BCD 2017

T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 7 75 ROOFING 3.1 THATCH FIRES and the role of wood-burning stoves ALISON HENRY and JIM GLOCKLING O VER THE past two decades there has been a significant increase in the number of thatched-roofed buildings destroyed by fire. Many of these were listed or in conservation areas. Fire in a thatched roof usually spreads slowly but is extremely difficult to extinguish, so damage is usually extensive. As well as the traumatic impact of losing a home and most of their possessions, many owners also have to bear much of the cost of reconstruction, as buildings are often not insured to their full rebuilding value. In addition to covering materials and labour, reconstruction costs include items such as VAT, fees for planning, listed building and building regulations fees, and temporary accommodation (which is often needed for longer if the building is listed, as it usually takes longer to obtain all the relevant approvals). Under-insurance can pose enormous problems for owners, requiring them to dig deep into their savings, take out a loan or, in some cases, sell the property. If the house is part of a terrace, neighbouring properties might also be damaged. When a historic building is seriously damaged by fire, the heritage cost should not be underestimated either. In many cases, the only surviving historic fabric is the outside walls and chimney stacks. Although re- construction usually replicates the form and style of the original building, for example, by reinstating a thatched roof and copying the design of the doors and windows, this only restores architectural value: the historic and archaeological values of the thatched roof and everything else that has to be replaced are entirely lost. Often the result is, essentially, a new building in imitation of the original. Greater understanding of the causes of thatch fires and the development of robust measures to prevent them are urgently needed to help reduce the numbers of thatched buildings lost to fire each year. CAUSES OF THATCH FIRES – THE EARLY THEORIES By the 1990s it was increasingly being recognised that, in most cases where a fire originated in a thatch roof, a wood-burning stove was in use when it started. These stoves burn at a higher temperature than open fires, and it seemed clear that there was some correlation between the increasing incidence of thatched-building fires and the growing popularity of wood burners. However, the actual methods of thatch ignition have been the subject of much debate. Research carried out in the 1990s was based on computer modelling rather than measurement of actual flue gas velocities and temperatures. The modelling suggested that heat from flue gases would be transferred by conduction through the chimney brickwork into the adjacent thatch. It was therefore concluded that if the thatch surrounding the chimney was very thick (as often occurs in old, multi-layered thatched roofs), it would provide sufficient thermal insulation to enable the brickwork to heat up to the point where the temperature at the interface with the thatch could reach 200 o C, which would be sufficient for the thatch to smoulder. The modelling further suggested that prolonged use of a wood burner could raise the temperature of the thatch to 400 o C, at which point it would ignite. This ‘heat-transfer theory’ gained widespread currency. Recommendations to reduce the risk of ignition included installing an insulated flue liner to reduce conduction of heat into the brickwork. In the late 2000s, forensic investigators noted that many thatch fires had started shortly after the wood burner was lit. In other cases fires occurred in single-coat thatched roofs, and in properties where the chimney was already fitted with an insulated liner. These fires could not be explained by the heat-transfer theory. As historic buildings continued to be lost to fire, it became clear that further research was needed. CURRENT RESEARCH In 2014, Historic England (formerly English Heritage) and NFU Mutual Insurance Society Limited (which insures many of the UK’s thatched properties) commissioned the Fire Protection Association to take a fresh look at all the possible causes of fire in thatched properties with wood-burning stoves. These include: • direct ignition – due to sparks, hot embers or burning debris falling from the chimney and landing on the thatch, whether during normal operation or as a result of a chimney fire • convection – due to transfer of heat by hot gases, for example through failed mortar joints or holes in perished brickwork or flue liner. • conduction – due to heat transfer through solid materials. To assess the problem two full-scale test rigs were constructed. The first comprised a 12kW wood-burning stove fitted with a rigid steel flue, instrumented to measure the operational parameters of wood burners and the speed and temperature of flue gases, and to understand the mechanisms of spark emission from the flue. The second rig included the same type of stove installed in a brick fireplace with a brick chimney stack. This could be used unlined or it could be fitted with various types of liner (insulated and non-insulated), including a damaged one. Again, the rig was fully instrumented. Because thatch is designed to shed water, it is extremely difficult for the fire services to extinguish burning thatch. (Photo: NFU Mutual Insurance Society Ltd)