Thatched properties and wood-burning stoves

Wise after the event? Following a chimney-related fire that destroyed the roof of a house in Leicestershire, reinstatement works included increasing the height of the chimney stack and fitting a bird guard to the pot (Photos: N Hill © Historic England).

The main recommendation of new guidance on preventing fires in thatched properties is that wood-burning and multi-fuel stoves should not be used in thatch-roofed buildings.

[edit] Introduction

Since the 1990s the number of fires in thatch-roofed buildings has risen significantly. In the past decade, more than 500 thatch-roofed buildings in England have been damaged or even destroyed by fire. Evidence strongly suggests that this is related to the increasing popularity and use of wood-burning and multi-fuel stoves. Recent studies have shown that these types of stove are more likely to cause fires in thatch roofs than any other form of heating, including traditional open fires.

How and why this occurs has long been a subject 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 within the surrounding thatch layer to set light to it. From the late 1990s this idea shaped much of the advice given by fire and rescue services, insurers, local authorities and industry on preventing fires in thatch roofs. But by the late 2000s, evidence gathered by forensic investigators showed that many fires could not be explained by this heat-transfer theory. In some cases fires had started shortly after the wood-burner was lit. In others, fires occurred where an insulated flue liner had already been installed in the chimney. It was clear that more research was needed.

In 2014 Historic England’s building conservation and research team and the NFU Mutual Insurance Society Ltd commissioned 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 to find ways to reduce risks. To assist in this, full-scale fire tests were carried out at the FPA’s laboratory in Moreton-in-Marsh, Gloucestershire.

Two full-size test rigs were used to obtain data on flue-gas temperatures and velocities during stove operation, and explore all potential methods of heat transfer and fire-raising. Aspects studied in the course of the two-year research programme included:

stove, flue and chimney design
fuels; methods of lighting; methods of refuelling
methods of heat transfer through chimneys
factors affecting spark emission from chimneys
ember transfer and performance of spark arrestors
effects of damaged flue liners and defective chimneys
effects of blockages due to soot and tar build-ups and birds’ nesting
investigation of near-pot-top fires as a means of fire-raising
occurrence of thatch fires (times of year and weather conditions).

Potential methods of heat transfer and fire-raising were investigated. These included:

1. Direct spark ignition.

2. Near-pot-top chimney fires.

chimney fire (ignition of deposited soot or tar)
bird’s nest or other blockages

3. Convection: escape of hot flue gases into thatch through defective mortar joints or masonry in the chimney stack.

unlined chimney without restriction or blockage near top of flue
unlined chimney with restriction or blockage near top of flue
chimney with ‘perfect’ uninsulated flue liner
chimney with defective flue liner

4. Conduction: transfer of heat from flue to thatch by conduction through the chimney stack.

unlined chimney
chimney with ‘perfect’ uninsulated flue liner
chimney with broken uninsulated flue liner
chimney with uninsulated liner touching brickwork
insulated versus uninsulated flue liners

The research findings proved that heat transfer by conduction was not the primary cause of fire in thatch roofs. With an unlined flue, even the most aggressive and prolonged operation of the stove failed to raise the temperature at the interface between chimney stack and thatch to a level that would cause ignition. Instead it was 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), and by hot flue gases leaking through defects in the chimney stack.

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. In addition, it was found that some stove designs were inherently safer than others. As a result of these findings new guidance on measures to reduce the risk of fire has been produced. The research report also suggests features that should be incorporated in an ideal stove installation.

[edit] New guidance

The fundamental recommendation to come out of this research is that wood-burning and multi-fuel stoves should not be used in thatch-roofed buildings. Where these types of stove are being used, the actions listed below, if taken together, will reduce the risk of fire. These precautions should be taken in addition to complying with the stove manufacturer’s operating instructions, building regulations requirements, and any insurance policy conditions.

1. Sweep chimneys frequently to prevent buildups 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 required 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.

2. Check that the distance between the top of the chimney pot and the thatch is sufficient.

Sparks and embers emitted 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 distances between chimney pot top and thatch. 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.

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 distance between chimney pot top and thatch is insufficient and it is not acceptable to increase it, the use of wood-burning and multi-fuel stoves is not advisable.

3. 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 should be used of a type that will not impair the function of the chimney, be liable to blocking or impede effective sweeping.

4. 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 to 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. 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 multi-fuel stoves is not advisable.

5. 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.

6. 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, and 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.

7. 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 it 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 full research report 'Fires in Thatched Properties with Wood-Burning Stoves' and the new guidance pamphlet for homeowners is available from https://historicengland.org.uk/advice/technical-advice/emergency-and-fire/fire-advice/reducing-fires-in-thatched-roof-properties/

This article originally appeared in IHBC's Context 157 (Page 33), published in November 2018. It was written by Iain McCaig, a senior architectural conservator working in Historic England’s building conservation and research team.

--Institute of Historic Building Conservation