Civil Engineering Reference
In-Depth Information
The durability of different roof coverings
Plant
Artificially fertilized (in years)
Natural (in years)
Reeds
30
50-100
Straw
10-12
20-35
Eelgrass
-
200-300
Bracken
-
8-10
(Hall, 1981; Stanek, 1980)
The long life span of eelgrass is due to its high content of salt, lime and sili-
cic acid. It is therefore not so readily attacked by insects - a particular prob-
lem in normal thatched roofs. The most stable of the different cultivated
grains is rye.
Strong sun generally causes splits and breaks down thatched roofs. They sur-
vive longer in northern Europe than further south. At the same time there can
also be a different life span between the north and south facing parts of the roof.
All organic material can return to earth as compost.
Straw
Thatching
When thatching with straw a series of battens (sways) are erected on the rafters at 30 cm
intervals. Bundles of straw are laid edge-to-edge on these battens, one layer on each
sway. Every layer is bound down by runners which are bound to the sways, preferably with
coconut twine. The completed roof is evened out using special knives to a thickness of
approximately 35 cm. The ridge is usually made with turf cut into 1-2 m-long pieces. On
the inside of the rafters it has been the custom more recently to place fire-resistant insu-
lation boards of woodwool cement. Good ventilation from the underside of the roof is
important. As with timber roofs, the rule of the steeper the roof, the longer it lasts, applies.
The usual slope in normal climatic conditions is 45°, while along coasts it should be up to
50°.
Wall cladding
This method of cladding has never been widespread. Traditionally the most usual mater-
ial was rye, which was bundled together and threshed without destroying the stalks.
Weeds and loose straws were combed out with a special comb. Then eight or nine hoops
were bound together into a yealm and trimmed with a knife.
When cladding a house with straw, it is usual to start at the bottom. Every layer should
be 30 cm high, and fastened by nailing the upper part to a batten. The bundle hangs down
to cover the first batten. Every layer is cut at the bottom to make it straight and even. As
long as the straw cladding is intact, it will give useful extra insulation, as it holds small
pockets of air.
