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Pine, oak, aspen and larch can be used as roofing. Roofs can either be covered with cleft logs or planks, or with smaller units such as shingles. All timber roofing methods have one common requirement: they must prevent water standing anywhere as this could lead to fungal attack. This requires reasonably steep roofs (the steeper the better) and timber that has a mature quality, rather than fast grown timber. It may even be necessary to impregnate the timber (Table 15.8).

The weight of a timber roof covering varies from 25-40 kg/m2 according to how the roof is laid and the type of timber. The insulation value varies slightly for the different types of timber, but is generally of no consequence. The use of timber roofs is often limited to small buildings in the countryside. This is because of the high risk of fire, especially when the roof is additionally treated with wood tar. Thick materials usually have better fire resistance than thin materials.

Any form of roofing has to be ventilated underneath. On non-insulated inland outbuildings, the roof covering can be laid directly onto battens fixed to the roof trusses. On housing and in areas exposed to hard weather it is necessary to have a good roofing felt under the battens and a double batten system to allow water to run down under the battens carrying the timber roofing.

It is to the timber's advantage if the roof surface is regularly treated with linseed oil or wood tar, preferably from beech, which has a lower content of aromatic hydrocarbons. An important reason for treating the wood is to increase elasticity and thus avoid cracking. Smaller timber

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Table 15.8 Longevity of timber roof coverings under favourable conditions in a dry and cold climate

Type

Lifespan (years)

Shakes, untreated on steep roof

More than 100 years

Shakes, maintained with wood tar, steep roof

More than 200

Shakes, maintained with wood tar, low pitched roof

More than 100

Cleft log roof, untreated

Probably very high

Plank roof, maintained with wood tar or linseed oil

30-50

Plank roof, pressure impregnated with metal salts

60-80

components, such as shingles and shakes, can be put into a linseed oil bath and warmed to a maximum temperature of 70 °C. In certain coastal areas, cod liver oil has been used.

Liquids for impregnation based on toxic mineral salts or oil and coal-based products (see Table 19.3), can be washed from roofs into local ground water and soil.

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