Roofing and wall cladding with grasses

Grass claddings have good insulation properties both with respect to heat and sound, and many different plant species can be used (see Table 10.8). Harvesting and laying methods for all coverings are labour-intensive, although parts of the harvesting process for reeds could be mechanized relatively easily. In Denmark, a mobile harvesting machine for straw roof coverings is in use. Here, the grain is removed without destroying the straw. During the three month long summer season this harvesting machine can produce straw for 200 roofs covering 180 m2 each.

The durability of thatch depends upon the growing conditions and how the plant was cultivated. A sunny climate without nightly frost is best. Straw and reeds which are used on the continent today are nearly all artificially fertilized, which produces enlarged and spongy cell growth resulting in a far shorter lifespan than necessary (Table 15.10).

Strong sun generally causes splits and breaks down thatched roofs -they survive longer in Northern Europe than further south, for instance. At the same time, there can also be a different lifespan on the north and south-facing parts of a roof. Eelgrass probably has the longest lifespan of all thatching materials. The most stable of the cultivated grain straws is generally said to be rye.

Table 15.10 Longevity of roof coverings and exterior cladding based on grasses, experiences of Northern Europe

Species

Unfertilized/fertilized naturally (years)

Artificially fertilized (years)

Bracken

8-10

Eelgrass

200-300

Heather

More than 25

Reed

50-100

30

Straw from rye/wheat

20-35

10-12

(Source: Hall, 1981; Stanek, 1980)

(Source: Hall, 1981; Stanek, 1980)

METHODS FOR THATCHING WITH STRAW

Roofing. When thatching a roof with straw a series of battens (sways) are erected on the rafters at 30 cm inter vals.The straw is bundled togetherand carefully threshed with-outdestroying the stalks.Weeds and loose straws are combed out with aspecial comb. The bundles are laid edge-to-edge on these battens, one layer on each sway. Every layer is bound down by runners that are fastened tothe sways, preferably with coconut twine.The completed roof is trimmed, using special knives, to a thickness of approximately 35 cm. The ridge is usually made with turf cut into 1 to 2 metre long pieces. On the inside ofthe rafters it has been the custom more recently to place fire-resistant insulation boards of woodwool cement. Good ventilation from the underside ofthe 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°, whilst along the coast it should be up to 50° (Figure15.30).

METHODS FOR THATCHING WITH EELGRASS

Roofing. (Figure 15.31) A layer of twigs (preferably pine or juniper) is placed on battens at 30 cm intervals. The eelgrass is worked and shaken to get rid of lumps and to make the straws lie in the same direction. Sections of eelgrass are then wrung hard to form 3 metre long scallops, in the same way one wrings water out of a floor cloth. The scallops continue out into a long, thin neck that acts as a fastening loop to the battens. The scallops are fastened tightly together on thefour to five lower battens, and the rest of the roof is built up with loose eelgrass laid in layers and pulled well together. By mounting a buffer along the roof's edge similar to the turf mound on a turf roof, it is pos-sibleto manage without scallops.The roof needsto settleforafewmonths before asec-ond layer is added.The total thickness is usually 60-80 cm, but there are examples of 3 metre thick roofs - which must be one of history's warmest roofs ever. After the final layer, the thatching is cut level with a special knife.The ridge is often covered with a long strip of turf. This could be replaced with a layer of eelgrass kneaded in clay. After a few years the roof will settle down and become a solid mass with the consistency of flaked tobacco. The time is then ripe for a new layer. Rain only gets through the outside layer and then trickles slowly down to the edge ofthe roof. At the same time the roof is open to vapour coming from the inside ofthe house.

Wall cladding with eelgrass wastraditionally most often used forgables,using10 cm thick layers of combed seaweed of about 60-70 cm in length. These bundles were stuffed between vertical battens at 30 cm intervals. Every layer was fixed by a horizontal branch woven between the battens. Finally, the gable was cropped with a long knife

15.30

Details of roof thatching with straw. Source: Grutzmacher, 1981.

15.30

Details of roof thatching with straw. Source: Grutzmacher, 1981.

so that it had a smooth even surface. Like eelgrass roofing, the eelgrass gable has a very high durability, but with time will settle, and cracks must be refilled.

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