All organic materials decompose when exposed to moisture and heat. Maximum durability is achieved by selecting the best materials, harvesting at the right time and by processing and using them according to best practice principles described in previous chapters. Service life can also be improved by preservative treatment.
Some species are more durable than others, and some parts of particular species, for example the sapwood of timber used for shingles, should be discarded. Research into conventional practice suggests that it is very unlikely, with the single exception of copper sulphate herbicides, that chemical preservation of grass or palm thatch is economic - treatments are either ineffective or excessively costly. In contrast, chemical treatment of wood or bamboo tiles is generally viable.
Before deciding to use chemicals it is advisable to consider other methods that may be equally effective, cheaper and less dangerous. Wherever organic materials have been traditionally used for roofing it is very likely that people will have devised non-chemical preservative techniques.
For example, in the Philippines it is common knowledge that bamboo which is to be used for roof tiles will better resist wood borer infestation and general decay if it has been soaked for a day in sea water. They also know that bamboo will be less likely to split if it has been harvested when the culms are fully mature. Similarly, farmers in England know that it is essential to allow newly cut thatching straw to dry thoroughly before it is stored ready for sale. Local advice is always recommended.
Preservatives improve durability by minimising or preventing microbial or fungal action, attack by insects and by making a material less prone to water penetration. The most effective treatments can be expected to double the natural life of a material. However, many chemicals are water soluble or are leached out by moisture changes and thus become ineffective within a few years.
Working with chemically treated materials poses a hazard to the roofing contractor. Special precautions must be taken to ensure that hands are washed before eating, and care must be taken to avoid inhaling toxic dust if mechanical saws are used to trim treated shingles.
There are hundreds of different commercially available chemicals and mixtures of chemicals. As they are sold under a variety of trade names it is important to check the precise composition of a particular product before specifying its use. Chemical use is controlled by legislation that differs from country to country, so advice must be sought from government authorities. Manufacturers instructions must always be followed as many chemicals are very toxic and environmentally damaging.
Danger symbols must commonly encountered in construction
Harmful or Irritant
Research on non-poisonous preservatives is continuing world-wide, and new reports of the toxicity of chemicals previously considered safe are constantly emerging. Provided that manufacturers instructions are used, it seems safe to use chemicals based on borax, soda, potash, wood tar (creosote), beeswax and linseed oil. But caution is still required as toxic solvents are sometimes used to make relatively innocuous chemicals easier to apply.
Absolutely to be avoided are treatments containing DDT (dichlor-diphenyl-trichlorethane), PCP (pentachloraphenol), Lindane (gamma-hexachloro-cyclohexane) and arsenic. Even very small quantities are lethal for people and animals, and they can cause long-term environmental damage. These chemicals are banned in many countries but are still available in some developing countries - just because they are on sale does not mean they are safe.
Guidelines for safe use
Safety depends on controlled storage, appropriate working conditions, protective measures, personal hygiene and careful disposal of waste, as well as training and experience. People working with chemicals must be aware of any risk they are taking and must be provided with every facility to prevent accidents.
Storage: maintain minimum possible stock levels in secure, well ventilated rooms under the control of a trained storeman.
Working conditions: insist on training, suitable equipment, good ventilation, no naked lights and no smoking, eating or drinking when using toxic chemicals.
Protective measures: keep all chemicals off the skin and particularly out of the eyes. Goggles must be provided as well as barrier creams, gloves or protective clothing to suit the degree of risk.
Hygiene: have facilities available to wash exposed skin in clean running water.
Disposal of waste: follow local regulations and manufacturer's directions.
WARNING: roofs that are treated with chemical preservatives must not be used to collect rainwater for drinking or irrigating food crops.
There are three basic categories of chemical preservative treatment and several different methods of using them.
Surface treatments, applied by brushing, dipping or spraying during processing and sometimes re-applied periodically to extend the roof life.
These are relatively simple, use inexpensive equipment and can be done on-site.
But they are the least effective as only the outer surface is coated and most chemicals will be washed off by rainwater. On-site treatment is also inadvisable as there is a greater chance of spillage and manual methods are more likely to expose workers to toxic chemicals.
Thus in most cases surface treatment is not recommended.
Penetrating treatments which are applied by soaking, sometimes assisted by vacuum, pressure or heat.
The simplest method is to soak timber or bamboo in an open tank. The material must be totally covered and soaked for several days to achieve effective penetration. It must be dried before it can be used. It is not economic to soak thatch due to the quantity of material involved.
Boucherie method of treating bamboo
Pressure impregnation of bamboo using borax in Costa Rica
The advantage of open tank soaking is its simplicity and inexpensive equipment. Small batches of material, for example for one roof, can be treated economically. However, this is offset by the dangers associated with open tanks of chemicals.
Many different techniques have been devised to increase the penetration rate and make the process safer. Impregnation in sealed vessels either under negative pressure (vacuum) or positive pressure is more effective than soaking. Similarly, increased penetration may be achieved by heating the chemicals. All of these processes require specialised equipment and careful management; if these facilities exist locally roofing contractors should use them in preference to other preservative methods.
Sap displacement, where preservatives are drawn into the material by capillary action.
Displacement methods are only useful for treating bamboo - timber for shingles and shakes is best treated by penetration methods. Two techniques can be used; both must be done with freshly cut bamboo.
Butt treatment is where bamboo culms are stood in a drum of preservative. The leaves act as a pump; as sap evaporates it is replaced by the chemicals. The process may take several weeks.
The Boucherie Method uses gravity pressure or a pump to force preservative into the bamboo. It can only be used with freshly cut bamboo so treatment should be started on the day that the bamboos are harvested.
Chemical treatment is generally economic for wood tiles. The safest and most effective methods require expensive industrial equipment. Bamboo can be treated, but there is no clear evidence that existing processes are economic. Despite many experiments and extensive research there is no conclusive evidence that thatch can be economically treated with preservatives. Best practices in cultivation, materials selection, harvesting and processing and finally in application are of primary importance before chemical treatment is even considered.
Berry R.W.; Timber in Tropical Building; Overseas Building Note 199, Building Research Establishment, England, 1993.
A Guide to the Safe Use of Chemicals in Construction; Construction Industry Research and Information Association. London, 1981.
Janssen J.: Building with Bamboo; Intermediate Technology Publications, London, 1995.