being filled with a suitable mortar mix, compressing the mortar
back well into the depth of the joint and bringing the pointing
flush with the masonry face
mass-masonry wall construction (which includes most walls built
before 1919) works on the basis that moisture entering a wall
is able to escape as water and water vapour as easily as possible.
Lime mortars can readily handle the transmission of water and
water vapour between the inside and outside of a masonry wall,
owing to the complex interconnected pore structure of masonry.
The use of cement mortars is widely recognised as being detrimental
to such buildings and structures as they can drastically alter
the way in which a wall handles water and water vapour. Cement
mortars tend to have a consistent and 'closed' pore structure
that traps water rather than allowing the building to breathe
(not necessarily a problem in modern cavity wall construction).
Any trapped moisture will expand if subjected to freezing conditions,
and mortars may ultimately fail, often causing damage to the surrounding
masonry in the process.
Masonry walls need to be maintained and
repaired, just as roofs and rainwater goods do. Where walls are
solid, without a cavity, keeping them in good repair is necessary
for the interior to remain functional and dry. Pointing is the
most common repair, and often one of the most poorly executed.
The two main reasons for lime-pointing repairs are:
- The original lime pointing has decayed over time and needs
to be replaced. This may be the result of gradual decay through
weathering, or failure as a result of poor maintenance. Blocked
gutters or overflow pipes, for example, lead to damp masonry
and can result in frost damage.
- The masonry has been re-pointed with a cement-based (or
other inappropriate) mortar at an earlier date, which is causing
problems to the building and/or the adjacent masonry and must
be replaced with a more appropriate mortar.
walling, whether ashlar or rubble stonework or brickwork, is generally
built straight and plumb, with joints struck off or pointed flush
with the masonry. This assists with rainwater run-off, thereby
keeping the wall as dry as possible. Because mortars, in principle,
should be sacrificial to the masonry substrate, they normally
weather at a faster rate, forming open or recessed joints in the
While open joints and recessed mortar pointing are often
a result of natural weathering, there is also a relatively modern
'fashion' for reinstating recessed pointing, in some cases significantly
recessed from the face of the masonry. This bears little resemblance
to the original character and appearance of mortar joints in traditional
walling and, in effect, provides ledges on which water can collect.
This reduces the weather-proofing ability of the wall and encourages
additional moisture to enter the masonry units and mortar joints.
Old lime mortar is often raked out and replaced when it is perfectly
sound: it is a common misconception that the softness of existing
lime mortars is a sign of failure. Most historic
mortars are inherently weak and can break or crumble when disrupted.
Furthermore, 'hard' mortars are not necessarily the most durable:
in fact the opposite appears to be the case. Research by A M Forster
at Heriot Watt University (2003) identified a relationship between
the hydraulicity of lime mortars and their water vapour permeability,
and it is now generally accepted that lower (compressive) strength
mortars are more likely to have higher rates of vapour permeability.
Vapour permeability is essential for any mass-masonry walls to
remain dry and functioning as intended. Where defective or open
mortar joints do exist, these can easily be pointed with an appropriate
and compatible mortar, to match any existing or original mortar
in colour, texture and performance.
A REPAIR STRATEGY
repair work, it is imperative that an understanding of the building
or structure is gained before specifying and undertaking any works.
For lime-based works this is particularly important. An original
mortar might have inherent defects. Buildings and their surrounding
environment may change over time: for example a residential building
may now be a roofless ruin. Repair mortars can, and sometimes
should, be designed to perform in a different way from original
mortars to meet new performance requirements.
The first stage,
prior to starting any pointing work, should always be a close
inspection and a survey of the existing masonry walls and their
condition. This should identify and differentiate between any
original mortars and later repairs. If possible, any inappropriate
mortars which are causing accelerated decay of the adjacent masonry
should be removed and replaced with more appropriate materials.
Sound, original mortars should be left in place: the principle
of minimum intervention applies equally to pointing as to other
conservation repair work.
squared rubble sandstone walling which has been repointed
in a modern dense cement-based mortar, accelerating its decay,
particularly at the stone/mortar interface.
