Download - Re Pointing Rubble Stonework
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RepointingRubbleStonework
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In traditionally built rubble stone walling it should be expected thatthe mortar that was used in its original construction will deteriorateover time and a need or repointing will eventually occur. I thewall was correctly built in the rst place this is only liable to be
required some time ater the rst 100 years. I repointed properly aurther century o lie can readily be expected rom the wall withoutmuch additional maintenance. Over the last ew decades theprocesses involved in doing this work efectively have not been ullyunderstood. As a result, much supercial and inappropriate workhas been carried out that will not last as long and will soon have to
be redone. Tis INFORM aims to guide building owners throughthe steps that are required or contractors to carry out repointingwork properly.
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The need for pinning stonesWhen traditional rubble stone masonry walls
were originally constructed it was common
practice to use a variety o small stones, called
pinnings, to make the larger stones secure
in the wall. Both the large and small stoneswere bedded, or set, in lime mortar (although,
occasionally, clay was used as a mortar).
Troughout the country a variety o materials
were used in constructing walls, and to make
them as structurally sound and weather-tight
as possible. Tis gave rubble walls distinctively
varied appearances across the country
depending upon what local practices and
materials were used.
Depending upon the region, the lime mortar
nish around the stones could be fush or
slightly recessed so that all o the stone aces
could be obviously seen. Elsewhere the mortar
can be ound to be progressively smeared over
the aces o the rough stones until the nal
nish almost emerges as a ull lime render.
In other areas additional lines o small stones
were pushed into the wet mortar or joint lines
were drawn onto the wall ace beore it set to
give the impression o squared stonework.
Irrespective o the nish, the underlying
rubble stones were generally originally built so
that the stones and smaller pinnings touched
each other. In that way the structural load o
the wall could be satisactorily carried to its
oundation whilst the mortar still had to set.
Tis building process is an important point
to bear in mind when carrying out repointing
work because, i it is ignored, the visual
appearance o the wall can be signicantly
altered.
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When repointing is requiredRepointing will be necessary when the
bedding or jointing mortar becomes washed
out rom the wall and the stones start to
become loose and risk alling out o place.
I let unattended to rainwater can readilypenetrate the heart o the wall and encourage
urther deterioration. I this is not repaired
the wall can ultimately collapse.
In recent times cement mortars have been
commonly used or repointing work but this
is not recommended as it can also damage
the stonework and interior nishes through
keeping water trapped inside the wall. Limebased mortars are much more appropriate and
eective or masonry repointing work as they
let the walls breathe.
Te successul use o lime mortars depends
to a great degree on good site practice.
Provision should be made at an early stage or
appropriate scaolding to be built so that the
work can progress in an appropriate manner.
Beore work starts it is always advisable to
take some detailed photographs o the area tobe repointed. Tis will give a visual reerence
o what the wall looked like beore any
changes are made to it, and the photographs
can be regularly used to check the quality o
the work against the original appearance as it
progresses.
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Site preparationLime mortar repointing work should not be
carried out during periods o heavy rainall or
in rosty conditions. Adequate protection will
need to be considered and this could require a
ully protected scaold. It is also necessary toensure that appropriate access, mortar mixing
areas, site protection, the correct equipment
and all health and saety requirements are in
place at all times.
Raking outo provide a sound base or the replacement
mortar it is necessary to remove any decayed
mortar rom the ace o the wall and romaround the stones. I, at some time in the
buildings past cement mortars have been used
to patch the wall it may be necessary to use
a hammer and chisel to careully break away
the cement and re-expose the underlying lime
mortar. Tis needs to be done with care to
avoid damaging the edges o the underlying
stones. Otherwise, raking out should be done
with a variety o thin metal tools which can
easily penetrate between the stones to remove
the loose or deteriorated old mortar.
Te aim o this approach is to reach theposition where sound mortar remains in
the depth o the wall. In the process it is
inevitable that some o the small pinning
stones will become displaced and all out
o the wall. Occasionally some o the larger
stones might also become displaced, especially
i the wall is in a poor state beore work starts.
Any displaced stones should be careully setaside or re-use in their original position when
the repointing work takes place.
o ensure all o the loose and decayed mortar
is removed the raked out areas should be
careully fushed out with water, taking care
to also fush the loose debris down the entire
wall ace rather than let it accumulate on
the aces o lower stonework. Te resultingdepth o the space into which new mortar
needs to be placed should be in the region o
two to three times the width o the original
joint. Anything less will not give the new
mortar sucient space to set and perorm
appropriately. In the raking out process,
power tools should not be used as they can
be dicult to control and can badly damage
or mark the remaining stonework. During
the raking out work it is also important to
ensure that all vegetation and any penetrating
roots are completely removed rom the wall
structure. Tis is necessary to avoid the risk outure damage to the repointed work should
the growth re-establish itsel.
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In some cases it may be necessary to
dismantle a particularly loose section o wall
so that it can be rebuilt during the repointing
work. Where this is required it is appropriateto paint a square grid onto the ace o the
stones beore dismantling work starts. Using
water-soluble emulsion paint, the loose
stonework should be marked with a square
grid and this area should be specically
photographed or uture reerence beore
work starts. Dismantling should occur so
that the stones are careully laid out beside
each other in the manner by which theywere taken apart rom the wall. Tat way,
using the painted grid lines and pre-work
photographs as a reerence, they can be
readily identied when the time comes to
rebuild them back into the original position
- careully removing the painted grid ater
rebuilding.
