Civil Engineering Reference
In-Depth Information
Recommendations to reduce fire
spread are as follows:
●
rendered metal lathing,
thermoplastic insulant: sufficient
metal fasteners to stabilise the
cladding, and fire barriers every
two storeys
Non-metallic mesh worked into base coat
Polymer modified cement base coat
4 to 6 mm proprietary mix
rendered metal lathing,
thermosetting insulant: sufficient
metal fixings to stabilise the
cladding
●
Final finish
glass fibre fabric-reinforced thin
renders, thermoplastic insulant:
fire barriers, which also support
the cladding, are required at every
storey
●
Figure 9.18
A third method of applying thermal insulation
Metal reinforced cementitious
render, 25 mm thick, provides
effective protection to
combustible insulation. The
reinforcement should be
independently supported
●
Noise and sound insulation
Rendered walls depend for the most
part on the mass of the background,
and reference should be made to
appropriate earlier chapters.
Traditional thick render
Metal mesh
Glass fibre reinforced thin renders
perform reasonably well over non-
combustible insulation, but offer
little protection to combustible
insulants
●
Polymeric insulants protected
only by unreinforced thin resin
coatings should not be used where
there is any risk of direct flame
attack
100 mm high strip of
non-combustible
insulation every two
storeys
●
Durability
It has already been noted that few
surfaces are truly self-cleaning under
the action of rainwater, since streams
of run-off will preferentially follow
certain routes rather than others.
Some rendered surfaces will tend to
dirty unevenly, and may be prone to
algal growth.
Field observations of renderings
have revealed that some failures
occur very early in the life of a new
building, while others only become
serious a year or more after the
building is occupied.The early
failures have mostly been shown to
be due to unsatisfactory site practices
and to the drying shrinkage
characteristics of the rendering
mixes, which could produce initial
cracking. Some types of failure have
been much more difficult to remedy
and and have been due to a
combination of faults.
Unsatisfactory storage of materials
on site, inadequate protection of
newly constructed walling against
inclement weather, and defective
workmanship all produce poor
adhesion between the rendering and
its background. The rough texture of
brickwork provides a good key for
the rendering undercoat, but newly
completed brickwork which has been
allowed to become rain-soaked has a
film of water at the brick and
Render stop bead
Combustible insulation
There will be some risk in fire of parts
of the cladding becoming detached
and falling to the ground, particularly
where plastics fixings play a
significant role. Metal fixings are less
likely to lose their integrity in fire. A
proportion of metal fixings should
therefore be included.
Insulating render systems of the
types given above do not appear to
cause an exposure risk to an adjoining
building, and there is little risk of fire
in overcladding entering a building
through window openings.
Where there is no cavity, but
combustible materials are used for
insulation purposes, it is advisable to
interrupt the sheets with a cavity
barrier, particularly where the
insulation is thermoplastic (eg
polystyrene) (Figure 9.19). Cavity
barriers are not required for systems
where there is no cavity as such and
non-combustible materials are used
for insulation.
Figure 9.19
A rendered metal lathing system - if
thermoplastic insulants are used, cavity fire
barriers are needed every two storeys
Fire
Plastics materials used for fixing
external rendered thermal insulation
systems to walls are likely to melt or
burn in a fire and this can lead to
deformation of the cladding and
extension of damage. Loaded fixings
fail at much lower temperatures than
unloaded fixings.
Experience of real fires is limited,
but laboratory tests on multi-storey
rigs have been carried out to assess
the fire performance of systems
incorporating combustible insulants
fixed directly to a masonry wall and
to compare these with that of timber
cladding. From the tests it is possible
to make the following general points.
