Reducing the Risk of Fires in Thatched Properties with Wood-Burning Stoves
Evidence strongly suggests that the growing popularity and use of wood burning and multi-fuel stoves have been responsible for an increasing number of fires in thatched roof buildings.
Moreover, recent studies have shown that these types of stove are more likely to cause fires in thatch roofs than traditional open fires. How and why have been a source of contention.
A widely-held view was that sufficient heat could be transferred from flue gases by conduction to the exterior of the chimney stack to set light to the surrounding thatch layer. However, evidence gathered by forensic investigators showed that many fires could not be explained by this ‘heat transfer’ theory. For instance, in some cases fires started shortly after the wood-burner was lit. In others, fires occurred where an insulated flue liner had been installed in the chimney. It was clear that further research was needed to understand the causes of fire in thatched buildings with wood-burning stoves.
New research funded by Historic England
In response, Historic England and NFU Mutual Insurance Society Ltd funded the Fire Protection Association (FPA) to take a fresh look at the relationship between wood burning and multi-fuel stoves, chimneys and thatch fires, and recommend ways to mitigate risks.
The three-year programme of research, which included full-scale fire tests, provided new insights and disproved the ‘heat transfer’ theory. Instead, it demonstrated that thatch could be set alight by high-energy sparks and embers ejected from the chimney, as can happen when a bird’s nest has been built in the flue. Flue-top devices described as ‘spark arrestors’ appeared to have little or no impact on spark mitigation, and might even encourage emitted sparks to remain in close proximity to the thatch.
The research also demonstrated that fires could be started by hot flue gases leaking into the thatch layer through defects in the chimney stack. In addition, it was found that some stove designs are inherently safer than others.
New guidance to reduce fire risk
As a result of these findings, this new guidance on measures to reduce the risk of fire has been produced.
The fundamental recommendation is that wood burning and multi-fuel stoves should not be used in thatch roofed buildings.
But where these types of stove are being used, the guidance lists actions that can be taken to reduce the risk of fire. These actions should be taken together, in addition to complying with the stove manufacturer’s operating instructions, building regulations requirements, and any insurance policy conditions.
Sweep chimneys frequently to prevent build-up of tar and soot that can lead to chimney fires
Chimney fires endanger thatch roofs in several ways: they can result in extreme chimney temperatures, the emission of flames from the chimney top and the ejection of burning material. The frequency of chimney sweeping depends on the fuel being burned and how often the stove is used. For example, the chimney serving a stove where wood is burned every day during the heating season might need to be swept four times a year.
Check that the distance between the top of the chimney pot and the thatch is sufficient
Sparks and embers emmitted during normal stove operation are generally low in energy and short-lived. The further they have to travel between the chimney top and the thatch, the less likely they are to cause a fire. Building Regulations Approved Document J (page 32; para 2.12) gives guidance on minimum chimney pot top to thatch distances. In some situations it may be advisable to increase the transit distance of spark to thatch, for example, by increasing the height of the pot top or reducing the thickness of the thatch, as appropriate. But the acceptability of this may be limited in listed buildings or conservation areas. In each case a balanced judgement will have to be made between the impact on significance and the public benefit of not losing a heritage asset to fire. Where the chimney top to thatch distance is insufficient and it is not acceptable to increase it, the use wood burning and multifuel stoves is not advisable.
Fit bird guards to prevent nest building in chimneys
When a wood burner is lit, a bird’s nest built of twigs inside a chimney will catch fire and be ejected from the chimney as heavy, high-energy burning embers. These embers are long-lived and can set thatch alight, even after travelling a considerable distance from the chimney top. Sweeping alone is not sufficient to eliminate this hazard, as heating and nest-building seasons overlap. Therefore, a mesh-topped bird guard of a type that will not impair the function of the chimney, be liable to blocking, or impede effective sweeping should be used.
Install a suitable flue liner
Defects in chimney stacks, such as open joints, cracks and missing bricks, which allow hot gases and sparks to escape into the thatch are another proven cause of fire. It is advisable for a CCTV inspection of the flue be carried out to identify defects. The threat can be eliminated by lining the flue. Ideally a rigid, twin-walled insulated stainless steel flue liner should be installed. But when access or chimney shape makes this impossible, a twin-walled flexible stainless steel liner should be used instead. In listed buildings, the impact on significance of installing a flue liner will have to be assessed and balanced against the benefit of reducing the risk of fire. Where it is not possible to install a suitable flue liner, the use of wood burning and multifuel stoves is not advisable.
Take care when lighting the stove
When lighting the stove, controls may be set to provide the maximum ventilation to help get the fire going. This creates conditions where unsuitable kindling materials, such as paper and card, can be lifted from the fire and ejected from the chimney top. To reduce this risk only firelighters and wood kindling should be used. Stoves should NEVER be used as incinerators to burn waste paper and rubbish. Stoves should never be left unattended when lighting, and the ventilation controls should always be returned to the normal operational setting as soon as the fire is alight.
Take care when refuelling the stove
When refuelling the stove, controls may be set to increase ventilation and boost the fire. It is essential that controls are returned to their normal setting as soon as the added fuel is alight. Stoves should never be left unattended until this is done. Failure to control ventilation will result in very high fire box and flue temperatures, as well as high flue gas velocities. This can lead to chimney fires (if tar and soot are present) or burning embers being lifted from the fire and ejected from the chimney top. In unlined flues, aggressive burning also increases the risk of hot gases escaping into the thatch through defects in the chimney stack.
Fit a stove pipe temperature gauge
If a stove is operated at too low a temperature, tar and soot may be deposited in the chimney and provide the fuel for a chimney fire. Operating temperatures that are too high can lead to chimney fires if soot and tar have built up. It can also lead to burning material being ejected from the chimney and increases the risk of hot gases and sparks escaping into the thatch through chimney defects. A stove pipe temperature gauge will enable you to ensure that the stove is working within safe limits. All members of the household (and guests) should be aware of the meaning of the gauge sections and know how to control the stove to maintain safe operating limits.
The report describes the investigations carried out, presents the findings, and recommends actions to mitigate risks. In addition, it suggests features that should be incorporated in an ‘ideal’ stove installation.
The Fire Protection Association, NFU Mutual Insurance Society Limited and Historic England have published a new guidance pamphlet for owners/occupiers of thatched buildings who use wood burning and multi-fuel stoves. It provides practical guidance on measures to reduce the risk of fire.