Thatch Fires
and the role of wood-burning stoves
Alison Henry and Jim Glockling
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| 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) |
Over 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 reconstruction 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°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°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. A series of tests has been carried out to:
- investigate the operational parameters of wood-burning stoves using traditional fuel types and a range of air venting levels throughout the life of the fire including initial ignition and later refuelling
- investigate the effects of various materials, such as paper, card and kindling, used to help light the fire
- study flue gas properties and methods of spark/ember transport and ejection
- measure heat transfer by conduction through brickwork, with a variety of flue configurations, including damaged and partially blocked flues
- measure the transfer of heat by convection (movement of hot gases) via imperfect brickwork and damaged flue liners
- assess the risk of thatch ignition due to a chimney fire.
RESEARCH FINDINGS TO DATE
The research is ongoing, and further tests will be carried out to investigate other means of thatch ignition related to wood-burning stoves and open fires. However, some significant findings have already come to light.
Spark ejection
Sparks were ejected from the flue during lighting and the early life of the fire, during refuelling and poking, and randomly during normal operation, perhaps associated with collapse of burning logs. The size and frequency of sparks were affected by the type of fuel and the volume of air used for venting. Some sparks were ejected into the air at high speed and flared momentarily, but others stayed alight while floating down towards the notional roofline. Sparks from cardboard and paper were larger and glowed for longer than those from kindling and logs, and some retained their energy during their descent.
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| The two testing rigs: the one on the left was used to establish the general operating parameters of wood-burning stoves, including flue gas temperatures and speeds. The rig on the right was used to evaluate the effects of heat conduction through brickwork, and movement of hot flue gases via defects in the brickwork or flue liner. (Photo: Alison Henry). |
More sparks were produced when the stove was being well-vented and therefore burning at high temperature. Measurement showed a direct relationship between stove temperature and flue gas velocity: the higher the velocity, the greater the risk of burning material being transported upwards and ejected from the chimney.
Some wood-burners allow aggressive ventilation, and are therefore potentially more dangerous than other models. For example, models in which all controlled ventilation occurs under the fuel bed may be operated in such a way that they generate high temperatures and flue gas velocities, thereby increasing the risk of lighter material in the fire bed being carried up and ejected from the flue. Stoves that have a separate ash-pan door below the main door which can be opened while the stove is in use are of particular concern, and manufacturers warn against venting them in this way.
Temperature of flue gases and chimney brickwork
During typical wood-burner operation, the temperature within the fire box was generally between 500 to 800°C, depending on the refuelling rate and degree of ventilation. However, temperatures dropped significantly with height. At the top of the stove/base of the flue, the temperature was approximately 200°C lower than in the fire box, and dropped by approximately 50°C per metre up the flue pipe. At the height corresponding to the ridge level (where the chimney would pass through the thatch), the outside surface of the metal flue during normal wood-burner operation was generally between 75 and 125°C.
During aggressive wood-burner operation, especially when the fire was vented beneath the fuel bed, temperatures in the wood-burner could reach 900°C. However, to maintain a temperature of 800 to 900°C typically required 4½ hours of extremely aggressive venting and constant refuelling (during which time it became impossible to approach the word burner without wearing protective clothing). The result was that, at ridge height, the external surface of the metal flue pipe reached 275°C, but the internal temperature of the chimney brickwork was 100°C lower.
Moreover, the external surface of the bricks (that is, the surface that would be in contact with the thatch) never reached more than 80°C, regardless of whether the wood-burner was operated with or without a flue liner. This is well below the temperature required to ignite thatch. This suggests that conduction of heat from the flue into the thatch via sound chimney brickwork and mortar is, on its own, unlikely to be a cause of fire in thatched buildings.
Ignition of thatch by convection
Cutting out a mortar joint or even a complete brick in the unlined experimental chimney, leaving the underside of the thatch exposed to the interior of the flue, did not result in ignition of the thatch so long as the chimney was unobstructed. However, once a partial blockage (such as a bird’s nest or soot accumulation) further up the chimney was simulated, hot flue gases were diverted through the defective brickwork and into contact with the thatch. Ignition soon followed.
