This talk is an extract from the Understanding Thatched Roofs course that Charles Chalcraft presents for RICS Training in Hampshire and in Suffolk. He also presents this course in Devon in a 15th Century thatched Church House. This is the only course in the country that trains surveyors how to survey thatched properties.
The Thatched Cottage is a dream that many people long for, it is a part of our history, it is an essential part of our landscape that attracts tourists from all over the world to our shores. Apart from thatch, roses round the door and the rural dream, the romance is incomplete without flickering firelight from the cast iron goddess of heating - the woodburning stove.
Former Chief Executive Marjorie Sanders and Dr Roger Angold former Chief Scientific Officer of the National Society of Master Thatchers have been collecting data over the last 17 years of researching fires in thatched buildings. The staggering statistic is the equivalent of 1 thatched house fire per week over the 17 years.
The annual insurance cost of chimney related fires alone in thatched properties exceeds £20,000,000.
This is a major loss of heritage properties, once rebuilt they are never quite the same.
This is the short title for this talk, it should include architects, loss adjusters, insurance companies, LABC inspectors, conservation officers, valuation surveyors, building surveyors, English Heritage, home owners and installers...... the list is extensive. We can all reduce the numbers of fires in thatched buildings when we understand the root causes. We need to protect our heritage.
Background: 70% to 80% of thatch fires are caused by multi fuel and woodburning stoves. Before these stoves came along, houses were heated by open fires that cooled down every night; there was a large chimney that cooled the smoke that eventually came out of the open stack.
The main cause of thatch fires is by heat transfer from the flue gases through the chimney liner that may be touching the brick flue that is surrounded by highly insulating thatch.
Modern woodburning and multi fuel stoves produce flue gases that can reach 600°C. Research has found that wheat straw will char when heat transferring through brick reaches between 180°C and 200°C, creating a "hotspot" within the thatch. Fire will take place where there is sufficient oxygen available. Fires within chimneys are caused by the use of wet, unseasoned logs, poor maintenance and unswept chimneys.
Electrics are the next major cause with cables that are not in conduit or sheathed in the loft that are being eaten by vermin; downlights in upstairs ceilings mostly above bathrooms; open light fittings in lofts; halogen outside lights located just below the eaves and fires originating in the house.
Other causes include:- workmen/plumbers; bonfires and arson.
Chimneys: Many of the chimneys are built with handmade bricks and lime mortar. With no protection from the rain, gradually some of the mortar on the inside of the chimney gets washed out. Tar will travel through the bricks. The outside of the stack above the roof will be maintained occasionally, the base of the stack in the loft may receive some attention but the part within the thatch will only be visible if and when the thatcher opens it up. There will have to be a good reason for doing this and most of the time it will remain without maintenance. Successive re-thatches deepens the thatch which creates greater insulation around the chimney and also reduces the height of the chimney above the thatch. Building Regulations B recommends a height of 1.8m for the terminal or pot above the ridge with a pot not exceeding 600mm. Spark arrestor cowls are not recommended. This twin cylinder of stainless steel mesh needs to be removed twice a year for cleaning. They become choked up and in the event of a chimney fire they can shower the roof with red hot stainless steel.
Chimneys at greatest risk are central chimneys that are surrounded by thatch, that on a multi coat roof that can be between 1m to 3m deep. This depth of thatch insulates the chimney.
Chimney Liners: In December 2012 the British Flue & Chimney Manufacturers Association issued a guidance that "Single skin liners must never be used with wood or multi-fuel applications"...........and that "for dwellings with a thatch or a combustive roof ... require specialist attention and should only be worked on by experienced installers. Attention is drawn to the HETAS guide on thatched properties. The home owner’s insurance company must be advised of proposed works."
Skin flexible stainless steel liners do not fit the purpose i.e. they do not provide adequate thermal separation from exhaust gases to the existing flue. There will be instant heat transfer into the surrounding fabric of the chimney.
The greatest risk is existing and new DIY installations of single skin flexible stainless steel liners, where they snake their way up the flue touching the brickwork. These may or may not be backfilled with micafil or equivalent granular insulation. There is no way to know without breaking open the chimney if the liner is touching the sides or not.
Be aware that the insulation in old twin wall steel liners decays yielding similar problems.
Suggested suitable liners include: pumice; clay; ceramic and concrete liners and new twin wall insulated steel all backfilled a suitable granular insulation.
