Decorative Masonry Repairs
Catherine Woolfitt
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Masonry joints through decorative elements and sculpture require special care in cutting out and placing
repointing mortar and in the design of a physically and visually compatible mortar. |
An understanding of the
characteristics of building stones
and their weathering and decay in
the external environment is fundamental to
effective repair of all types of historic masonry.
Specifiers need to anticipate the physical and
visual impact of any proposal to clean and
repair a historic building. They should also
bear in mind that cleaning and surface repair
normally require listed building consent and
all aspects of the proposed work, whether
aesthetic or technical, need to be considered.
Historic stonemasonry facades can present
a wide spectrum of challenges, and variations
in stone type, date, extent of weathering and
style of construction are all significant factors.
Broadly, the older the building and the greater
the extent of past repair and alteration, the
more complex and challenging the remedial
work is likely to be. Heavily decayed surfaces
can be particularly demanding, especially
when they are in exposed or inaccessible
locations, occur consistently across a facade, or
pose a threat to health and safety.
This article focusses on minor repairs to
decorative ashlar masonry – that is to say,
accurately dressed blocks with relatively fine
joints. It therefore excludes works to rubble
masonry, and major structural repair work
and concentrates on facades of functioning
buildings, excluding the ruined monument
context. Other masonry types and contexts
may present different challenges and will often
demand a different approach from ashlar.
SURVEY AND UNDERSTANDING
Condition survey is the first essential step
in the process of developing and planning
remedial work. The aim is to gather as much
information as possible about the nature of
the masonry, the building stones used and the
history of past repair work, and it is usually
carried out in several stages depending on
the level of access and whether cleaning is
involved.
At the planning stage, a preliminary
survey from ground level with the aid of
binoculars may be sufficient to produce
remedial work documents (a specification,
schedule of work and budget costs), all of
which will normally be provisional to some
degree. For this reason it is common practice
to include a provisional sum for unforeseen
work, repair items which are difficult to detect
from ground level.
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The Whitehall elevation of the Foreign and
Commonwealth Office: survey should include
assessment of differential weathering and decay
across a masonry facade, from sheltered, recessed
areas to severe exposures, such as copings and cornices. |
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The preliminary survey should consider
both the individual masonry elements and the
complete elevation, and an understanding of
surface condition and soiling is important.
Studying soiling patterns will reveal how
rainwater is shed, how masonry joints
function and where they fail to function
properly due to localised decay.
Variation
and irregularity in the deposition of soiling
may relate to natural weathering but can also
result from stonemasonry defects. Patches
or streaks of clean stone in the midst of
soiling typically indicate localised rainwater
washing, which may relate to exposure, the
orientation of the facade, or environmental
conditions such as the prevailing wind and
proximity to other buildings or trees.
Streaks
can also indicate the presence of defects such
as fractures or eroded perpendicular joints
(the ‘perpends’) in cornices or mouldings
which will eventually admit rainwater.
With time, rainwater penetration will
promote decay of the stone arrisses at open
joints. Once the mortar joint is completely
eroded, characteristic rain-washed streaks
will form on soiled stone surfaces below.
It is normal practice to carry out cleaning
work in advance of masonry repair for two
reasons. Firstly, depending on the extent and
nature of surface soiling, it can be difficult to
identify and assess defects without cleaning first. Secondly, repairs made to a moderately
to heavily soiled surface can be highly
conspicuous – a patchwork of light blocks
set within a dark ground. Indeed, even where
stonework has been cleaned, repairs in new
(matching) stone may appear lighter in colour
next to weathered, historic stone which will
have its own patina.
Typically a pre-cleaning inspection
is carried out as soon as scaffold access is
available. This is a general assessment to check
for potential problems such as open joints
and fractures that may admit water during
wet cleaning work. Detailed survey normally
proceeds after masonry cleaning so that all
defects are readily identifiable.