If sound original
mortars are found, these may provide evidence of how the joints
were originally finished. In most cases, joints will be flush
with the masonry face, but there may be evidence of 'lining out'
or colouring of the mortar, for example, which should be fully
investigated. Such evidence, when properly understood, should
be used as a guide for repair work. Appropriate and representative
mortar sampling and analysis can reveal the constituents of original
mortars and assist in developing compatible specifications which
will closely match the colouring and texture of the original mortars.
matching original mortar should never be used as an excuse for
removing sound original mortars as, with experienced interpretation
and an understanding of the relevant factors, it will always be
possible to achieve a suitable match.
In many of our oldest buildings,
ordinary brickwork and rubble stonemasonry were finished with
an external coating of lime (a render or harling) and often limewashed.
Evidence of such finishes can often be found on closer inspection
of the walls. However, Victorian rubble masonry was often intended
to be left exposed, as was fine brickwork of all periods.
were as much functional as decorative, and where such coatings
are removed these can often reduce the 'weatherproofing' (not
'waterproofing') ability of the wall to keep the inside of buildings
dry. Clear evidence of their use should always move the emphasis
away from pointing and onto reinstatement of the coating itself.
The appropriate specification and application of such coatings
are critical to ensure the walls function as originally intended.
Specialist advice can be sought where it is not clear what the
original wall finishes were, to avoid a conjectural restoration
of repair mortars should always be based on holistic evaluation
of the building, which can then lead to the determination of performance
requirements for the specific situation. Performance requirements
cover a range of issues including the durability of the mortar,
its ease of use and compatibility with original and/or other surviving
historic materials. In some cases there may be a conflict between
specific requirements, and judgement will be required in achieving
a final specification. To determine whether new materials will be compatible
with surviving historic materials, information on both old and
new materials must be available. Analysis of surviving historic
mortars should be carried out to provide relevant information
on their constituents and performance. Technical information and
performance data available for new materials can then be evaluated
against information on existing materials. Issues to consider
include the material's performance in use, ease of application and compatibility.
mortar should have:
- Adequate vapour permeability
- An appropriate degree of capillarity for the proposed use
- A water absorption rate not significantly greater than the
- An elasticity that reflects the built condition and scale
of the works
- Sufficient tensile strength to suit the construction requirements
- Compressive strength to suit the construction requirements
(usually quite low)
- A bond strength sufficient to achieve a good wind- and water-tight
bond, never greater than the host masonry, nor so feeble as
to result in the separation of the mortar, leading to capillary
ingress of water at the masonry/mortar interface.
- Have appropriate workability characteristics when fresh
to allow the work to be undertaken correctly
- Remain workable for a sufficient length of time to allow
- Achieve an adequate degree of frost resistance at a sufficiently
early age to avoid potential freeze/thaw risk.
for protection and curing should be taken into account at
the specification stage.
WITH ORIGINAL HISTORIC MATERIALS
- Have a vapour-permeability similar to, or greater than,
that of adjacent historic materials
- Be visually compatible with surviving mortars and/or with
the original appearance of the building
- Reflect the historic integrity of the original materials
and methods of construction where practical.
THE RIGHT BINDERS AND AGGREGATES
are available in a wide range of types, suitable for different
applications and locations. The two main types are nonhydraulic
and (natural) hydraulic limes. In line with the classification
of cements, building limes are classified by BS EN 459-1:2001
in terms of compressive strength (N/mm2 at 28 days).
Non-hydraulic lime and sand mortars are most commonly made from
a lime putty containing at least 90 per cent calcium lime (CL90).
