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 Canada, fire-resistance tests follow CAN/ULC-S101. Ratings for common assemblies can be taken from listings by testing laboratories such as ULC (UL Solutions) and Intertek, or from Appendix D of the NBC, which gives fire-resistance ratings for generic assemblies of wood, steel, concrete and masonry based on test data and calculation. Part 9 of the NBC also contains tables of typical rated assemblies for houses and small buildings.

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 separations and fire compartments

The NBC uses the term fire separation for a construction assembly that acts as a barrier against the spread of fire. A fire separation may or may not need a fire-resistance rating; where the code requires a rating, the assembly must provide it. The code requires fire separations in many places, including between major occupancies, around each dwelling unit and suite in some occupancies, around exits, around vertical shafts, around service rooms and storage garages, and at each floor assembly in most buildings. A fire compartment is an enclosed space separated from the rest of the building by fire separations with the required rating.

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 labelled 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 Canada are tested to CAN/ULC-S115, which gives F ratings (flame), FT ratings (flame and temperature), FH and FTH ratings (with hose stream) for penetrations and joints. The NBC generally requires an F rating not less than the rating of the closure required for the fire separation, and an FT rating for some penetrations in higher buildings and specific locations. Fire dampers follow CAN/ULC-S112 and fire doors CAN/ULC-S104 and 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 NBC handles spatial separation in Subsection 3.2.3 for Part 3 buildings and Subsection 9.10.15 for houses. The key distance is the limiting distance, measured from the exposing building face to the property line, the centre line of a street or lane, or an imaginary line between two buildings on the same property. The exposing building face is the part of the exterior wall facing one direction that is within a single fire compartment or the whole wall if unsprinklered. Tables give the maximum percentage of unprotected openings, such as windows and doors, based on the area of the exposing building face, its height-to-width ratio, the limiting distance and whether the building is sprinklered. At short limiting distances, the wall must also have a fire-resistance rating and noncombustible cladding. Sprinklered buildings may generally have twice the unprotected opening area of unsprinklered ones.

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 NBC requires noncombustible cladding for many buildings close to property lines, limits combustible components in noncombustible construction, and allows combustible cladding on taller buildings only if the wall assembly passes the CAN/ULC-S134 full-scale exterior wall fire test. Construction managers should treat any substitution of cladding, insulation or air barrier on exterior walls as a code matter.

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 NBC uses flame-spread ratings from the CAN/ULC-S102 tunnel test. Most walls and ceilings must have a flame-spread rating not more than 150; exits and some corridors require 25 or 75 for walls and ceilings, and high buildings and some occupancies add smoke developed classification limits. Floor surfaces in exits are tested to CAN/ULC-S102.2.

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.