Nesting birds
A bird’s nest significantly increased the risk of setting light to the thatch. When the hot flue gases came into contact with the nest, the twigs did not burn because there was insufficient oxygen inside the flue. Instead they were converted to charcoal in a process known as pyrolysis. As charring continued, the nest lost its structural stability and collapsed.
Charcoal is very light, so some of the smaller fragments were lifted up by the rising flue gases, and ejected from the top of the chimney. Immediately on contact with the oxygen in the air they burst into flames and rained down on the thatch surface. These burning fragments were much larger than the sparks emitted during normal or even aggressive wood-burner operation, and had sufficient energy to ignite the thatch very quickly.
There is often a spate of thatch fires if there is a cold snap in late spring, and it is possible that this may be related to nest building at this time of year.
CONCLUSIONS
The research has demonstrated that ‘heat transfer’ (conduction of heat from flue gases via sound brickwork) is very unlikely to be a prevalent mechanism of thatch ignition. It is clear that aggressive venting of a woodburning stove increases the temperature and velocity of flue gases, promoting ejection of sparks and burning brands from the chimney, some of which may have sufficient energy to ignite the thatch when they land on it. Stoves that allow aggressive venting can achieve higher operating temperatures and flue gas velocities and therefore can potentially increase this risk. Partial blockages in the chimney, whether due to soot accumulation or a bird’s nest, increase the risk of hot gases penetrating chimney defects and igniting the thatch. A bird’s nest in the chimney can also increase the risk of fire due to pyrolysis and ejection of burning nest material.
RECOMMENDATIONS
Additional research is needed before comprehensive advice on reducing the risk of thatch fires can be issued. In the meantime, the following recommendations for the safe use of wood-burning stoves will help reduce the risk:
- The wood-burner installed must be appropriate for the size of the space to be heated. Over-specifying the stove output increases possible operating temperatures.
- Wood-burners should always be lit and operated in accordance with the manufacturer’s recommendations. Models that have a separate ashpan door should never be operated with the ash-pan door open.
- Do not use paper or cardboard to help light a fire. Fire-lighters and dry kindling produce fewer sparks.
- Burn only fuels recommended by the wood-burner manufacturer. Do not use softwood, unseasoned wood or joinery/carpentry off-cuts, or use the fire to burn paper and card.
- Stay with the wood-burner during lighting and refuelling, and reduce venting once the fuel is alight. Maintaining strong ventilation during normal burning increases the temperature and velocity of flue gases and may increase the risk of thatch ignition by various means. Operating the wood-burner with a bright flame indicates that the fire is oxygen-rich and operating at higher temperatures than necessary.
- Install a stove pipe thermometer to ensure that the wood-burner is operated at the correct temperature. These indicate when venting needs to be reduced to lower temperatures and reduce flue-gas velocities. It also shows when the stove is being operated at too low a temperature, which can increase the accumulation of soot or tar on the inside of the flue, providing fuel for a chimney fire.
- A flue liner is recommended for all properties where the condition of the flue brickwork or mortar may be in question; this will reduce the risk of hot gases escaping from the flue into adjoining thatch.
- A bird guard or bird deterrent should be fitted to the top of the chimney to prevent birds nesting. The device must neither impair the operation of the flue nor be capable of becoming blocked, nor should it hinder thorough chimney sweeping.
- Chimney flues should be swept at least once a year, but more often if the fire is used frequently. Check requirements with your insurer.
- A camera survey should be undertaken every 12 months, even in chimneys that are fitted with a liner, to observe their condition and suitability for continued use.
- Increasing the distance between the top of the chimney and the surface of the thatch reduces the risk that sparks or burning material ejected from the chimney will ignite the thatch. A minimum distance of 1.8 metres between the top of the chimney and the thatch surface is recommended. This may be achieved by adding a chimney pot or additional courses of brick or stonework (or both). Rarely, removal of the most recent layers of thatch may be justified to help increase effective chimney height.
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Further Information
K Benjamin, ‘Fires in thatched buildings: A survey of 148 fires between December 2008 and May 2016’, Burgoynes, London, 2016
English Heritage, Practical Building Conservation: Roofing, Ashgate, Farnham, 2013
