Reprint from Thatchers Standard Issue No3 Autumn 2003
“A Timely Warning” by Marjorie Sanders, M.Phil, C.BioI, M.I.BioI, Churchill Fellow & Roger AngoId, Ph.D, M.A.(Cantab), B.Sc, C.BioI, M.I.BioI, FRMS
Any week now, the first cold snap of autumn will arrive together with the first of the season's thatch fires. Fortunately the research relating to causes of fires in thatch (carried out in the early 1990's) has been generally accepted and has not courted the controversy related to choice of materials and other issues affecting thatch longevity. Judging by the high level of fires in thatch as the very cold end of spring turned into summer this year, this message needs to be repeated over and over.
Prevention is essential! detection is almost always too late !
The major cause of preventable thatch fires can be associated with particular usage patterns of enclosed multi- firel stoves. It has been shown that houses with a deep thatch and a central chimney with a single brick skin are most at risk. Easy to recognise warning signs of potential problems have been identified. Unfortunately, attitudes to the risk of fire are largely based on anecdotal evidence and real hazards are underestimated; while other risks, for example, flying sparks from a chimney are negligible.
The provisional 1996 UK fire loss statistics (the most recent year for which data has been collected) show that 68,800 fires occurred in domestic dwellings; 68 of these were in thatched properties. This equates to one fire for every 320 dwellings compared to one fire in 735 thatched houses. This latter is because the owners of thatched properties are generally very fire-conscious. Insurance company and fire brigade records show that the outbreak of a fire in a thatched property is most likely to occur in early autumn or late spring. With the exception of arson, fires seldom occur when the property is unoccupied and are most frequently detected in the evening. Historically, these fires have been attributed to sparks from the chimney or burning brands. While the majority of fires are chimney related, it may not be fires in the chimney or sparks that start a thatch fire.
An Insurance company view: "Modern enclosed solid fuel appliances are designed to burn efficiently and cleanly. Stove manufacturers aim to increase temperatures to promote ‘clean-burn’, which is favoured by consumers. Chimney engineers recommend “the hotter the fire, the cleaner the burn” - that is, there will be fewer emissions into the atmosphere and tar deposits are less likely to occur within the flue. Solid fuel stoves can generate flue gas temperatures in excess of 300°C and as high as 600°C, which cool at a rate of only 1oC for every metre of chimney height.
When these appliances are fitted into elderly chimneys and there is only a four-inch layer of brick between the flue and the thatch, then the thatch will be especially vulnerable to the risk of heat transfer. Modem chimney linings, if faultlessly fitted, can prevent flue gas leaks, but they do not significantly reduce the temperatures achieved by heat transfer within the thatch when the appliance is in constant or regular use. It is useful to know that thatch adjacent to the chimney can reach 85% of the flue gas temperature after only one day of continuous use. Intermittent use of the appliance will not necessarily allow the temperature within the thatch to drop significantly between periods of use because deep thatch around a chimney is such a good insulator. The critical temperature of 200°C can be achieved and sustained with relative ease.
Traditionally, when combed wheat reed and long straw thatch is repaired or maintained, the existing thatch is stripped back to sound material and a new spar coat is fixed over the older layers. In this way the depth of thatch invariably increases over the years, and where the thatch abuts the chimney it can reach a depth of 1-2 metres. There can therefore be a considerable surface area where the thatch is vulnerable to heat transfer from the flue.
The research report has identified that homes most at risk from heat transfer are those with unlined single skin brick chimneys rising
centrally through a deep thatch that have been converted from their original use, serving open fires, to accommodate enclosed solid fuel
stoves.”
- 1 :735 John Albion Insurance (1999)
Appearance versus hazard safety
When the first chimneys were invented and even “retrofitted” to thatched buildings, the chimney top was designed to extend 6 feet above the ridge (it was believed this would allow sparks to either cool or be blown harmlessly away). Over time, repairs to the thatch and spar coating have added additional layers these have built up round the chimneys creating the potential fire hazards described above. Armed with this information it is easy to spot at risk thatch during any drive round traditional villages. Tar covered spark arrestors make the problem even worse.
