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Pointing
Analysis of Original Mortar
Before carrying out re-pointing work on Historic buildings
or structures, determine the type of mortars used in the original
building phase. Make sure when removing samples for analysis
that they are from an original phase of work, and are not
from later remedial works.
The samples should be representative of the bulk of the remaining
material. Analysis is needed to determine correct binder,
aggregate size, colour and type.
Survey
Carefully survey the wall to determine the pointing style
and condition of the mortar. Notes should be made of open
joints, decayed mortar, vegetation and general extent of re-pointing
required. Protection
Before cutting out of defective mortars, windows, doors, drains,
vegetation etc., should be covered and protected. Special
care should be given to friable cut masonry and other special
features such as antique glass etc.
Cutting out of Defective Mortar
Considerable care must be taken when removing decayed mortar
from historic masonry. When the mortar being removed is decayed
or crumbly its removal can be carried out using hand tools.
Plugging chisels, masonry chisels and old hacksaw blades used
in conjunction with a club hammer will easily remove the mortar.
Never use chisels that are wider than the mortar joint as
this may result in damage to the adjacent masonry. The removal
of cement pointing will require greater care if damage to
the adjacent masonry is to be avoided. If wide joints exist,
a series of holes may be drilled through the centre of the
joint, which will allow the joint to break inward when tapped
with a chisel. On finer joints a tap with a sharp masonry
chisel on the top and bottom of the joint will help to break
the bond to the masonry at the edge of the joint.
The use of power tools or air chisels should only be carried
out under strict supervision and by experienced personnel.
Remember that the percussion action of these tools may loosen
masonry.
Where lime mortar requires a chisel to remove it, it should
be questioned whether its removal is necessary.
Depth of Joint
As a general rule joints are cut out to a depth of twice the
height of the joint, quite clearly this rule applies to brickwork,
but in the case of rubble masonry this could result in overly
deep joints, which may de-stabilise sections of masonry. Therefore
a minimum depth of 25mm will allow a good body of re-pointing
mortar. Deep joints must be built up in consecutive layers
with a minimum of 4 days between coats. Notes should be taken
if pinning’s are removed in the cutting out process
and these should be retained for re-use.
Cleaning the Joint
Once the joints have been cut out, they should be thoroughly
cleaned. Brushing out with a small brush will remove loose
mortar and dust, once this has been carried out, the joints
should be flushed out with water. This must always proceed
from the top, working down the building, ensuring all traces
of debris are cleaned down. Where there are deep joints or
voids, care must be taken not to flood the core of the wall.
Applying the New Mortar
Before applying the re-pointing mortar, ensure that the joint
is well damped down. The new mortar should be stiff and not
sloppy, as this will result in shrinkage and leave a smudged
and dirty finish. The new mortar should be placed into the
joint using a pointing iron or a plasterer’s small tool
and well packed into the joint. Pointing trowels should be
avoided, as this will not allow pressure to be applied across
the whole of the joint.
Where there is evidence of stone pinnings or wide joints,
the pinnings (small stone chips) should be packed into the
new mortar. These will help to avoid overly large joints,
which may result in shrinkage. Pinnings will also support
weak areas of masonry.
Finish Style
The style of finish will depend on evidence left of the original
work, but take care not to replicate styles from later remedial
work. On brickwork, struck, weather struck, penny point, flush
finish and tuck, are all common. On rubble masonry, it is
unlikely to have been a distinct style, as the remains of
the mortar are likely to be nothing more the struck off bedding
mortar. Therefore, a joint, which is flush or slightly recessed,
will blend with the masonry. The finished joint should be
finished by beating the wall with a stiff churn brush once
the mortar is fairly stiff. This process also compacts the
mortar and leaves an open texture, which will aid evaporation
of moisture.
Aftercare
New pointing work should be protected from frost, rapid drying
and direct rain for a minimum period of 7 days. See ‘Aftercare’
statement.