Civil Engineering Reference
In-Depth Information
with some pozzolanas added to give a
render which developed considerable
strength and durability over time.
Practice on the continent of
Europe has over the years arguably
been well in advance of UK practice
with specification and application of
external renders. The relevant
features of good practice are well
known - the protection by a
flashing of even the smallest
projection, throwing the render onto
the wall surface, ensuring a rough
textured finished surface, using
cement:lime:sand (not cement:sand)
mixes, careful selection of well
graded gritty sands, and using gauge
boxes rather than the inaccurate and
inconsistent batching 'by the shovel'
still commonly practised today!
Failure of renders continues to be a
fairly frequent subject of requests for
help from the BRE Advisory Service
(Figure 9.9).
In Scotland, building materials,
designs and traditional building
practices differ somewhat from those
used in other parts of the United
Kingdom. The traditional Scottish
rendering mix, used successfully on
brickwork for many years, was a 1:3
cement:sand mix finished with a
roughcast or harling in which the
final coat, containing a proportion of
fairly coarse aggregate, was thrown
on as a wet mix and left as a rough
textured finish
(233)
.
overhangs protect the top joint and
avoid rainwater percolating behind
the render (Figure 9.10). Conversely,
great care needs to be taken with both
specification and workmanship when
using clipped eaves and verges where
the top joint is not protected.
Window sills and heads
Hood moulds and window surrounds
may be found constructed in brick
and rendered over to simulate stone,
and this practice dates at least from
the mid-sixteenth century.
Figure 9.9
Adhesion failure on a rather rough looking
brick substrate
Relief or incised decorative
modelling in renders, sometimes in
contrasting colours, may sometimes
be encountered, particularly on half-
timbered work. This technique, now
rare, is called pargetting or parge-
work. Specialist advice will be
necessary on its conservation.
The larger
the better
Figure 9.10
Eaves should offer protection to the head
of the rendering
Characteristic details
Heads can with advantage be bellcast.
Basic structure
As already noted in Chapter 2.6,
daub used over wattles in early times
essentially consisted of a mixture of
clay and chalk, and possibly lime,
reinforced with chopped straw or
reed. These mixtures by themselves
were not durable in rain or frost, but
when covered with a coating of lime
plaster became reasonably durable if
the annual limewashing was
attended to and the building had
generous overhanging eaves. Any
shrinkage gaps opening between
timbers and the daub would tend to
be closed by the dry clay swelling
when wetted.
Since medieval times, changes in
local practices with respect to mixes
have developed only slowly. Until
the invention of Portland cement,
mixes were extensively lime based,
Insulating renders
Insulating renders can combine a
measure of insulation with a more or
less acceptable rendered finish.
These renders, incorporating
lightweight aggregates such as
polystyrene or expanded minerals
(perlite) in a cement-based mix in
thicknesses of 25-75 mm and
without reinforcement, have had
some application in low-rise housing
in the UK since 1978 and in high-rise
since 1983. They have had mixed
results.
Drip
Stainless steel or
plastics bead
Abutments
Eaves and verges
Notwithstanding the effects of high
wind loadings, the larger the
overhang the better, particularly in
conditions of severe exposure. Large
Sill projects into wall.
Non-tiled sill should
preferably be stooled
DPC under sill
Figure 9.11
Rendering around a window opening
