Civil Engineering Reference
In-Depth Information
●
Rendering is one method of
increasing the exposure rating to
wind driven rain of a wall by one or
two categories, that is to say a wall
which would otherwise be suitable
only for sheltered exposures
(20)
can
be upgraded to severe or very
severe
●
Exposure to marine or polluted
environments may lead to attack
on the cement content of renders
and an increased rate of corrosion
of metal lathing
●
Rendering did not reduce the
passage of water into aerated
concrete backgrounds to the same
extent as observed with clay brick
backgrounds. Rendering does,
however, help to prevent rain
penetration through the joints
Thermal properties
Insulating renders are available to
increase the thermal insulation value
of solid walls, and at the same time
provide increased resistance to
deterioration (Figures 9.16 and 9.17,
and Figure 9.18 on page 254).
Precautions to be taken include the
avoidance of water vapour transfer
from inside the building condensing
on the external insulation system by
the careful selection of permeabilities
or the provision of vapour control
layers as appropriate
(40)
.
Although evaporation rates were
generally much lower than rates of
absorption, there was no
significant build-up of water
within the materials
●
Rainwater absorbed intermittently
did not penetrate deeply before it
was lost by evaporation
●
Rendered walls exposed to driving
rain as well as pollution may streak
differentially. Flint or calcareous
dry dashes have the best self-
cleaning properties, though some
flints contain iron which can lead
to staining
●
Cavity
Breather membrane
Breather membrane
Metal lathing fixed directly with pins
Metal lathing on battens
Renders undoubtedly reduce rain
penetration into walls. The following
points observed during experimental
work by BRE will have a bearing on
their effectiveness.
●
Renderings with mix proportions
of 1:1:6 and 1:
1
⁄
2
:4
1
⁄
2
were very
effective in reducing the passage of
water into brick backgrounds, and
the addition of a dry-dash finish
further improved the performance.
Cracking, or loss of dash by
erosion, might significantly alter
this protective property
First undercoat
3 to 6 mm Designation I
First undercoat
3 to 6 mm Designation I
Second undercoat
10 to 14 mm Designation II
Second undercoat
10 to 14 mm Designation II
Final coat Designation II, III or IV
Final coat Designation II, III or IV
Figure 9.17
Two methods of applying thermal insulation
Case study
Stucco failure on heritage dwellings
During one particular winter, large areas of
stucco became detached from a number of
properties. From the information available, the
weather had been of a higher than normal
rainfall accompanied by freezing conditions.
At this time some of the stucco became
detached and fell to the ground. In the majority
of cases the stucco had failed at the more
exposed parts of the buildings (ie chimneys,
parapets, mouldings, sills and porticos).
The BRE Advisory Service was asked to
investigate. It was apparent also that the
properties had suffered structural movements
which had resulted in the cracking of walls,
particularly around window openings, parapet
abutments and porticos.
Subsequent investigations revealed that
the remainder of the stucco was badly cracked
and hollow. Cracks in the stucco had allowed
the ingress of water into the brickwork and
behind the render. The freezing of this water
had produced the damage to the stucco, as
well as deterioration to the brickwork.
The original designs of the chimneys,
parapets, sills, mouldings and porticos did not
provide adequate protection from the
weather. The parapet walls should have been
protected by a coping that had a good
overhang with a drip to throw the water clear
of the top of the stucco. The brickwork and
the stucco below the coping should have
been protected by a damp proof course.
The parapets had no protection, in fact the
stucco was taken up over the top of the
parapet and had cracked in a manner that
was typical of shrinkage movements, sulfate
attack and structural movements.
To avoid a recurrence of the frost
damage to the stucco would require careful
inspection. The defective areas should be
hacked off and re-rendered while at the
same time reconsidering the design of the
exposed detailing. After completion of the
repairs, inspections should be repeated at
regular intervals.
It might be possible to effect temporary
repairs to loose, hollow and cracked areas
by injecting polymer resins or cementitious
grouts, and protecting the upper faces of
the cornices, mouldings and string courses
with a proprietary flashing material; but the
satisfactory performance, even of such an
extensive repair programme, could not be
guaranteed.
Figure 9.16
Applying external thermal insulation to solid
walled houses
