Civil Engineering Reference
In-Depth Information
Birch bark
Bark from birch trees was the most usual waterproofing method until the mid-twentieth
century. It is laid in six to 16 layers with the outside upwards, and the fibres following the
fall of the roof to carry the water to the eaves. The more layers there are, the better the
waterproofing.
The layer of turf over the bark layers must be at least 15 cm deep to prevent the bark
from drying out and splitting. A roof angle of 22° is the lowest possible for this sort of
waterproofing. This is a very labour-intensive technique and is dependent upon a limited
resource.
Marsh-prairie grass
Marsh-prairie grass laid on thin branches was the usual waterproofing layer used by immi-
grants in the drier areas of the USA.
Tar and bituminous products
These have also been used, to a certain extent. In Germany during the 1930s a building
with a flat concrete roof was coated with coal tar and then a 10-20 cm-deep layer of
earth was laid on top. The roof has kept well through the years (Minke, 1980). Coal tar
is not particularly good environmentally because of its high content of polycyclical aro-
matic hydrocarbons (PAH). Using a pure bituminous solution is a better solution, but
there is little evidence as to how durable this would be. If using bituminous felt there
should be at least three layers, but the durability is probably relatively low because of the
acidic activity of the humus in the earth. A high proportion of quack grass in such a roof,
Agropyron repens
, would be inadvisable. Polyester reinforced bituminous felt is often
used as an underlay for other plastic membranes. The material does not then come into
direct contact with the earth.
Corrugated asbestos sheeting
This was used a great deal during the 1950s, but is no longer produced. This is due to the
associated health risks and its limited life span.
Steel and aluminium sheeting
These cannot be used, because they are quickly eaten away by the acidic humus.
Slate and tiled roofing
It is actually possible to lay a turf roof on top of a sloping roof covered in slates or tiles,
but it is unlikely to be an economical or resourceful use of materials.
Bentonite
Bentonite is a type of clay which expands when it comes into contact with water and
becomes a tough and clay-like mass which prevents water penetration (see p. 269). This
material is used in tunnel building and can also be used under a turf roof. The depth of
earth must be at least 40 cm to give the clay enough pressure to work against. This
restricts the use of this method to larger buildings with flat roofs. It would still need a layer
of bituminous felt underneath.