CL90 dry hydrate (commonly known as builders' lime) is generally
used as a plasticiser in a cement mortar (1:1:6 or 1:2:9 cement:
lime: sand mortars), and is not generally suited to making lime/sand
mortars. This is because lime (calcium hydroxide) carbonates by
reacting with airborne carbon dioxide (to become calcium carbonate)
in the presence of moisture. This reaction is part of the curing
process. If supplied as a lime putty, the lime is protected from
the air because it is saturated with water, and covered with a
film of water from the moment it is made. In its dry hydrate form,
on the other hand, the lime is supplied in bags or sacks as a
dry powder, and some premature carbonation is inevitable. Its
performance is therefore less predictable.
the stone has weathered back completely, leaving a 'lattice'
of cement pointing visible.
limes (NHLs) come in a range of strengths from NHL 2 (feebly hydraulic)
and NHL 3.5 to NHL 5 (eminently hydraulic), which are suitable
for a range of different applications and building elements. NHLs
come as a dry hydrate powder (as with cement). It should be remembered
that while strength development is relatively fast for cement
mortars, lime mortars strengthen over a much longer period of
time, and this must be taken into account when specifying work.
The correct choice of aggregates can be critical to the success
of a lime mortar. Aggregates should always be well graded and
varied according to the joint widths. Generally, the maximum aggregate
particle size should be around one third of the joint width. Particle
shape also affects cohesion, and sand should be sharp. BS EN 13139:2002
Aggregates for Mortar is the current standard, rarely referred
to, which replaced the more familiar BS 1199 and 1200:1976. Aggregates
must be suitable for their purpose, whether for new work or for
conservation repair work, and this is the first priority. However,
achieving a visual match is also essential, particularly where
old and new mortar are to co-exist, as in patch pointing. Texture
as well as colour is therefore important, and for best results
the aggregate of the existing mortar should be analysed and graded.
The choice of aggregate will also affect the mortar mix ratio,
since particle shape and grading will change the void ratio. In
principle, there must be sufficient binder to bind all the aggregate
particles together: too little binder will result in a weak mortar
with high capillarity and poor workability, too much binder will
increase the risk of shrinkage cracking when the mortar dries
out. Typically, a nominal volume ratio of one part binder to 2.5
or three parts of sand would be expected where a sharp, well-graded
concrete sand (5 mm down) is being used. The minimum binder to
sand ratio can be checked by calculating the void ratio of a particular
sand. This can be achieved simply by drying a sample of aggregate
and then measuring the amount of water required to fill all the
a 'top-down, bottom-up' process. Clearing out and preparation
of joints should always be carried out from the top down, and
pointing should be carried out from the bottom up to take account
of the effects of gravity.
Any cementitious or other non-original
and inappropriate mortars should be carefully removed, avoiding
damage to adjacent masonry. If a cementitious mortar cannot be
removed without damaging the masonry, consideration should be
given to leaving it in place. Defective lime mortars (those which
are friable or have become detached from the adjacent masonry)
should be raked out to a sufficient depth where sound mortar exists.
However, care should be taken to ensure that sound, original lime
mortars are left in place, in accordance with the principle of
Where masonry is relatively impervious and
a suction bond may be difficult to achieve, joints may need to
be raked out further, (say) at least twice the depth of the joint
width, to provide a level of mechanical anchoring of the new mortar
into the joint.
such as plasterers' small tools, half hacksaw blades and specially
made steel hooks can be used to avoid damage to the stone arrises
and widening of the joints. Large chisels and any tools wider
than the joint width itself should not be used. The use of power
tools is frowned on by many conservators, but they should not
be completely disregarded. There are some power tools available
now which can be used successfully to remove existing mortars
(particularly where cementitious), such as those with oscillating
blades. However, where historic masonry is concerned they should
only be used by the most highly experienced craftsperson owing
to the ease and speed with which they can damage masonry.
loose and friable material must be removed prior to placing new
mortar, as it requires a sound surface to adhere to. A stiff bristled
brush should be suitable for clearing stone surfaces of unwanted
material, followed by lightly spraying water (not under high pressure)
into the joints to remove any remaining dust and debris. Preparation
of the wall surfaces generally should include thorough cleaning
down and removal of all loose material, dust, etc, and damping
down before starting work.
Control of suction between the new
mortar and the substrate is required. The more absorbent the stone
or brick, the more wetting down it will need, in order to prevent
water being drawn from the newly-placed mortar into the stone.
If too much water is lost, the mortar will shrink excessively,
become friable and ultimately become detached from the substrate.
