Civil Engineering Reference
In-Depth Information
nutrition in the water coming from the area. Pure cellulose should be composted
under controlled conditions.
Birch bark
The bark of birch trees has been widely used as a waterproof membrane under
turf roofs. It has to be kept permanently damp to prevent it cracking. The pieces
of bark were taken from large birches, conveniently known as roof birches. A
roof had between three and twenty layers of bark, depending upon the required
durability. The bark is very resistant to rot and can be used as waterproofing in
other potentially damp areas, e.g. foundation walls. Because it prevents damp
and spreads moisture evenly, it is better than asphalt paper for protecting built-
in beams. During the rebuilding of the Church of St Katarina in Stockholm dur-
ing the early 1990s, 300-year-old birch bark was found at the end of inbuilt
beams. They were exceptionally well-preserved. The same method was there-
fore used in the rebuilding. In 1948 the Danish engineer Axel Jörgensen wrote:
'Building traders should set up an import of birch bark from Sweden or Finland,
so that we could once again use this excellent protective medium' (Jörgensen,
1948).
Bark should be removed as carefully as possible, so as not to damage the tree's
layers. The tree can then continue to grow, though it may not produce more
building-quality bark. Bark is loosest during spring, and the best time to take the
bark is after a thunderstorm (Høeg, 1974). Bark has also been used as insulation
in walls, especially cavity walls, where its considerable resistance to rot and its
high elasticity produces a stable wall.
Peat and grass materials
Many peat and grass species have considerable potential as climatic materi-
als, for thermal insulation and air and moisture regulation. Loose fill,
boards, blocks and matting of bog peat and straw represent good thermal
insulation materials. Many types of plants have good moisture-regulating
properties, and some even have a high resistance to rot, such as flax, jute and
moss.
Plant products often make suitable thermal insulation because, in a dried state,
they contain air and have a stable structure that deters settling. In the case of
straw, fibres or stalks are used after the leaves have been removed. Eelgrass,
lichen, moss and peat can be used in a dried state. Parts of cocoa and maize
plants contain cellulose, which makes good building materials.
