Civil Engineering Reference
In-Depth Information
Plastic
There are many different plastic materials on the market for this particular function, such
as PVC or polyester sheeting with fibreglass reinforcement. The best product from an
ecological perspective is polyethylene sheeting of about 0.5-0.7 mm thickness. This is
an oil-based product but is relatively free from pollution when in use. When burnt it does
not emit any poisonous gases. The polythene sheeting available today is mainly for slop-
ing roofs. It has studs or small protrusions on its surface which 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 break down plastics. The durability is unknown as there are no examples
that have been in use for a long period. On flat roofs, reinforced PVC sheeting is the most
common material. The plastic barrier is normally laid on top of a layer of bituminous felt.
Flashing
Flashings around chimneys and pipes that go through the roof are usually of lead
or copper. The use of these materials should be kept to a minimum for environ-
mental reasons. Slates can be used around chimneys on turf roofs (see Figure
15.14).
Climactic conditions affecting turf roofs
Sun
Strong solar radiation can cause the planted surface to dry out, especially if it is on a rel-
atively steep roof facing south. If this angle is less than 20° there is no problem. For steep-
er roofs in drier climates the roof needs to be shaded or needs a thicker layer of earth giv-
ing a high water- and warmth-storing capacity.
Table 15.4:
The uses of different waterproofing layers
Material
Amount of work
Life span
Areas of use
Bark from birch
Very high
Long (30-100 years)
Sloping more than 22°
Bituminous felt
Low
Medium/low
depending on type
of soil
All roofs
Corrugated asbestos sheeting
Low
Medium
Sloping more than 15°
Steel/aluminium sheeting
Low
Short
Sloping more than 15°
Slate/tile roof
Medium
Long
Sloping more than 20°
Bentonite clay with
bituminous felt
Low
Unknown
Flat roofs
Polethylene sheeting
with bituminous felt
Low
Unknown
Sloping more than 15°
Polyvinyl sheeting with
bituminous felt
Low
Unknown
All roofs
