Fire spreads quickly within roof structures
Wood-structure roofs contain ventilation openings, cavity spaces and attic spaces. The purpose of these is to keep the structures dry, but they also make extinguishing a fire that has entered such structures virtually impossible. While a cavity space heats up rapidly due to its small size, poor air inflow decelerates the burning rate. Gases are discharged through the openings in the structures, spreading first as incombustible gases. Sometimes a thick layer of incombustible gases may build up under the roof and ignite when it comes into contact with a flame spreading through cavity spaces or some other ignition source, causing fire to spread rapidly across a large area.
Cavity space charting is important
One of the most important aims of a building’s fire safety assessment is to chart any cavity spaces or hidden spaces found in various locations. External walls and roofs contain ventilation openings, the purpose of which is to keep the building dry while suspended ceilings situated under roofs and intermediate floors house ventilating ducts, pipework and power and other cables.
How to prevent a large fire during repairs?
The risk of fire can be reduced by minimising ignition risks. Alternative working methods to hot work are a primary means of achieving this. Moreover, arson can be prevented by ensuring that combustible material is stored far from external walls.
Before hot work can be initiated, a hot work permit is required. This requires that the hot work site is inspected, the fire risks involved are assessed and protective measures are defined based on these. Workers must have sufficient knowledge of the safe performance of hot work in the said location. The hot work site and its immediate surroundings are inspected. All combustible structures and areas are protected, including lead-throughs and the backs of sheets, panels and wall and eaves flashings. The attic and ventilation and other intermediate spaces are inspected and protected if necessary.
Ventilation and heating device fans near the hot work site are switched off to make sure that sparks do not enter the channels. Channel openings are carefully covered. Protective measures must be taken at least 10 metres from the hot work site, bearing in mind that sparks generated by disc cutting may spread even further than this. The spark zone can be limited by surrounding the hot work site with protective walls made of an incombustible material that absorbs sparks.
During hot work, it must be ensured that the surroundings do not catch fire and that structures that may convey heat to inner structures or adjacent facilities, such as roof gullies, other lead-throughs and various types of flashing, do not heat up. Hot air blowers or other burners must not be left unsupervised when in operation. The hot work site must be equipped with extinguishers that are suitable for cavity space fires. First-aid extinguishing preparedness must be maintained throughout the hot work and during the subsequent fire-watch.
Improving fire compartmentation in connection with renovations
Special attention must be paid to the compartmentation of cavity roofs in order to prevent major damage. Roof structures must be compartmented at partition walls so that single cavity spaces are broken into smaller sections. A sufficiently high firebreak is built above the roof from bricks, for example, their height depending on the roofing and the fire load of the facilities. With respect to large fire compartments, firebreaks must also be built on the roof elsewhere than at partition walls. Dividing a cavity space into smaller sections using low, light-structure partition walls is usually not a successful solution since fire will spread along the roofing above such walls.
Fire spreads through hot combustion gases discharging through under-roof structures or the openings in them. The roofing must be fire resistant (type K1) for several metres (five metres) around the firebreaks. Under-roof spread of the fire can be prevented through smoke traps and smoke venting. The tightness of compartmentation structures in suspended ceilings’ hidden spaces should be ensured. Cable and other lead-throughs must be sealed immediately after installation.
Extinguishing plan
A detailed extinguishing plan must be drawn up for cavity spaces. This involves considering the general extinguishing principles for structures containing cavity spaces, after which a detailed plan is drawn up for the most probable ignition areas. Extinguishing must be practised. Firebreaks are planned in advance since firebreak creation requires preparatory work, and suitable equipment is arranged for such work. Moreover, working methods must be tested, for example in case holes are required in the roofing, and the necessary equipment must be acquired either for the building or the fire services in question.
Choosing extinguishing methods
A smoke vent is made close to the fire pocket to discharge overpressure from the cavity space and thus prevent the fire from spreading. Replacement air vents are made so that the gas flows can be directed towards the fire pocket. This decelerates the spread of hot and combustible smoke gases and creates more time for creating firebreaks.
Sometimes a dry fire water pipe equipped with open nozzles can be installed in a cavity space. In case of fire, the dry fire water pipe is activated in order to limit the spread of hot smoke gases. Firebreak locations must be planned and marked clearly on the structures.
Fire hydrants or dry risers and fire extinguishing equipment cabinets are installed on roofs on which fires are difficult to extinguish. In major risk concentrations, large roof areas are equipped with fire water monitors in order to ensure efficient fire extinguishing. Fire ladders or stairs are needed at least every 50 metres on external walls.
Fire services and property owners must co-operate in order to determine fire-extinguishing methods for areas involving a fire risk, and acquire a sufficient amount of equipment.
Pekka Kallioniemi
Bibliographical reference (in Finnish, with English abstracts):
Reports on the fire safety of cavity spaces by VTT Technical Research Centre of Finland:
http://www.vtt.fi/inf/pdf/tiedotteet/2002/T2128.pdf
http://www.vtt.fi/inf/pdf/tiedotteet/2003/T2202.pdf
http://www.vtt.fi/inf/pdf/tiedotteet/2004/T2249.pdf