Impervious stones may require minimal or no damping down.
re-pointing, the mortar should be sticky but not wet. A suitable
mortar should stick to the underside of an inverted hawk (or trowel).
An appropriate pointing tool should be chosen to suit the width
of the joint, thus preventing spreading of mortar or staining
on the masonry faces. The mortar should be firmly pressed into
the depth of the joint with the pointing tool. Where pinning stones
exist (normally found in rubble-stone mortar joints over 10-15
mm) they should be hammered in any joints to force the mortar
well back into the depth of the joint and to reduce the volume
of mortar present in one location.
In general, when mortar has
stiffened up, it should be firmly compacted into the joints by
beating with a stiff bristle brush. This will help eliminate any
initial shrinkage cracking and ensure that the mortar is fully
compacted into the joint with a good bond to the surrounding masonry.
should then be lightly scraped back with the edge of the pointing
tool or similar, to provide a rough, open-textured surface which
is ideal for carbonation and curing, and for maximum evaporation
of moisture from the joints once fully cured.
surface of the mortar will result in surface laitance caused by
lime particles being 'worked' to the surface, and forming an outer
crust which may restrict carbonation of the mortar behind. This
can also result in lime leaching if subjected to rainfall before
the mortar has sufficiently cured.
Sample panels of pointing should
always be produced at an early stage in the contract, as these
will serve to clarify final details of specification, define the
standards of workmanship and finish, and provide control
samples for guidance of the contract work.
require adequate protection until they are fully cured, and inadequate
protection is a common cause of failure in lime work. These mortars
should not be expected to cure as quickly as cement-based mortars.
masonry with poorly executed cement pointing; an attempt to
lightly rule some horizontal joints (albeit in a random fashion)
does little to take the eye away from the rather synthetic
grey/blue colour typical of OPC mortars.
In drying conditions, new lime pointing will need to be dampened
regularly (by lightly spraying) to prevent rapid drying. Over-rapid
drying will result in shrinkage cracking due to rapid loss of
water and will inhibit curing because the lime will only react
with carbon dioxide in the presence of moisture. For hydraulic
limes, retention of moisture within the mortar for the first few
days is critical to ensure that the hydraulic set takes place
because its hydraulic constituents react with water itself: thereafter,
the remaining carbonation set requires cyclical light damping
and slow drying.
The length of time required for the mortar to
cure is also variable and will depend on environmental conditions,
type of mortar used, its finish, and the mass of mortar in any
one location. Curing can be accelerated by good working practices
with pinning out and finishing joints (as described above). Adequate protection
can usually be achieved by close covering the new work with hessian
and polythene sheeted panels placed against the face of the new
In conclusion, it is ultimately the role of specifiers and
users of lime-based mortars to ensure that appropriate materials
and methods are used for the finishing of historic masonry walling,
whether lime pointing or otherwise. Understanding the nature of
traditional materials and building methods will assist by providing
a starting point for developing suitable repair strategies and
specifications, working within current legislative controls, taking
account of the relevant philosophical issues and ensuring they
are appropriate to modern day working practices. General advice
and guidance for lime pointing must be approached with caution.
It is important that regional, local and building specific variations
are considered to ensure that local and vernacular traditions
affecting the character and appearance of our historic built environment
are fully considered. The use of standard specifications and solutions
are clearly not suitable for lime pointing, as for other building
conservation repair work.
- BS EN 459-1:2001,
Building Lime: Definitions, specifications and conformity criteria, British Standards Institute, London, 2002
- Scottish Lime
Centre Trust, Historic Scotland Technical Advice Note 1, Preparation
and Use of Lime Mortars, Historic Scotland, (originally
by Pat Gibbons, 1995) Edinburgh, 2003
- Stafford Holmes
and Michael Wingate, Building with Lime: A Practical Introduction,
ITDG, London, 2002
- Philip Hughes,
SPAB Information Sheet 4, The Need for Old Buildings to Breathe,
SPAB, London, 1986
article is reproduced from The Building Conservation Directory, 2007
MSc IHBC was, at the time of writing, Senior Buildings Consultant with the Scottish
Lime Centre Trust. He now runs his own building conservation and traditional masonry consultancy.
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