Birch bark was the most commonly used waterproofing material for turf roofs until the mid-twentieth century. It is laid in six to 16 layers with the outside facing upwards, and thefibresfollowingthefall of the roof to carry the water towardstheeaves.The more layers there are, the better the durability.
The layer of turf over the bark layers must beat least 15 cm deep to prevent the bark from drying out and splitting. A roof angle of 22° is the lowest possible slant for this sort of waterproofing. Using birch bark for waterproofing is a very labour-intensive technique and is dependent upon a limited resource.
Marsh-prairie grass laid on thin branches was the usual waterproofing layer used by immigrants in the drierareas of the USA.
Tar and bituminous products have also been used, to a certain extent. In Germany, a building with a fat concrete roof was constructed in the 1930s. It was coated with coal tar and then a 10-20 cm-deep layer of earth was laid on top. The roof has lasted through the years (Minke, 1980). Coal tar is not particularly good environmentally because of its high content of polycyclic aromatic hydrocarbons (PAH). Using a pure bituminous solution might be better, but there is little evidence as to how durable this would be. If using bituminous felt there should be at least three layers, although durability is probably relatively low because of the acidic activity of the humus in the earth. 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.
Plastic.There are many different plastic products on the market for this particular function.The best product from an ecological perspective is polyethylene sheeting of about a 0.5-0.7 mm thickness. This is an oil-based product but is relatively pollution free when in use. When burnt it does not emit toxic gases. The polythene sheeting available today is mainly for sloping roofs. It has studs or small protrusions on its surface that stop the turf from sliding down, and is claimed to be resistant to humus acids. As the plastic is underneath earth, it is not affected by ultraviolet radiation or large changes of temperature, which have a tendency to breakdown plastics.The durability is unknown as there are no examples that have been in use for a long period. On fat roofs, reinforced polyvinyl chloride sheeting is the most common material. A membrane of ethylene-propylene rubber (EPDM), often blended with polypropylene, can also be used.
Bentonite is a clay containing montmorillonite. It expands in contact with water to become a tough and clay-like mass that prevents water penetration (see page 262). This material is used in tunnel building and can also be used under a turf roof. The depth of earth must then be at least 40 cm to give the clay enough pressure to work against. However, this method is restricted for use on larger buildings with flat roofs. It would still need a layer of watertight polyethylene or bituminous felt underneath. A concrete surface can be primed with liquid bitumen.
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