Civil Engineering Reference
In-Depth Information
these can lead to large or small cracks which, in a damp area, can lead to fungus
attack.
Many different paints give a timber wall this sort of protection. The best of
these are probably the pure linseed oil paints, which penetrate well into the
wood. The effect of this layer depends upon reapplication at regular intervals.
A positive parameter in this treatment is the fact that the paint is considered
water-repellent. Research has shown that this treatment does not necessarily
make the timber last longer, however, and in certain cases it can have a directly
negative effect by retaining moisture in the material and not letting it escape.
pH-regulating surface-coat or impregnation
Substances which regulate pH are an effective way of preventing or removing
mould attack. Mould will not grow if the pH level is higher than 6.0. The same
can be said of insect attack. Exceptions are the fungus
Merulius lacrymans
and the
longhorn house beetle. The pH-regulating materials to use are alkalines such as
clay, cement, lime and waterglass. They are not poisonous in themselves, so they
do not cause problems in the indoor climate of the building.
Waterglass as a pH-regulating coat
Waterglass is very alkaline and in addition forms a coat so hard that insects cannot pene-
trate it to lay their eggs. Waterglass is, however, not waterproof when used on timber, and
can therefore only be used indoors or on protected parts of the building. Waterglass needs
a rough surface - it does not bind well to a planed surface. It is dissolved in boiled water
and applied to the wood with a brush. It can also be applied to straw, using a solution of
one part waterglass to two parts water. Waterglass is very open to water vapour. It is very
fire-resistant and was therefore often used in loft structures in old town houses.
Poisonous surface-coats or impregnation
Experience has shown that timber with a high content of tar and resins lasts
longer than timber with a low content of the same. This is partly because the tim-
ber is harder and partly because these substances have ingredients which are poi-
sonous to fungus and certain insects. These natural fungicides and insecticides
consist of, or are similar to, different types of tannic acid. Traditional types of tim-
ber protection aim to increase the quantity of such materials by covering the tim-
ber with tar. Extract from bark has also been used to impregnate oak, birch and
spruce, with good results. This method was once so popular that bark extract
became a major Norwegian export. Over 2000 years ago the Chinese tried using
salt water as an impregnating agent, because the salt's action on the wood was
slightly antiseptic. More recently, metal salts have been used for impregnation,
and wood tar has mostly been replaced by derivatives of oil.
