Civil Engineering Reference
In-Depth Information
because it contains salt and a large amount of lime and silica. Sheeting material
made of eelgrass is considered more fire-resistant than the equivalent timber
fibreboards.
In excavations made in Lauenburg, Germany, there are indications that build-
ings were thatched with straw as long ago as 750-400 BC. In Denmark this sort
of roof is believed to have been in use for at least 2000 years, also, particularly on
the islands of the Kattegatt, eelgrass has been traditionally used for roof cover-
ing and wall cladding.
The use of thatched roofs has decreased considerably since the turn of the
century. This is partly due to insurance companies demanding higher premi-
ums due to the higher fire risk, and partly because of the mechanization of
agriculture. Straw that has gone through a combine harvester is unusable. In
Germany and the Netherlands, reeds have almost become non-existent
through land drainage. In Europe today the raw material is imported from
Poland, Bulgaria and Romania. Even Denmark has difficulty supplying its
local needs.
In England, Germany and the Netherlands thatching is still a living craft.
Further south, roofs built from plants still dominate many cultures. In India, for
example, 40 million houses are covered with palm leaves and straw.
Ecologically speaking these materials are very attractive. They are constant
resources which are otherwise never used. The production processes do not
require much energy and produce little pollution. In buildings the products
usually have no problems. Sheeting products often have adhesives added, such
as polyurethane glue at 3-6 per cent by weight. This reduces the environmental
quality somewhat. As waste, the pure products can be composted or energy
recycled. For the products containing adhesives filters are required for the
fumes that come from their incineration, and waste has to be deposited at certi-
fied tips.
Roof and wall cladding with grass
Many different types of grass can be used for roofs and walls. Harvesting and
laying methods for all coverings are labour intensive, although parts of the
harvesting process for reeds could be mechanized relatively easily. The har-
vesting of eelgrass could also be made more efficient. In Denmark, a mobile
harvesting machine for straw roof coverings is already in use. Here, the grain
is removed without destroying the straw. During the three month long sum-
mer season this harvesting machine can produce straw for 200 roofs covering
180 m
2
each, but it is generally difficult to see any way of making the actual
thatching process more efficient. Thatched roofs are and will always be
labour-intensive.
