Civil Engineering Reference
In-Depth Information
To avoid problems with moisture, building materials should be dried accord-
ing to standard practice, and concrete, earth and timber structures should be
allowed to dry before they are used with moisture-tight materials. Good ventila-
tion design is important for an enclosed structure.
Rain
External cladding and roof coverings, discussed in greater detail in the next
chapter, cope with rain. There is also a need for special components, partly to
protect exposed parts of the building such as pipes going through the envelope,
partly to carry the water away from the building. Such components are often
made of metal sheeting, and are either built on site or prefabricated in a factory
and transported to the building.
Ground moisture
The site for the building should be dry and well-drained. It is advisable to keep
the natural level of the water table and keep all rainwater within the site without
using the public drainage system. There is little need to overload the public sys-
tem unnecessarily, and a stable water table is necessary to keep the local flora and
fauna in a state of balance. Topography, soil and other site conditions can easily
come in conflict with this strategy, but it is important to find a foundation system
that suits the site.
Perimeter walls and slab foundations of concrete will always be exposed to
moisture. This can be reduced by a layer that breaks the capillary action of water
from the ground plus a watertight membrane, but it is always difficult to stop a
certain amount of moisture entering the fabric of the building. Concrete slabs
directly on the ground are problematic. There have also been a whole series of
damp problems with organic floor coverings such as timber, vinyl sheeting, etc.,
laid directly onto the concrete, even where there is a plastic membrane in
between. As insurance against such problems, concrete slabs on the ground
should have mineral floor coverings such as slate or ceramic tiles.
All structures normally have a damp-proof membrane between the foundation
and the rest of the structure, usually consisting of bitumen felt.
Air moisture
Air moisture is almost entirely produced inside the building by people, animals
and plants, or from cooking and using bathrooms; this can damage the structure.
Air moisture tries to penetrate the external walls and condense there.
Air's moisture content and condensation risk
The lower the temperature, the less water vapour air can hold. At 20°C air can hold
14.8 g/m 3
of water vapour, while at 0°C it can only hold 3.8 g/m 3 . If the internal air at
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