Facade fire

A facade fire is a fire that spreads over or within the external wall of a building, usually upwards from storey to storey. New Zealand's Verification Method C/VM2 describes the scenario as a fire source that exposes the external wall and leads to significant vertical fire spread. The fire source can be a fire inside the building whose flames project out of a window, or a fire outside, such as a burning rubbish skip against the wall.

Two routes of spread are distinguished. In the first, flames leaving a window heat the wall above and enter the building again through openings on a higher floor. In the second, the external wall itself takes part in the fire, when combustible cladding or cavities in the wall carry the flames upwards. Facade fires threaten escape routes, firefighters and neighbouring property, and can involve falling debris. Codes such as C/VM2 therefore limit the combustibility of external wall systems and the vertical distance a fire may travel, which C/VM2 sets at 3.5 m.

How a facade fire starts

One source of a facade fire is a fully developed fire inside a room. After flashover the fire is usually ventilation-controlled: there is more pyrolysis gas than the incoming air can burn, so part of the gas burns outside the room in flames from the openings. This external flaming is typical of post-flashover fires, and the flames rise along the outside of the building because of buoyancy. Wind speed and direction can change the extent of the flames projecting from windows, especially when there are openings on both the windward and leeward sides.

The heat from these flames can be severe. Experiments on full-scale rooms with a wall extending up to two storeys above the window measured heat fluxes of up to 200 kW/m² on the wall above. In Oleszkiewicz's tests with a window 2.6 m wide and 1.37 m high, the heat flux 0.5 m above the window was roughly 210 kW/m² for an ideal fire size of 10.3 MW. For a fire of 5.5 MW it was only about 25 kW/m², as read from the published graph. The heat flux fell with height, to roughly 55 kW/m² at 3.5 m above the window for the largest fire.

Heat flux on the wall above a 2.6 m by 1.37 m window for four fire sizes in the room, from Oleszkiewicz's tests. Based on Lattimer (2002), p. 2-293.

The flame length above a window depends on the heat release rate of the part of the fire that burns outside and on the size of the window. A relation by Yokoi describes this and is treated under external flaming. Fires can also start outside the building. C/VM2 names a burning rubbish container or skip in close contact with the facade as a design fire source that could ignite the wall and spread fire upwards.

Routes of vertical spread

The first route runs from window to window. Flames from an opening heat the wall and the openings above, and the fire can enter the next floor even if the wall itself does not burn. Platt's probabilistic fire spread model expresses this route simply. The probability that fire spreads up the facade via windows is taken as the probability that the external flame is at least as high as the spandrel. In New Zealand, C/VM2 (second edition, 2025) aims to prevent projecting fire plumes in unsprinklered buildings from reaching unprotected areas on upper floors within 1.5 m vertically of the plume source.

The second route runs over or through the wall. Flame spread over a surface is much faster upwards than sideways or downwards. Typical orders of magnitude are about 0.1 cm/s for lateral or downward spread on thick solids and 1 to 100 cm/s for upward spread. In upward flame spread, the flames and the buoyant flow heat the material ahead of the burning zone, and this wind-aided spread can accelerate. Low-density materials are a particular concern, because for a given flame heat flux the spread rate increases as the density of the fuel decreases. A material with a low thermal inertia, such as lightweight insulation, heats up quickly at the surface; if it is combustible, this contributes to rapid ignition.

Hidden spaces form a third path. Concealed spaces behind exterior cladding on the inside of facades are treated as enclosures when fire spread is analysed. C/VM2 therefore requires cavity barriers at each floor level to stop fire spreading through cavities in an external wall. A cavity barrier is a construction that closes openings within a concealed space against the passage of fire or restricts fire spread within such spaces. Fire can also travel from a low building to a higher wall. C/VM2 covers the case of a fire breaking through a lower roof that is not fire-rated and exposing a higher external wall next to it.

Routes of vertical fire spread on a facade: flames from an opening re-entering a higher floor (left) and fire spread over cladding and through the cavity behind it, interrupted by a cavity barrier (right). Based on Ministry of Business, Innovation and Employment (2025b), pp. 48–49, and Ramachandran (2002c), p. 3-392.

Examples from high-rise fires

Two office fires in the United States show the window-to-window route on a large scale. On 4 May 1988, a fire in the 62-storey First Interstate Bank Building in Los Angeles started on the 12th floor and burned out four floors. The fire spread upwards mostly as flames extended about 9 m (30 ft) up the face of the building and through the gap between the floors and the glass curtain wall. Some spread also occurred through floor penetrations and shafts. The fire moved up at about 45 minutes per floor and burned intensely for about 90 minutes on each level, so that two floors were heavily involved at any time.

On 23 February 1991, a fire on the 22nd floor of One Meridian Plaza, a 38-storey office building in Philadelphia, eventually consumed the 21st to 29th floors. Vertical spread came from unprotected openings in floors and shafts, failing fire-rated floors and flames lapping through windows on the outside of the building. The fire climbed at about 90 minutes per floor and was stopped only at the 30th floor, which had working sprinklers. NIST concluded that in both buildings vertical fire spread was by external flame exposure. The flames broke the windows from the outside and ignited the office furnishings on the floor above.

Materials and wall systems

The fire behaviour of a facade depends on its materials and on how they are put together. C/VM2 (second edition, 2025) requires the entire external wall cladding system to consist of non-combustible or limited combustible materials, or to meet the acceptance criteria of a recognised test or classification method. In C/VM2, a combustible material is one that is neither non-combustible nor limited combustible. Combustibility can be assessed with tests such as AS 1530 Part 1 or ISO 1182, and brick, concrete and steel count as non-combustible.

