Civil Engineering Reference
In-Depth Information
Table 15.6:
The life span of different timber roof coverings under favourable
conditions in a dry, cold climate
Type
Life span (years)
Shakes:
no impregnation with steep roof
More than 100
maintained with tar, steep roof
More than 200
maintained with tar, shallow roof
More than 100
Cleft log roof
Probably very high
Plank roof, maintained with tar or linseed oil
30-50
Plank roof, pressure impregnation
60-80
often limited to small buildings in the countryside. This is because of the high
risk of fire, especially when the roof is treated with tar. Thick materials usually
give a better fire resistance than thin materials.
Any form of roofing has to be ventilated underneath. On non-insulated
inland outhouses, 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 nec-
essary to have a good roofing felt under the battens and a double batten sys-
tem to allow water to run down under the battens carrying the timber roof-
ing.
The materials for a roof need to be carefully chosen and the angle of the roof is
critical. The steeper, the better. The stave churches have falls of up to 60°. Still
much older shakes can be found on the wall than on the roof.
Timber is the roof covering with the least negative effect on the environment
in terms of the use of resources and pollution during the production process, as
long as it is not impregnated.
It is to the timber's advantage if the roof surface is treated with wood tar,
preferably from beech, or linseed oil. Smaller timber components such as shin-
gles 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 instead
of linseed oil. The oldest preserved shingles are to be found on the walls of
Borgund stave church, Norway. They have been regularly painted with wood tar
every fourth year since the late Middle Ages.
Liquids for impregnation based on poisonous mineral salts or oil- and coal-
based poisons (see Table 19.3), will be washed out into local groundwater or soil.
The cleft log roof
This consists of half-cleft trunks laid over each other. This type of roof is very widespread
in Finland and Sweden. Cleaving the timber gives a much more damp-resistant surface
than sawing and chopping (see 'Splitting', p. 168). This roof has a longer life span than oth-
ers, as long as drainage is adequate. The lower layer is often made of planks instead of
