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Single-Storey Frames, Shells and Lightweight Coverings 213
These sheets tend to attract dirt because of the coarse texture of the surface, which is not
easily washed away; thus the sheets can become unsightly quite quickly.
Flat roof membranes, laid directly over thermal insulation material, will experience
considerable temperature variations between day and night. In consequence, there is considerable
expansion and contraction of the membrane, which in time may cause the membrane
to tear. Solar radiation also causes oxidation and brittle hardening of bitumen
saturated or coated materials, which in time will no longer be impermeable to water. The
durability of a roofing membrane in an inverted roof (upside down roof) is much improved
by the layer of thermal insulation laid over the membrane, which helps protect it from the
destructive effects of solar radiation and less extreme variations in temperature. The useful
life of bitumen impregnated felt membranes is from 10 years, for organic fibre felts up to
20 years and for high performance felts up to 25 years: this can be extended by using an
inverted roof construction. Mastic asphalt will oxidise and suffer brittle hardening over
time, which, combined with thermal movements, will give the material a useful life of
around 20 years.
Safe maintenance
The Construction (Design and Management) (CDM) Regulations 2007 require that buildings
should be designed so that they can be constructed, maintained and demolished safely.
One in five construction-related accidents is caused by falls from, or through, roofs (HSE,
1998). Care should be taken when designing structures to ensure that falling through sheeting
materials and from the roof is recognised as a hazard and the risks of such occurrence
are reduced. Provision should be made to prevent falls, including adequate access for plant
and equipment. Safety rails should be used to prevent falls over the edges of roof structures.
Harnesses, fall arrest systems and safety nets do not prevent falls but do reduce the risks
of injury in the event of a fall. Inclement weather poses a significant risk to those working
in exposed positions and at heights. Work at heights should not continue during high winds
or conditions that make the risks unacceptable. Debris netting (as well as safety netting)
or birdcage scaffolds may be used to offer protection from falling objects and allow work
to continue in the zone below the roof area. Debris shoots should also be used to ensure
that waste, which presents a hazard if it falls, is quickly removed from the roof. Consideration
must be given to maintenance operations once the roof structure is complete. Guarded
walkways, access platforms, safety rails, etc., will be needed to ensure safe access.
Fire safety
Particular attention should be given to the internal and external fire spread characteristics
of sheet materials in relation to the overall design of the building. A further cause for
concern in framed buildings is concealed spaces, such as voids above suspended ceilings,
roof and wall cavities. Cavity barriers and smoke stops should be fitted in accordance with
current regulations.
Resistance to passage of heat and ventilation
Resistance to the passage of heat is provided by thermal insulation materials, either separate
from the sheeting material or as an integral part of the sheet in composite panels. Consideration
must be given to thermal bridging in steel-framed buildings, especially at junctions,
and care is required to avoid condensation. The principles of condensation, or rather the
manner in which it can be avoided within the roof and wall structures, were discussed in