For combustible claddings, the first edition of C/VM2 used small-scale tests in the cone calorimeter. Samples were exposed to 50 kW/m² for 15 minutes, and the cladding was accepted if its peak heat release rate stayed at or below 100 kW/m². This limit was chosen to give a low probability of accelerating flame spread over the surface. The commentary warns that small-scale tests are not suitable for all materials, particularly where the fire behaviour depends on connections or joints. A table in the first edition set two performance levels. Level A allowed a peak heat release rate of 100 kW/m² and a total of 25 MJ/m², and level B 150 kW/m² and 50 MJ/m².

Composite products show why the complete construction matters. In corner fire tests on combustible linings, one of the materials was an 88 mm sandwich composite with a balsa wood core and glass-fibre facings. The heat fluxes measured in the corner were slightly higher than predicted by correlations derived with non-combustible walls. For products whose fire performance depends on connections or joints, the C/VM2 commentary advises larger-scale tests instead of small-scale tests.

Testing facades

Because the behaviour depends on the complete system, facades are often tested at large or medium scale. The first edition of C/VM2 accepted a large-scale facade fire test such as NFPA 285, ISO 13785-1 or the vertical channel test. The test had to show that vertical flame spread stays within 3.5 m above the fire source. Current large-scale facade tests expose the wall to heat fluxes in the range of 50 to 90 kW/m². The second edition no longer names test standards such as NFPA 285 and BS 8414; it refers instead to a separate document, the Building Product Specifications.

Test facilities reproduce the window fire described above. Oleszkiewicz's facility consisted of a burn room with a window and a target wall above it, next to a vertical channel test apparatus. An intermediate-scale calorimeter developed in Canada for facade systems exposes specimens of 1.22 by 2.08 m to a radiant heat flux of up to 30 kW/m² and measures their heat release rate. Fire resistance tests of facade elements use a separate temperature curve. The external fire exposure curve represents a fire in the open air; it levels off at about 680 °C, whereas the standard curve keeps rising.

Design and regulation

In New Zealand, C/VM2 (second edition, 2025) contains design scenario VS, external vertical fire spread. Building Code clause C3.5 requires that fire does not spread more than 3.5 m vertically in buildings taller than 10 m, or in buildings with upper floors that contain sleeping uses or other property. This aims to keep escape routes tenable until occupants have left and to protect firefighters working in or around the building. The scenario applies to multi-level buildings of more than 10 m and to other multi-level buildings with sleeping uses, other property or external escape routes on upper floors.

The design fire for the facade materials is a radiant heat flux of 50 kW/m² on the facade for 15 minutes for buildings of importance levels 2 and 3. For importance level 4 the flux is 90 kW/m² for 15 minutes. Some damage to the exposed area is accepted, but the cladding must not spread flames significantly beyond it. According to the commentary on the first edition, the 3.5 m limit accepts that fire may reach the floor immediately above but not two floors above. The value of 50 kW/m² represents the heat flux from a fire plume projecting from an opening, although higher fluxes are possible.

For spread via openings, C/VM2 offers three options in unsprinklered buildings: an automatic sprinkler system, fire-rated construction such as aprons and spandrels, or calculations of radiation from the projecting fire plumes. In a sprinklered building this route is addressed by reducing the risk of flashover, and external drenchers are not needed. For a lower roof next to a higher wall, the options are sprinklers below the roof or fire-rating the underside of the roof within 5.0 m of the wall. A third option is fire-rating the wall up to 9.0 m above the unprotected roof.

The three parts of design scenario VS in C/VM2 (second edition, 2025) and their means of compliance. Based on Ministry of Business, Innovation and Employment (2025b), pp. 47–49.

In Australia, the Fire Safety Verification Method includes a similar scenario VS. It compares the risk to life from a fire affecting the external wall, including penetrations, cladding and attachments, with that of a reference building to the deemed-to-satisfy provisions. The handbook refers to Verification Method CV3 and to AS 5113. It notes that an external wall system tested to AS 5113 may still contain combustible elements whose risk needs further mitigation. Typical measures listed include control of combustibility, distances between vertical openings, non-combustible and fire-rated spandrels, horizontal projections from the facade and enhanced sprinkler protection.

Consequences and limits of protection

A facade fire can reach several floors quickly, which matters for the evacuation strategy of a high-rise building. The Australian data sheets note that phased evacuation, in which floors leave one after another, may be sensitive to rapid fire spread to several floors via the facade. The evacuation management strategy should describe what to do in that case. The criteria of C/VM2 likewise refer to tenable escape routes and to the safety of firefighters working in or around the building.

Facade fires also threaten the surroundings. The Australian scenario names falling debris and spread to adjacent buildings as consequences of fire spread over the external wall. Fire spread between separate buildings across a boundary is, however, a different design scenario. Both C/VM2 and the Australian method treat it separately from vertical spread.

Fire resistance and facade fire spread are different properties. The fire resistance of building elements is tested with a standard temperature curve. The facade scenarios, in contrast, address the combustibility of the whole cladding system, including penetrations, attachments and cavities, and the flames from openings. Sprinklers inside the building protect the facade only indirectly, by reducing the risk of flashover in the room of origin.

See also

• Fire spread
• Fire spread between buildings
• Stay-put strategy
• Glass breakage in fire

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Australian Building Codes Board. (2020b). Handbook: Fire Safety Verification Method (Print version 1.1).
• Australian Building Codes Board. (2022). Fire Safety Verification Method Standard 2022.
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• Ramachandran, G. (2002c). Stochastic models of fire growth. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-381–3-401). National Fire Protection Association; Society of Fire Protection Engineers.
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