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Rendering
Introduction
External lime coatings should be applied in relatively thin
coats, therefore any defects in the form of hollows should where
possible be corrected before the application of the render coats
by building with a similar material with similar
properties if the original material is not available
Defects should not be corrected through coats of “dubbing
out” ( Thick applications of render / mortar ). if it
can be avoided. Areas
of varying thickness are prone to shrinkage, carbonation and
curing problems. Careful background preparation plays a vital
part in the weather resistance capability of the wall.
Materials to be used in background repairs, should, wherever
possible, be matched to the existing fabric. In doing so, the
repair will be compatible and produce a similar performance
pattern. Where previous remedial work has taken place with
unsuitable materials, (i.e. dense cement mortars or bricks),
these should be removed and repaired with matching lime mortars.
Wall Preparation
The background must be free from dirt, grease and vegetation.
These elements should be removed before repairs
are underway. The removal of biological growths should be
thoroughly carried out, as any remaining lichen, algae, etc,
could grow back and attack the bonding between lime finishes
and the background.
Guttering roof coverings etc should be
checked and repaired as necessary so as rain water will not soak
into the finished render.
Dense impervious surfaces such as flint and engineering
brick will need a harling coat to adhere and bond in preparation
for subsequent coats.
Friable surfaces such as cob will need consolidation before application this is achieved by a thorough dampening to reduce suction followed by a thrown or harled coat .The harling material is a combination of aggregates and lime, mixed into a slurry consistency and applied by casting vigorously at the wall.
On
the day of application surfaces should be dampened ( but not wet
). A harling coat is then applied to the wall by casting vigorously
from a harling trowel or even a coal scuttle shovel. It
can also be applied by mortar spray or Tyrolene gun. It should
then be cured for at least 4 days to set hard before the application
of the first scratch coat of render
First Coat
The first coat of lime render is applied by use of a laying
on trowel or float, the coat is applied to a thickness of
approximately 8-10mm thickness, coats much thicker than this
will result in shrinkage cracks, in 2 coat work, this coat
will need to be straightened by use of plastering straight
edges, once flat the coating should be left to stiffen up,
sometimes known as tightening, as this happens, the render
should be compacted and compressed by scouring the render
with a wood or polyurethane float, i.e., rubbing up in a circular
motion. The over working of the surface should be avoided
as this will draw lime to the surface. During the scouring
process any shrinkage cracks should be closed, by pushing
the crack back with the edge of the trowel and rubbing new
material into the depression.
Once this process has been completed the surface is keyed
by use of a scratcher in a diagonal pattern. The inclusion
of hair or other fibres in the backing coat mixes will greatly
aid the minimising of shrinkage cracks and is strongly recommended.
Second Coat
Before the application of the second coat, the first coat
should be left for a minimum of 4 days, during which time
the first coat should be checked for shrinkage cracks and
also kept damp to avoid rapid drying out. Before applying
the second coat the first coat should be damped down, making
sure the water is absorbed into the render and not sitting
on the surface. The second coat is applied using the same
tools as the previous coat,
The coat thickness should be 8mm and no thicker, the coat
is applied left to stiffen and then scoured up to the required
finish, as before over scouring should be avoided. Good curing
once the work has been completed is essential if shrinkage
cracks are to be minimised [See Curing and
Protection]
After care
After the application
of lime renders, controlled curing and protection will be
needed to ensure maximum strength and durability are achieved.
The lime putty will stiffen initially due to absorption of
moisture into the wall and evaporation to the air and will
cure and harden as a result of Carbonation’, (the re-absorption
of carbon dioxide)
The NHL Natural Hydraulic Lime will set more quickly than
lime putty as a result of a chemical
reaction with the clay content known as the ‘hydraulic
set’. ‘Carbonation’, (the re-absorption
of carbon dioxide), also takes place. This process is best
achieved in warm and moist conditions, which allows the new
works to dry slowly. Therefore, during and after completion
of the work, it is essential to ensure ambient conditions.
Rapid drying by the sun, wind or artificial heat will all
have a detrimental effect on the final outcome of the lime
finishes. Temperatures below 5ºc will slow the carbonation
and hydraulic setting process and frost conditions will damage
un-carbonated areas, through the action of freeze-thaw (expansion/contraction)
resulting in feeble and crumbly finishes.
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Excessive shrinkage is a result of rapid drying, and this
can lead to separation between coats and background and cracking.
Rapid drying of the surface of new mortars, can also lead
to the pores of the mortar becoming blocked with fine material,
transported to the surface by the passage of water evaporation
too quickly from the mix, this will inhibit the carbonation
process taking place deeper into the new mortar.
The best way to control and protect the carbonation process
is to form a microclimate for the new work. Where the new
work is scaffolded, this can be a reasonably simple job. Scaffold
netting is very useful for reducing the effects of wind. In
addition to this in warm or hot conditions, damp hessian can
be placed against the new work and then covered by sheeting
to stop rapid drying. New work should be damped down for a
minimum period of 10 days after completion and longer if possible.
The emphasis should be on damping down as opposed to saturating
new work. Provision should be made for damping down over weekends,
holidays etc. In cold weather, the work must be protected
from frost attack, by using thermal blankets e.g. polystyrene
sheets. Hydraulic plasters/mortars will stand up to cold conditions
after 3-4 weeks of hardening. It should be remembered that
prolonged periods of cold temperature will slow the overall
hardening process and extended periods of protection will
be called for.
Any cracking occurring after the first few days of application
can be remedied by scouring the surface with the wood float
around the area of cracking to fill and compact the crack.
Health and Safety
Lime mortar by its nature of application carries a risk factor,
and therefore personnel should wear protective equipment,
particular attention to be given to eye and skin protection.
Eye wash should always be on hand. Gloves should be worn when
working with any mortars or plasters.
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The Building Lime Company | Tel: 01794 884294