Preparation of mortarIn preparing the repointing mortar it is
important to batch the volume o the
lime and sand materials accurately so
that successive mixes can ollow the same
proportions. It is also necessary to understand
the behaviour o lime mortar and how it
sets to use it appropriately. In gaining this
understanding it is important to recognisethat regional variations in the mortar mix can
exist and that these should be copied so that
the appearance o the nished work matches
the original. o do this properly it may be
necessary to obtain some specialist support to
analyse what the original mix consisted o to
ensure that the replacement material will look
right and perorm equally well.
TampingTe process o repointing starts with the need
to push new mortar back into the heart, or
core, o the wall in a process called tamping.
Using appropriate tools the mortar should
be pressed well into the spaces created by
raking out so that it comes into direct contact
with the original mortar thereby eliminatingany voids. Te mortar mix used or tamping
should be the same as that used or the nal
repointing.
PointingDuring this process it is important to replace
any missing pinnings or loose stones back
into their original position and to ensure that
all the replaced stones make physical contact
once more. In doing so, this gives the wall
back its ull structural strength and original
appearance. Any missing pinning stones will
generally be revealed by the remaining widejoints, and these should also be replaced as
work proceeds. o ensure that the pinnings
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are correctly positioned in the wall the
mason will probably have to drive them into
the spaces between the large stones using
the wooden shat o the hammer. Tis is
preerable to using the metal hammer head as
there is a real risk that the metal could shatter
the small stones and render them useless.
As work progresses care also needs to beexercised to nish o the appearance o
the wall in such a way as to match the
original. Te surace nish and texture o
the lime mortar is also important to the nal
appearance. When the mortar has started to
set, dabbing the surace with a wet sponge,
or gentle stroking with a bristle brush, can
be eective in avoiding an overly smooth
nish. Reerence to the pre-work photographs
can assist in creating the right appearance,
the nal joint size and nish on the rough
stonework. Tis can be dicult to control
as sometimes only a ew extra millimetres o
mortar thickness can lead to the creation o a
wider surace nish as the mortar smears on
to the ace o the rough rubble stonework so
particular care is required at this stage o thework to get the nish right.
Lost pinning stonesTe appearance o the wall can be
dramatically altered i pinning stones are
missed out during the tamping stage o
the work. Tis can oten happen as recent
repointing practices have tended to ignorethe time consuming eort that is required
to replace the small stones. Consequently, the
volume o stone that was originally used in
the wall is much reduced, only to be replaced
by a similar volume o mortar.
Tis can give the alse impression that only
large stones were used to build the wall in the
rst place, and that they were surrounded bywide mortar joints. Tis approach not only
creates the wrong appearance but it is also
a alse economy. Because o the increased
amount o mortar that has to be used and will
be exposed to the weather, there is a greater
risk that it will decay aster and the expected
lie o the repointed work will be much
reduced.
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Principal author: Ingval Maxwell
Published by Technical Conservation, Research and Education Group, January 2007Historic Scotland, Longmore House, Salisbury Place, Edinburgh EH91SH
Tel: 0131 668 8638 Fax: 0131 668 8669
www.historic-scotland.gov.uk email: [email protected]
TeHouse1/07Producedfromsustainablematerial
Weather conditionsAs noted , tamping and repointing work
should not be carried out in extreme
weather conditions, and particular care
needs to be exercised i work is being
carried out when there is a risk o rost.In such cases, some orm o insulation
should to be provided to protect the
wall ace that has been worked on.
Usually this is provided by hessian
sheets. Equally, care needs to be exercised
during repointing works when heavy
rain is expected. Tis is to avoid the
resh mortar rom being washed out o
the wall ace and appropriate protectionmay need to be put in place to prevent
that rom happening. In extremely hot
weather intermittent gentle spaying with
clean water, or covering the work with
dampened hessian, will help prevent too
rapid drying.
PerformanceMany lime mortared original rubble walls
have stood in good condition or over
a hundred years and in numerous cases
or much longer, whilst only requiring
minimal maintenance. By recognising
how these walls have perormed in the
past, and understanding the original
techniques that were used in building
them, appropriate repointing workshould ensure their continued survival or
another century or more.
Useful contactsHistoric ScotlandLongmore House, Salisbury Place, Edinburgh,EH9 1SH0131 668 8600
www.historic-scotland.gov.uk
Historic Scotland TCRE GroupPublications Department0131 668 8638
Historic Scotland Conservation Bureau &Technical Enquiry Service0131 668 [email protected]
Historic Scotland Investment andProjects Team0131 668 [email protected]
Historic Scotland InspectorateListed buildings 0131 668 [email protected]
Further readingAN 1 Preparation and Use of Lime MortarsHistoric Scotland, Reprint 2005,ISBN 1 903570 42 5
Mortars in Historic Buildings Literature ReviewHistoric Scotland, 2003, ISBN 1 903570 82 4Te Repair of Historic Buildings in ScotlandHistoric Scotland, 1995, ISBN 0 9517989 2 8Te Conservation of Architectural Ancient Monuments
in Scotland Guidance on PrinciplesHistoric Scotland, 2001, ISBN 1 903570 32 8
All photos I Maxwell