Civil Engineering Reference
In-Depth Information
Timber frame
●
It will be necessary to adapt the
rendering method to suit the
original form of construction
(Figure 9.22)
40 mm cavity
or 20 mm if lathing backed
with breather membrane
Metal lathing
Vertical battens
First undercoat
3 to 6 mm
Designation I
Metal lathing
Metal frame
●
As for timber frame. It may be
necessary to provide secondary
framing members between the
stanchions of the metal frame, and
they should be of stainless steel
(preferred) or galvanised steel.
Lathing should be fixed with
stainless steel wire ties of 1.2 mm
diameter at 100 mm centres
(Figure 9.23).
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
Rendering over cross-braced studs
Rendering over rigid sheathing
Figure 9.22
Rendering on timber framed building
Control of suction
●
Open textured surfaces may have
such high suction in hot weather
that the final water:cement ratio
will be affected and the coat
weakened
●
The wall surface should be wetted
but not soaked in dry weather to
reduce suction
Bonding agents
●
Wire ties
Metal frame
To promote adhesion, SBR latexes,
or acrylic polymers should be
applied, mixed with cement or
cement and sand, and well
scrubbed into the surface
●
Render should be always applied
directly over the wet bonding agent
Render coats
EML
Walls to be rendered must not have
been subject to prolonged rain in
the previous 48 hours
●
Where subsequent coats are to be
hand applied, it will be necessary to
provide a key. This should be done by
combing or scratching the set surface
of the rendering with a comb set with
teeth about 20 mm apart. The scratch
lines should be wavy in shape, and
should penetrate half way through the
coat.
Mechanical keys
●
All raked joints should be recessed
10-12 mm
Figure 9.23
Rendering on metal framed building
Metal lathing
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Expanded metal lathing fixed in
accordance with BS 8000-10
(245)
can be used over most surfaces
Batching
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When rendering mixes are
batched with dry sand (as
opposed to damp sand), the mix
tends to end up weaker than mixes
batched with damp sand. Cement
content should therefore be
increased about 20% to
compensate
●
Where renderings are to be used on
metal lath - and there is therefore
no suction in the background, and
coats are of the same nominal mix
- the undercoat tends to be weaker
than subsequent coats because of a
higher water:cement ratio. More
cement should be added to the
undercoat mix (as above) or a
slightly weaker second undercoat
used
For exposed conditions, austenitic
stainless steel to BS 1449
(246)
Grade 304 will be suitable
Daywork and movement joints
●
since daywork joints will normally
show in most kinds of unpainted
renders, consideration should be
given to their siting (eg behind or
alongside architectural features).
Movement joints should be
provided to coincide with any
movement joints provided in the
main structure
●
Spatter or stipple coats
●
Spatterdash is thrown on the wall
surface whereas stipple is stiff
brush applied, well scrubbed in
before the stipple is raised
●
The mix proportions for both are
1 part cement to 1-2 parts coarse
sand, mixed to a thick cream
In the case of rendering on metal
lathing, the frequency of
movement joints should be in no
case more than bays of 5 m width
●
A bonding agent is necessary for
stipple, but optional for
spatterdash
●
Curing is by dampening over two
days
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