Thatching long straw to a base coat. Note the old flashing still in position around the central chiney and the progress of the new
ridge above the existing ridge in further reducing the chimney height above the thatch. If this chimney is in active use the risk of fire in
the thatch has been significantly increased (English Heritage, thatch & thatching - a guidance note (2000)).
Conservation officers are now assiduously implementing the English Heritage Thatch & thatching: a guidance note (2000). Much weight is given to preserving materials and styles by replacing like for like. Whatever the strengths and weaknesses of this policy one fundamental issue of building protection is overlooked: the consideration of sensible fire prevention strategies.
The guidance note does refer to building regulations and fire safety including the care that is needed in the use of multi-fuel stoves. The section on fire safety, concludes “It would be difficult at this juncture to summarise all the work which is going forward, but the key to safety in thatched buildings lies not in changing the behaviour of the thatch itself but in sensible assessment of the risks, particularly those which modern life brings into old buildings”. Further on it is recommended that “chimneys, where they project into coatwork, should be carefully examined for soundness, especially below the level to which the thatch will rise after re-thatching and where necessary locally repaired.”
Validated information is available to conservation officers to understand the mechanisms of heat transfer through chimneys into the thatch; it is their behaviour that needs to change when specifying design, material and methods.
Priority should be given to limiting the depth of thatch close to chimneys. This should never exceed one metre. Protection of the thatched heritage from fire risk should be more important that the ephemeral nature of the material used in its maintenance. Once a listed property has been severely fire damaged, why bother to restore it in the previous style and materials? However, it should carry a health warning of the dangers inherent of not moving with the times.
An early 17'" Century listed property being re-thatched in Sept. 2f/O3. Material and style have local conservation officer approval.
flexible metal liner was installed August 2002. Whatever happened to cleaning down to a firm base for spar coating and a replacement flush
ridge? Is this property moving further into the “at risk” category as a result of these “improvements”?
The same property 26th January 2006. Fire fighters did a brilliant job in damage limitations. 15 fire appliances and 80 firemen were
required to bring the fire under control. The fire is definitely chimney related, the cause is possibly an inappropriately installed
flexible metal chimney liner. Another avoidable and tragic loss. The Society has set up a working party jointly with the fire authorities,
chimney engineers, insurers and conservators to reduce this threat to our thatched heritage.
This re-thatch took place in September 2003. In the “Standard” No 3, page 24, there is also a photograph of this chimney taken during the re-thatching process. The caption includes the question An early 17th Century listed property being re thatched in Sept. 2003. Material and style have local conservation officer approval. A flexible metalliner was installed August 2002. Whatever happened to cleaning down to a firm base for spar coating and a replacement flush ridge? Is this property moving further into the “at risk” category as a result of these ‘improvements”? The same property 26th January 2006. Fire fighters did a brilliant job with damage limitation. 15 fire appliances and 80 firemen were required to bring the fire under control. The fire is definitely chimney related, the cause is possibly an inappropriately installed flexible metal chimney liner. Another avoidable and tragic loss. The Society has set up a working party jointly with the fire authorities, chimney engineers, insurers and conservators to reduce this threat to our thatched heritage. “ Is this property moving further into the “at risk” category as a result of these “improvements”? This months front cover, unfortunately, provides the answer.
Conclusion
After a year the property was re-thatched and re-furbishes, drying out of the plaster meant that the owners were another three months before they could move out of rented accommodation back into their home. The restoration has transformed the property into the perfect homes and gardens idea of the rural dream. However during the Insurance assessment the cause of the fire was not identified or indeed rectified, which means the inappropriate badly installed flue liner is still in place and still has the potential to repeat the fire event. The owners are aware there is a problem and do not use the fire where the problem occurred. However, in September 2009 the property has gone on the market, described as recently refurbished to a very high standard with special features of an inglenook fire place and a wood burning stove. Under the present circumstances chances are new owners will fall in love with the property and in particular the special features and will be totally unaware of the danger still lurking in the chimney.
This is not an isolated incident where a property is repaired at the insurers expense with the problem still inherent in the building. There may well be a case here for insurance companies to ask the question regarding the past fire history of any new property they are asked to insure on proposal forms, and to follow through the cause and remediation that has taken place.
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