Responsibility for inspection, scheduling
and documentation should be clearly set
out in the contract documents. All survey
work should be carried out by specialists
experienced in the conservation and repair of
decorative masonry, and the detailed survey
and condition assessment is best carried out
by the contractor and specifier jointly, as this
has a number of advantages: repairs can be
agreed and measured more quickly, unusual
or difficult repair issues can be resolved
jointly, and minor intrusive investigation to
understand the extent or nature of defects
can be carried out immediately. Contract
documents should set out the criteria for stone
repair and replacement (discussed below).
COMMON DEFECTS, BUILDING
STONES AND REPAIR TYPES
Stone masonry defects should be assessed
individually and repairs tailored to suit the
exposure and the building element. This is
especially important for elaborately detailed
buildings with decorative mouldings and projections such as cornices.
Repair types commonly include:
- stone indents or ‘piecing-in’ repairs
- mortar or ‘plastic’ repairs
- the installation or repair of
existing lead weatherings
Stone indents can be designed to suit all
conditions, but mortar repairs are sometimes
more practical, particularly for smaller defects
or in sheltered areas. However, as a general
rule, mortar repairs will not be sufficiently
durable for repair work on severe exposures
(unprotected copings, cornices, ledges, other
projecting mouldings/elements) although they
may be suitable in more sheltered locations
(with reinforcement, depending on the size
and location) and can be used if necessary
at ground level where their condition can be
more easily monitored and they can be more
readily renewed.
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Relative degrees of exposure and weathering: exposure is a key criterion for scheduling and specifying
individual repair methods and materials. Stone in the red zones typically exhibits a greater degree of
weathering and deterioration. To be durable and effective, repairs must be designed to suit the exposure. |
Repair and replacement stone should be of
matching type. If the original stone is no longer quarried and reclaimed stone is unavailable,
the replacement should be the nearest possible
match, both geologically and visually.
Understanding of the nature of
the building stone and its weathering
characteristics is fundamental. The range
of building stone types used historically
across the UK is wide, and properties vary
considerably even within a geological group.
For instance among the English Jurassic
(oolitic) limestones, which include the Bath and
Portland groups, there is considerable variation
in weathering characteristics and durability.
Best masonry practice dictates that the
natural geological bedding planes of the stone
should be observed and stones laid with the
correct orientation of the bedding planes,
relative to their location in the building. For
example, ashlar facing stones in a flat wall
face should be laid with their natural bedding
planes parallel with the ground, not face
bedded, while blocks used for projections
(cornices, any element with a vulnerable
soffit) should be laid with their bedding
planes vertical and at right angles to the
building face to avoid the risk of delamination
of layers.
Failure to understand the natural
features and weathering of building stone,
the gradual erosion of surfaces that occurs
naturally versus the accelerated deterioration
that results from masonry defects, can result
in flawed condition assessment and incorrect
specification of repair work. It may also lead to
the loss of historic fabric if masonry is wrongly
condemned for replacement rather than repair.
TRADITIONAL MASONRY DETAIL
AND PROTECTION AGAINST
RAINWATER PENETRATION
Allied to the understanding of building
stone, is knowledge of traditional masonry
construction, which is also fundamental.
Survey should include the study of original
detail, how masonry blocks were worked and
dressed, how joints were finished (and their
original width), and whether the masonry is
functioning correctly.
Defects and failure of
the joints must be identified as joint treatment
is an integral part of masonry repair work.
Stone decay at the arrises of open, eroded
joints is a very common defect, especially
on projecting elements such as cornices and
mouldings, and compromises the integrity
of masonry and its ability to shed rainwater.
Traditional saddle joints were designed to
direct rainwater falling on cornices away
from the perpend joints, with the stone profile
raised at the joint shoulders. On deep and
large cornices, decayed, open joints promote
deterioration of the soffit, so grouting and
repointing is essential.
Particular attention
should be paid to all protective masonry
details that shed rainwater. These include
the drips (located on the lower leading edges
of cornices and other projecting mouldings)
and falls (sloping surfaces) on copings and
cornices. Where these details are decayed or
damaged, any water-traps which form will lead
to stone decay, and it is essential to reinstate
their weathering function.
