealant not listed for the penetrating item, oversized holes, multiple items crowded into one opening, missing mineral wool packing, and penetrations added by later trades after firestopping is complete. Good practice is to assign firestopping to a single qualified contractor, require each trade to coordinate penetrations, label rated walls above ceilings, keep a log of penetrations with the listed system used, and inspect before ceilings are closed. Some owners require third-party firestop inspection.

Spatial separation and exterior walls

Fire can spread from building to building by radiant heat through windows and other openings in exterior walls, and by flames from openings igniting combustible cladding. Codes therefore limit the area of unprotected openings in an exterior wall, require minimum fire-resistance ratings and noncombustible cladding as the wall gets closer to the property line, and in some cases require sprinklers or protected openings.

The IBC handles exterior walls in Section 705. Fire separation distance is measured from the building face to the closest interior lot line, the center line of a street or public way, or an imaginary line between two buildings on the same lot. Table 705.5 sets the required fire-resistance rating of exterior walls by fire separation distance, occupancy and construction type. Table 705.8 limits the area of unprotected and protected openings in each story as a percentage of the exterior wall area of that story, according to the fire separation distance and whether the building is sprinklered. Exterior walls containing combustible materials, foam plastic insulation or metal composite panels have additional requirements, including testing to NFPA 285 for some walls in Types I to IV.

Exterior cladding has received close attention since the Grenfell Tower fire in London in 2017, which killed 72 people when flammable cladding spread fire up the outside of a residential tower. The IBC requires NFPA 285 testing for exterior walls with combustible components, such as foam plastic insulation, metal composite material, high-pressure laminates and water-resistive barriers, in buildings of Types I to IV of any height, with some exceptions. Construction managers should treat any substitution of cladding, insulation or water-resistive barrier on exterior walls as a code matter, because the tested assembly is only valid with the tested components.

Interior finishes and flame spread

Fire can also spread across the surfaces of walls and ceilings. Codes limit the flame spread and smoke development of interior finish materials, with stricter limits in exits, corridors and assembly and institutional occupancies. Paint, wallpaper, panelling, acoustic tiles and decorative materials can all be interior finishes for this purpose.

The IBC uses classes from the ASTM E84 (UL 723) tunnel test: Class A (flame spread index 0 to 25), Class B (26 to 75) and Class C (76 to 200), all with a smoke-developed index not over 450. Table 803.13 sets the required class by occupancy and location, with Class A or B required in exit enclosures and corridors in many occupancies, and reductions for sprinklered buildings. Textile wall coverings and expanded vinyl need room corner tests.

Keeping ratings intact during construction and renovation

Fire separations are built by many trades over many months, and each can damage them. Electricians cut holes for boxes, plumbers drill floors for pipes, mechanical contractors cut openings for ducts, and tenants later add cabling. On renovation projects, existing rated walls are often penetrated or partly removed. A construction manager can protect the ratings by:

• Marking rated walls above ceilings with stencilled labels stating the rating, as many codes now require.
• Requiring a penetration permit or log for any new opening in a rated assembly.
• Scheduling firestop installation and inspection after all trades have finished penetrations but before ceilings close.
• Checking that fire door hardware, closers and glazing match the labels and the door schedule.
• Coordinating fire damper locations and access doors with the ceiling and wall finishes.
• Photographing rated assemblies and firestops before concealment for the close-out record.