Thatched roofs and chimneys
NOW AVAILABLE:
Chimney temperature monitoring system. This simple to operate system provides an early warning to the dangers of high temperatures in the brick work adjacent to thatch. See text below for more information.
Traditional thatch maintenance techniques often require a spar coating to be applied over existing thatch. Under these circumstances, and over a long period of time, the depth of thatch can reach up to 2 metres. 1 metre is not uncommon.
Under these conditions the single skin of a brick chimney 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. 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. Poor brickwork will allow even more temperature transmission in this critical part of the chimney.
Once a sustainable temperature of over 200~C has been reached within the thatch, conditions are favourable for the first stages of slow breakdown and charring. 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.
Because these conditions occur within the deep body of thatch, a fire generated in this way may be well alight before discovery. Prolonged high temperatures can lead to the ignition of thatch even with an intact chimney. Where a faulty flue allows gases to escape the risk is increased.
It is recommended to monitor the temperature of flue gases by placing a magnetic thermometer onto the flue at about eye level, so that excessive temperatures can be controlled.
A more advanced system of flue temperature monitoring is available from us, which is a must for owners of such properties. it consists of a probe that is drilled into the chimney brickwork as close to the thatch as it can be. This is connected to a control unit in the loft (so a power supply is requird in the loft) which then has a telephone type cable running down to a display box fitted in or near the fireplace. The display box uses a traffic light principle for warning of too high a temperature in the chimney brickwork. A constant digital read out is visible too. Should the temperature get too high ( set limits are pre-programmed) then a two stgae alarm is sounded. The system also has a battery back up in the event of power failure. Priced at £450 + delivery it could save a fortune.
Therefore, chimneys in thatched houses should have an insulated rigid liner, and, in the case of solid fuels, be swept twice a year for coal burning fires/appliances and quarterly if wood is burnt. Soot and tar from wood fires builds up rapidly. Tar not only acts as a fuel for any chimney fire but will also degrade old lime-based mortar in the stack itself, increasing the risk of fire.
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. 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.
Spark arrestors on the flues should be treated with caution because they can clog and restrict the flow of flue gases. They MUST be cleaned regularly. The best termination is a straight, open chimney pot with nothing to obstruct the escape of flue gas. This photo shows a combination of spark guard and "UFO" anti-downdraught cowl.... not an ideal combination for this type of roof.
Good quality smokeless fuel is preferable to ordinary coal or peat and any wood should be resin free (hard woods) and very well seasoned and dry to prevent the build up of tars.
Any building work requiring ladders or scaffolding resting on the thatch, in particular the ridge, should be padded (for example rolled old carpet). This will minimise possible damage. Any building work that will result in rubble or mortar dust being shed onto the roof should have that area completely covered for the full length of the roof so that no building materials, such as paint or mortar, become lodged in the thatch. The lime in mortars is particularly harmful.
Always follow the advice of your insurance company. They may place certain restrictions on the use of open fires and stoves within your property.
Check points
§ 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.
§ 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.
§ 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.
§ 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.
§ 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.
Flue Lining
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 it touching 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 touching. It must also be properly insulated with a positive barrier to minimise heat loss.
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.
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.