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Above left: understanding the weathering characteristics of natural building stone is an essential part of survey work.
Weathering has accentuated the natural bedding planes of the limestone and eroded the softer beds of this
late-19th century masonry. Progressive decay is especially problematic in the case of projecting elements where
there is an increased risk of failure and detachment of fragments. Above right: corroding ferrous metal cramps can cause
considerable damage in the form of cracking and
spalling, as visible in this Portland stone chimney
where extensive stone indents are now required. |
In some cases the installation of lead
protection may be an option. Lead sheet
weatherings have been extensively used
historically to protect cornices, pediments,
copings and other exposed details from
weather. They can prolong the life of
stonemasonry if properly detailed, with
welted joints and a drip to throw rainwater
(in accordance with Lead Sheet Association guidance) and may offer an alternative to
extensive stone replacement.
The installation
of leadwork involves cutting a chase into the
masonry above to fit a cover flashing, and this
will need to be weighed against the potential
benefit of protection that lead can provide. In
addition, weatherings will inevitably have some
impact on the appearance of the building (see
illustration), although this may be relatively
minor when seen from ground.
Cracks
Visible defects commonly include cracks
and fractures which may range in scale from
hairline to 10mm or more. It is important
to determine the cause of fractures and
whether they relate to hidden elements such
as imbedded ferrous metal fixings, or to
structural movement.
Dog cramps were widely
used in historic masonry to provide additional
lateral restraint where necessary. They were
often placed relatively near the stone face and
when corrosion occurs the expansion of rust
leads to characteristic fractures and spalls
occur, normally at the upper corners of the
affected blocks.
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Lead has been used extensively on this Grade I listed
building to protect vulnerable historic masonry, such
as the Bath stone copings and cornices. |
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The use of non-destructive
testing methods like metal detection can
be helpful to locate concealed cramps and
reinforcement of historic repairs. Gentle
tapping (‘sounding’) with a small metal tool
helps to detect detachment where there are
visible spalls, fractures or other defects.
Old cement repairs
Past repairs, as well as visible stonemasonry
defects, should be checked. Unfortunately,
many historic buildings exhibit a legacy of
historic repair work in unsuitable materials,
often in excessively hard and impermeable
mortars such as the ubiquitous ordinary
Portland cement (OPC). In the worst cases
large areas of masonry were refaced with
OPC mortars.
Commonly used in the past,
as an inexpensive and easy repair option,
this practice unfortunately still occurs, often
without regard to original joint lines and
other historic features.
This kind of repair
exacerbates the decay of more porous and
permeable underlying stone, and failure
eventually occurs at the interface between
the two. Such large scale re-facing of stones
should never be considered a repair solution or
alternative to correctly detailed stonemasonry
repairs (indents, replacement units) based
on traditional masonry skills.
Unfortunately,
however, past repairs of this kind, from small
to large scale, are often so extensive that the
best that can be practically achieved is to
renew those that have failed and detached, and
are evidently causing problems, and leave those
that remain well bonded, provided they are not
posing a health and safety risk.
PRINCIPLES AND PRACTICE –
REPAIR OPTIONS, CRITERIA FOR USE
& OTHER ISSUES
One principal aim of remedial work is to effect
repairs with minimal loss and alteration of
historic fabric. Maintaining the integrity of
masonry and restoring essential detail, such
as weatherings, where damage or loss has
occurred are also key objectives.
In practice,
replacement may be the best option if the
structural stability or weathering function of
individual stones is completely compromised
and cannot be restored with localised surface
repair. However, isolated and discrete defects
on a block, such as a spall from a corroding
ferrous cramp or decay adjacent to an open
joint, can often be repaired using stone indent
repairs whereby a new stone is cut and shaped
to fit the damaged area.
For sheltered or relatively small areas lime
mortar repairs may be used. A range of lime binders are available, from high calcium lime
(with or without additives such as pozzolans
to enhance performance) to natural hydraulic
limes (NHLs) graded from 2 to 5 based on
strength, to suit various exposures.
For both indents and mortar repairs the
damaged surface must be carefully prepared.
