Chalk Cob wall rendering
Introduction
Cob Wall Repair
COB WALL PROJECTS
Cob walls should always have a well maintained roof/capping and where possible the plinth should be exposed and repaired re-pointed as necessary using suitable lime mortar. Or as the old saying goes the wall should have a good hat and boots to keep the moisture out.
A cob wall in good condition in a sheltered situation does not need render coats for protection. A render finish even lime can never look as good as cob.
Cob Chalk and Earth walls are by
nature of a soft friable composition.
Rendering and repairs should be carried out with Lime Putty
mortar 3:1 or Round tower NHL2 mortar 2.5:1
External lime coatings should be
applied in relatively thin coats, therefore any defects in
the form of hollows
must be corrected before the application of the render coats
by either rebuilding with cob material ( normally
available in the immediate area) or with cob blocks
available from the building lime company.
Defects should not be corrected through coats of “dubbing
out” ( Thick applications of render / mortar ). 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 if possible without
causing more damage and repaired with matching lime mortars
or chalk / chalk cob
Cob 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, will grow back and attack the bonding between
lime finishes and the background.
Cob and earth walls have a friable surface which needs to be
prepared before render or plaster can be applied. The wall
surface should be wet down the day before application. On
the day of application it 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 left for at least 4 days to set hard
before the application of the first scratch coat of render
The harling material is a combination of aggregates and
lime, mixed into a slurry consistency and applied by casting
vigorously at the wall.
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 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 2 will set more quickly 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 5șc 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.
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
Harling 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.