Woodburning and multi-fuel stoves: Modern woodburning and multi fuel stoves produce flue gases that can reach 600°C. Continuing on the theme of the cast iron goddess of heating. Stoves are often selected as the focal point of the room and where there is a large inglenook fireplace it will naturally require a stove that is in proportion to the fireplace, with pleasing mouldings and shape, perhaps with an additional cowl. However there seems to be little matching of stove output of heat to the size of room. This leads to having to have windows open to get rid of surplus heat. It also pumps considerable quantities of very hot flue gases up the chimney. I recently surveyed a property where the size of room required a stove with an output of 4.2Kw. The delightful Norwegian stove in the inglenook was a multi-fuel rated at 11Kw, with a single skin flexible liner!
Electric cables in the loft should comply to British Standard 6207 Part 1 (1995) “Wiring systems using mineral insulated cable or PVC cable should run in conduit conforming to any of the following BS31 (1940), BS 4568 Part 1 (1970) or BS EN 50086 Part 1 (1994). For more details download (ESC) Electrical Safety Council Technical Guidance “Electrical installations in Thatched Buildings”. Cables should run along the middle of the loft away from the tops of the walls and conduits should be attached to the inside face of rafters or purlins. This will place them out of the way of sharp thatcher’s shearing hooks and crooks.
All cable circuits in the roof space should be covered by an RCD not exceeding 300mA.
Wire netting will be found on many roofs to prevent bird from pulling out the thatching material. There is a special way of fitting netting so that, in the event of a fire, it can be removed very easily. No electrical source should be nearer than 400mm and the netting should not the nailed or stapled into the wall plate. Hazel spars will hold netting tight when driven up into the thatch under the eaves; they pull out easily as required.
Fire Precautions: There are a number of precautions that can be put in place to reduce the numbers of thatch fires. Home owners have the most to lose – apart from the loss of their home and some personal items, the average time to live in a caravan following a fire and rebuilding the house is 2 years. It boils down to common sense, using dry hardwood logs (minimum of 2 years old or less than 20%moisture), no nearby bonfires, complying with ESC, HETAS and the Dorset Model recommendations. Chimney heat baffles can be fitted; these are 6mm sheets of aluminium that form a heat barrier between the outside of the chimney and the thatch. There should be smoke detectors in the loft that are linked to the other smoke detectors in the house. You can now buy chimney heat monitors that are fitted to the outside of the brickwork within the thatch that will warn homeowners about raised temperatures in the flue. There are automatic fire extinguishers that are positioned in the loft. Chimneys should be swept at least two times a year. There is now a recommendation that all woodburning and multi fuel stoves should be checked by a HETAS engineer once a year. (get a Fire Safety Assessment).
The Dorset Model: This is part of Building Regulations Part B and requires new dwellings to fit a fire barrier. The rafters are overdrawn with a minimum of 30 minute fire barrier (integrity & insulation), the barrier to be water resistant. 50 x 25 battens recommended on micro-porous board to allow the thatch to breathe. The thatch is treated as being sacrificial – this is where the roof burns and the fire does not enter the house unless the barrier is damaged. The use of flexible material or cavity foam as a fire resisting barrier is not acceptable for the Dorset Model.
Fire Barriers: In practice there are fire barrier boards and there are flexible fibreglass based fire barrier membranes. You may find rockwool thatchbatts fixed between rafters and there is a chemical fire retardant that is sprayed onto the outside surface of the thatch that slows ingress of oxygen in the event of fire. Unfortunately there are some fire curtain products that are marketed as suitable for use as a fire barrier under thatch. These tend to disintegrate at around 800ºC, when thatch burns at over 1000ºC and they are not micro porous.
Survey recommendations to purchasers.
Establish type, suitability and condition of the chimney liner – this may require invasive procedures including CCTV inspection.
Establish the stability and condition of the chimney stack especially between the thatch and the chimney.
Calculate suitability of existing stove to the size of room.
Electrical cable survey of loft. – re wire in conduit.
Fit 300mA RCD for all circuits in roof space.
No downlights in upstairs ceilings, if present then they require intumescent hoods, exchange for LED lights.
Ideally no woodburners/multi-fuel stoves.
No spark arrestors.
Chimneys swept twice per year.
Woodburner checked once per year by Hetas engineer.
Understanding Thatched Roofs Courses:-
HETAS; ESC; Dorset Model etc:-
National Society of Master Thatchers: http://www.nsmtltd.co.uk