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T W E N T Y S E C O N D E D I T I O N
T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 5
1 5 3
SERV I CES & TREATMENT :
PROTEC T I ON & REMED I AL TREATMENT
4.1
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.
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. 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
Lead has been used extensively on this Grade I listed
building to protect vulnerable historic masonry, such
as the Bath stone copings and cornices. Leadwork
which has been properly detailed in accordance
with the guidance of the Lead Sheet Association
can prolong the life of stonemasonry, in some cases
providing an alternative to extensive repair or
replacement work.
Finish is important: these clean new replacement
stone facings jar with the original masonry and
mechanical saw marks disfigure their surface.