Lack of surface preparation and lack of
adequate reinforcement or armature are two
of the most common causes of failure of past
repairs. The stone surface should be cut back
to sufficient depth that the indent or mortar
will remain safely in place and the edges of
the repair cavity undercut to hold the repair.
Although it is often possible in the case of
larger indent repairs to use mechanical (disc)
cutters to remove the bulk of the damaged
stone, the final dressing of the repair cavity,
the back and sides, should be done by hand
with sharp masonry chisels to form well-cut,
neat edges to the repair. The interface between
a stone indent and repaired stone should be
very fine, not a masonry joint, for maximum
durability. Good indent repairs require a high
level of masonry skill.
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Dentils in a cornice which had been poorly repaired in the past with machine-cut dentils that were larger than the originals and inadequately bonded to the
underlying stone. |
After removing the failed replacements, the stone was dressed back to enable a
more effective indent repair with a new section of dentils worked as a single piece
of stone. |
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The new indent unit loosely slotted into the cavity, ready for fixing |
The finished piece after fixing with new stainless steel dowels and grouting at the
interface |
Trials and exemplars
should be part of any repair contract to ensure
at the outset that the required standard can
be achieved. Historic indent repairs of very
high quality are often found on historic
masonry buildings, and are testament to what
is possible. These historic repairs often have a
curved profile, following the edge of a defect to
minimise repair size and avoid the mechanical
appearance that can result when indents are
cut square.
The following general rules should be
observed when carrying out historic masonry
repairs:
- Historic masonry joints must be followed
in all repair and replacement work. If
repairs bridge masonry joints, evidence
of the original construction (joint details)
will be lost, and there is a risk of the
repair fracturing along the joint lines.
- Stone indents to exposed edges, for
example on the nosings of cornices,
steps, and copings, should be
dovetailed in place to secure them.
- In general, indent repairs should be
anchored by dowels (normally corrosion
resistant stainless steel or phosphor
bronze) inserted into fine drillings into
the indent and the back of the repair
cavity, and bonded with resin designed for
this application. Resin should be used in
the drilling only and not smeared across
the stone surfaces at the interface.
- Lime grout is the appropriate material
for bonding the stone interface,
which must be completely filled to
avoid rainwater penetration using
grout specifically designed to suit the
exposure, stone type and colour.
- Both stone indents and replacement
stones should follow the original
profile of mouldings as far as these can
be determined. This can be difficult
with weathered surfaces and evidence
should be sought in the least weathered
areas and profiles recorded onto a
permanent template, such as zinc.
- Mechanical saw marks on stone faces are
completely out of character and should not
be accepted. Stone repair faces should be
finished by hand and dressed to replicate
original tool marks of the historic masonry.
- Masonry joints finished as part of
stone repair and replacement work
should also be visually and physically
compatible with the existing.
Provided these general rules are followed to a
high standard using good masonry skills, repair
work, and in particular indent repairs, should
provide a good long term solution and enhance
rather than detract from the historic masonry.
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Recommended Reading
G Allen et al, Hydraulic Lime Mortar for
Stone, Brick and Block Masonry, Donhead,
Shaftesbury, 2003
J Ashurst and FG Dimes, Conservation of
Building and Decorative Stone, Butterworth-Heinemann,
Oxford, 1998
British Standards Institution, BS 8221-1:2012
Code of Practice for Cleaning and Surface
Repair of Buildings, BSI, London, 2012
Ian Brocklebank (ed), Building Limes in
Conservation, Donhead, Shaftesbury, 2012
English Heritage, Practical Building
Conservation: Stone, Ashgate, Farnham, 2012
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The Building Conservation Directory, 2015
Author
CATHERINE WOOLFITT ACR MIfA is a
conservator and archaeologist specialising
in the care and conservation of ancient
monuments and historic buildings, with
particular emphasis on architectural detail
and sculpture in stone, fired clay and lime-based materials.
She established Catherine Woolfitt Associates in 2008 and provides technical
advice, consultancy and teaching services in
the conservation of the built heritage.
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