24 C O N T E X T 1 1 3 : M A R C H 2 0 1 0 Throughout most of its length, the heat from flue gases flowing up the chimney transfer through the brickwork, and dissipate from the outer surface of the chimney structure. Within the body of the thatch, however, the heat can not dissipate quite so easily. During the colder months, the fuel burners will operate for much of the day. This allows a build-up of heat at the thatch-chimney interface, which can not fully cool before the heating process starts again the following day. The temperature of the thatch at this interface continues to rise over time to temperatures in excess of 200oC. This is a critical temperature for thatch, leading to its decomposition and auto-ignition. It will ultimately lead to the full involvement of the roof by fire, unless it is prevented from doing so by external intervention. Methods have been developed to remove or reduce the risk of ignition, to reduce the potential damage caused by fire, and to improve fire-fighting techniques for thatched properties. In the hierarchy of control measures, prevention must come first. The most obvious prevention method would be to retain the original open fire place within the property. Where solid fuel appliances are fitted, they should be the subject of a building regulations application (Part J) and be installed by a suitably qualified engineer, such as a HETAS engineer. The requirements for lining the chimney and providing suitable separation between the flue gases and any combustible material are clearly outlined in Approved Document J.This is available for viewing online at www.planningportal.gov.uk/uploads/ br/BR_PDF_ADJ_2002.pdf. Regular cleaning and inspection of the chimney and its lining, as well as the burning of seasoned wood, all help to reduce the likelihood of unwanted fires. Fitting a flue pipe thermometer approximately 600mm above the appliance will provide a good indication of the working temperature of the stove. This will prompt the vigilant homeowner to adjust the damper when appropriate, to moderate the gas temperatures. If the appliance has been installed in accordance with the regulations, and is used with understanding, the potential for a thatch fire to start in this way is reduced to as low as reasonably practicable. Methods for detecting a thatch fire in the early stages include temperature probes inserted into the thatch adjacent to the chimney, single-point temperature alarms installed on the chimney brickwork within the loft space, and digital cable detection attached to the outer face of the chimney. The practicality of each of these devices will relate directly to their ability to assess the temperature within the insulated area at the heart of the thatch, at its interface with the brickwork of the chimney. The author has made contact with fire and rescue authorities around England andWales, and has put the question to specialists in fire safety and fire investigation. So far, no evidence has been submitted which supports or refutes the functionality of these devices in a real fire. Modern thatched properties should be built to the Dorset model, which provides a fire barrier between the structural roof timbers and the layers of thatch. This allows for burning thatch to be removed from the roof before the fire can spread to the roof members and to the remainder of the property below. When an older property is in need of re-thatching, the practically minded would see the benefit in stripping all the old thatch back to the structural timbers in order to retrospectively apply the Dorset model.The lowest levels of thatch, however, are rich in evidence of lifestyles, crop types and thatching techniques, sometimes dating back to medieval times. The conservation-minded will quite rightly overrule, and this layer will normally remain in place while the newer thatch is laid on top. Fire and rescue services are increasingly moving towards compressed air foam systems (CAFS) to tackle thatched roof fires.This foam is applied through probes inserted into the heart of the thatch. The foam is a simple wetting agent added to fire-fighting water, allowing it to cling to the surface of the reed or straw, cooling the thatch and restricting the spread of fire. While the initial outlay to provide CAFS fire-fighting appliances is large, it is expected that the long-term reduction in the fire-fighting resources needed to tackle thatch fires in the future will far outweigh the cost. It will provide a safer working environment for firefighters, and will result in significantly reduced costs, discomfort and inconvenience for the owner of the thatched property. Paul Turnbull works with the Hampshire Fire and Rescue Service. Heat can transfer through chimney brickwork to produce a hot spot capable of causing smoulder in the thatch.The diagram shows the temperature profiles within the thatch resulting from an average flue gas temperature of 300oC and a thatch depth of one metre. Key to the colour temperature profiles: Red 250oC Yellow 200oC Green 150oC Pale blue 100oC Mid blue 50oC Dark blue Thatch at ambient temperature The Dorset model: a fire barrier introduced between the roof structure and the thatch and battens. In a thatch fire the barrier provides protection to the building below. Installation is practical only where the thatch is replaced entirely. Diagram courtesy of Marjorie Sanders and Roger Angold, National Society of Master Thatchers
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