Cover Image
close this bookThe Basic of Biomass Roofing (GTZ - ITDG - SKAT - CRATerre-EAG, 1997, 36 p.)
View the document(introduction...)
View the documentIntroduction
View the documentGrass thatch
View the documentPalm thatch
View the documentWood tiles (shingles and shakes)
View the documentRoof sheets with organic fibres
View the documentTreatment of biomass materials: preservation
View the documentFire protection
View the documentSources of further information
View the documentKey questions
View the documentRecommendations
View the documentBASIN
View the documentBack cover

Fire protection

Fire is a serious disadvantage with all roofs covered with organic materials. Apart from sandwiching the roof covering between two incombustible layers there is no way of making them completely fire proof. Thus thatch and shingles are rarely permitted for closely spaced urban buildings and organic based sheet materials must conform to flame penetration and flame spread standards.

However, various techniques have been proven to be effective against the most likely sources of roof fires.

Building design

Electrical wiring should carried out by trained contractors. It must always be properly insulated and earthed. Particular care must be taken if cables pass through roof voids and attics which are rarely visited as rodents and insects may damage insulation, so metal or reinforced plastic conduit should be used.

Chimney design must isolate hot flue discharges and must release them at least one metre above the highest point of the roof. Chimneys should preferably be built of masonry and roof timbers should not be built into the chimney.

Cooking arrangements must minimise the chance of accidental fire.

Buildings should be protected with lightning conductors.

An outside tap permanently fitted with a long hose-pipe is advisable.

These are common-sense precautions which also have to be complemented by fire-conscious household activity.

It is also important to realise that skilled workmanship is likely to produce a roof covering that is less fire prone. This is particularly evident in the case of thatch; poor workmanship is likely to produce a loosely laid thatch which is more combustible than a densely compact layer of thatch.

In addition, there are several techniques of minimising the combustibility of the roof material and for reducing the likelihood that a fire will take hold in an organic roof.

Surface coating

Various methods have been devised to cover thatched roofs with an incombustible coating. In India this was done with a cement/sand slurry, or a bitumen-stabilised earth plaster. But these add to the cost of the roof, increase its weight and completely hide the original roof. Their major drawback is that they crack, thus letting in water which causes more rapid decay of the thatch.

Chemical treatment

Most fire retardant chemicals are water soluble; insoluble ones are much more expensive. Both types are rarely used because they are uneconomic. Strictly speaking, most chemicals act as flame retardants (reducing the intensity of fire) rather than fire proofing agents.

Dipping is the best way of treating roof materials, be they thatch or shingles. But these are washed off the surface within a year and humidity changes can leach the chemicals out of the material.

Borax and boric acid, diluted in water, are the least expensive, safest and most widely used chemicals. When a fire starts on a material treated with these substances they fuse to form a vitreous layer over the combustible surface and so cut off the supply of oxygen for combustion. However the soluble salts are leached out of the dry material within the roof coating by changes in ambient humidity.


Fire-protection for external fire-hazards

Treatment cost depends on a number of factors, but can be expected to add up to 50% to the cost of the roof covering.

Incombustible underlay

Fire needs a continuing oxygen supply. Partial reduction of combustibility is achieved by fixing a fire proof lining (either metal foil sheet or gypsum/cement boards) under the roof, nailed to the rafters. The effect is to isolate the roof from the interior of the building. It is important to ensure that the thatch or tile covering is still ventilated so that a damp roof can dry rapidly. This type of underlay is obligatory for thatch in several European countries where fire risk is predominantly from inside a building.

Sparge pipes

Perforated metal pipes fixed to the top of each side of a roof and connected to a water supply controlled by a tap at ground level extinguish roof fires very effectively and can be used to damp a roof when there is a threat of fire. They are obligatory in parts of Southern Africa. But they require water pressure capable of delivering over 2 litres of water per m2 of roof area per minute - more than the average domestic supply rate - so special arrangements have to be made.

Conclusion

Fire retardant treatment is desirable for organic roof coverings. As chemical treatments are likely to be very expensive, the best approach is fire conscious design of these buildings combined with common sense, especially when there is an increased risk of a fire.

In communities which have many houses roofed with combustible materials it is prudent to have a local fire-fighting service and emergency plans should fire break out.

Further Information:

Elkins G.H.J.; The reduction of the Fire Hazard of Thatched Roofs; Fire research Note no. 525, Fire Research Stn. Borehamwood, UK, 1961

The Prevention of Fires in Thatched Roofs; CSIR Note no. X/BOU 2-5, National Building Research Institute, South Africa, 1971.

Fire resistance - Applications of Boric Acid and its Salts; 1945. From Borax Holdings Ltd, Borax Hse, Carlisle St, London SW1.