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CICO Chimney Linings Ltd

CHIMNEY RELATED FIRES IN THATCHED PROPERTIES

On safety grounds all stoves, open fires and other solid fuel appliances must be installed correctly by competent installers, and used as intended with the correct fuel, in all properties. However thatched properties pose particular concerns which require specific, added safety measures to minimise the risk of fire.

In recent years accepted opinion as to the cause of chimney related fires in thatch has changed somewhat.

Traditionally it was thought that the majority of such fires were started by embers and sparks landing on the thatch, either in normal use or as a result of a chimney fire.

However, whilst these remain as danger areas, recent research and investigations by RHM Technology have revealed that a far more common cause is heat transmitted from within a chimney serving a solid fuel stove, through the brick of the chimney itself, to the underside of the thatch. In controlled tests scientists at RHM Technology proved that charring and slow combustion can occur in thatch surrounding a chimney at a temperature of only 200 degrees C.

A single skin of brick can, after a length of time, allow as much as 85 percent of the flue gas temperature to pass to the outer surface of the chimney and into the thatch that surrounds it. When you consider that modern high efficiency stoves can generate flue gas temperatures in excess of 300 degrees, and as high as 600 degrees in some cases, you see that the critical temperature of 200 degrees could be achieved and sustained with relative ease.

The situation is compounded by the common practice of overlaying existing thatch with new when repairs and maintenance are carried out. This leads to an increase in thatch thickness over the years and it is not uncommon for thatch next to a chimney to be as much as 1-2 metres thick. This hugely increases the surface area of thatch in the critical area around the stack.

Furthermore, the condition of the chimney brickwork within the thatch layers is usually the worst in the chimney as it is not easily accessible for maintenance, and is situated just below the cooling point of the chimney and thus suffers from internal corrosion as well. Obviously poor brickwork will allow even more temperature transmission in this critical part of the chimney.

By taking sensible precautions, using appliances correctly with the right type of fuel and with properly installed flue liners in place, you can minimise the risk of a chimney related fire in your thatched property. In particular the following points should be borne in mind

  1. The installation of a solid fuel appliances and flue liner is subject to Local Authority Building Control, and must be carried out by a competent person using suitably tested and certified materials to a safe standard of work. Appendix J (Document J) to the Building Regulations gives detailed advice on all aspects of this work, including requirements for air supply, distance from combustible materials, chimney termination heights, flue lining methods, hearth sizes and other related matters. The installer must fully understand the requirements of the work he/she is undertaking.
    As an alternative, solid fuel organisations such as HETAS run approved competent person schemes where trained installers can self-certify their own work, often to higher standards than those laid down in Document J.
  2. Have your installation checked periodically by a competent, qualified person, accredited by a suitable organisation such as HETAS or similar. The condition of flues and brickwork can be examined by sophisticated CCTV camera equipment to find any defects within the installation. We would recommend that such an examination is carried out annually.
  3. Have your flue cleaned by a qualified, competent chimney sweep accredited by the Guild of Master Sweeps or the National Association of Chimney Sweeps at least twice a year where coal is the fuel, and even more frequently where wood is burnt.
  4. Ensure appliances are maintained in good order, and are installed and used in accordance with the manufacturers instructions. Stoves should not be run in a slumbering, shut down condition as this promotes soot and tar build up which increases the risk of a chimney fire.
  5. Fit a flue thermometer to the outlet pipe of your stove to allow you to manage the way the stove is used and keep the temperature to a reasonable level.
  6. Efficient combustion requires a good air supply. A dedicated air vent of the correct size should be fitted into any room containing a stove or open fire.
  7. Make sure combustible material is kept well away from the fire, flue, canopies and any connecting stovepipes. There must be a non-combustible hearth extending at least 300 mm in front of the fire or stove. Do not store logs close to a stove. The back of a wooden Bessemer beam must be well away, and be properly sheathed and protected from any heat source. Document J gives minimum allowable distances.
  8. Fit smoke alarms at strategic points around the chimney stack, particularly in the loft space if there is one, as high up and close to the apex of the roof as possible. Loft smoke alarms should be linked to those within the habitable part of the property. Smoke alarms should be tested regularly.
  9. Only use correctly seasoned, dry deciduous wood from a reputable source. Do not use resin-rich softwoods or building timber as these spark easily and also lead to increased tar build up. Never use a stove to burn household rubbish, including paper and cardboard.
  10. A suitable flue liner, of the correct size, must always be fitted to a fire or stove in a thatched property. The installation should be carried out by a competent person, accredited by an organisation such as HETAS or similar. Materials used must be tested and certified as fit for purpose.
  11. Although there are a number of different methods of lining a flue, only those that are rigid, insulated, and capable of being correctly supported and centralised are considered suitable for thatch. Thus flexible stainless steel is generally no longer accepted as there is very little control over its route within the chimney once fitted, leading to the possibility of touches against the chimney brickwork. Where there is no alternative other than to use a flexible liner it must be fully bracketed, centralised and supported along its length so as to remove the possibility of touches. It must also be properly insulated with a positive barrier to minimise heat loss.
  12. .As an incorrectly fitted flue liner can actually lead to a chimney or thatch fire we would repeat our earlier advice to use only accredited competent installers.
  13. The uppermost termination point of the flue should be at least 1.8 metres (6 feet) above the thatch level, and at least 2.3 metres ( 7 feet 6 inches) horizontally from any thatched roof surface. Where the termination is too low it is not acceptable to fit a tall pot to the top of the chimney as this promotes flue gas cooling and tar build up. Therefore raising the stack height is often the only way to meet this criteria. As the aforementioned practice of adding layers to the existing thatch when repairing also raises the thatch height it should be avoided, and thatchers should always be aware to keep the thatch height as low as possible.
  14. Spark arrestors should not be fitted as they can, in time, become a fire hazard when they clog up with tar and soot from the flue gas. The best termination is a straight, open chimney pot with nothing to obstruct the escape of flue gas.
  15. The space between a flue liner and the inner chimney wall (the annular space) should be adequately vented top and bottom so as to allow cooling air to remove heat before it is transferred into the brick.
  16. The condition of chimney brickwork should be periodically checked, and maintained at every opportunity, particularly when hidden brickwork is exposed during re-thatching. Damaged or eroded brickwork should be replaced and made good, and if necessary the upper stack taken down and rebuilt through the thatch area.
  17. Where no chimney exists, such as in a thatched barn, twin wall insulated rigid chimney systems are normally used. Advice must be sought from the supplier or manufacturer before fitting and the normal air gap of 50mm around these systems must be substantially increased through the thatch, which must also be properly sheathed by a purpose made shield unit. Again, only accredited competent installers should be used.

CICO operate a national chimney lining network installing the full range of flues, liners, and other chimney related services. Most branches offer a full inspection service including thatched properties by CCTV if required. Please fill in the contact request form if you require this service.

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