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414 Barry’s Advanced Construction of Buildings
of masonry as rain screen, a cavity and an inner leaf that provides adequate thermal resistance
to the passage of heat, and an attractive finish.
Precast concrete wall panels act in much the same way as brick by absorbing rainwater.
Because of the considerable size of these panels, there have to be comparatively wide joints
between panels to accommodate structural, thermal and moisture movements. The joints
are designed with a generous overlap to horizontal and an open drained joint to vertical
joints to exclude rain. Non-absorbent sheet materials, such as metal and glass, cause driven
rain to flow under pressure in sheets across the face of the wall, so making the necessary
joints between panels of the material highly vulnerable to penetration by rain. These joints
should at once be sufficiently wide to accommodate structural, thermal and moisture
movements and serve as an effective seal against rain penetration. The materials that are
used to seal joints are mostly short-lived as they harden on exposure to atmosphere and
sun, and lose resilience in accommodating movement. The ‘rain screen’ principle is designed
to provide a separate outer skin, to screen wall panels from damage by wind and rain and
deterioration by sunlight, and to improve the life and efficiency of joint seals. This can also
enhance the aesthetic appeal of a building.
Durability and freedom from maintenance
The durability of a wall structure is a measure of the frequency and extent of the work
necessary to maintain minimum functional requirements and acceptable appearance.
Walls of brick and natural stone will very gradually change colour over the years. This
slow change of colour, termed weathering, is generally accepted as one of the attractive
features of these traditional materials. Walls of brick and stone facing require very little
maintenance over the expected life of most buildings. Precast concrete wall panels which
weather gradually may become dirt stained due to slow run-off of water from open horizontal
joints.
Panels of glass will maintain their finish over the expected life of buildings but will
require frequent cleaning of the surface and periodic renewal of seals. Self-cleaning glass
helps to reduce the frequency and cost of cleaning. Of the sheet metal facings that can be
used for wall structures, bronze and stainless steel, both expensive materials, will weather
by the formation of a thin film of oxide that is impermeable and prevents further oxidation.
Aluminium weathers with a light coloured, coarse textured, oxide film that considerably
alters the appearance of the surface, although the material can be anodised to inhibit the
formation of an oxide film or coated with a plastic film for the sake of appearance. Steel,
which progressively corrodes to form a porous oxide, is coated with zinc, to inhibit the rust
formation, and a plastic film as decoration. None of the plastic film coatings are durable
as they lose colour over the course of a few years on exposure to sunlight, and this irregular
colour bleaching may well not be acceptable from the point of view of appearance to the
extent that painting or replacement may be necessary in 10–25 years. In common with
other thin panel materials, there will be a need for periodic maintenance and renewal of
seals to joints between metal-faced panels. Rock-based, painted panels endure all weather
elements, with coatings providing UV protection, an anti-graffiti finish, and are also
self-cleaning.
Fire safety
The design of cladding and curtain wall construction must take into account fire safety.
Primary concerns are the internal spread of fire across the surface materials of walls and