and Temporary Works for Historic Buildings
|Temporary support designed to spread loading across vulnerable foundations
great deal of time and effort is put into developing schemes for the conservation
of historic buildings but it sometimes seems that scaffolding and temporary
works, the means by which the conservation schemes are successfully completed,
receive scant attention. If not erected properly and with due care and
attention to detail, these works can cause a great deal of damage to historic
basic processes of design and erecting scaffolding and temporary works
to an historic building are not greatly different from those necessary
when any other existing building is affected. The purpose of this article
is to highlight some of the important points which need special attention
if damage to historic fabric is to be avoided.
non-historic buildings are concerned, damage caused by improperly erected
scaffolding and temporary works, whilst being tiresome and causing unnecessary
expenditure, can often be repaired without serious harm. Where historic
fabric is concerned any damage is permanent and significant detail may
be lost. Once scarred, an important facade is scarred forever.
badly erected shoring work, whether or not it is to an historically important
building, has the potential to allow the collapse of part of the building
with disastrous and possibly fatal consequences. Experience indicates
that when things go wrong it is usually due to lack of attention to seemingly
and temporary works must be capable of being constructed without the need
for major intervention into historic fabric. This must be borne in mind
by designers and erectors of scaffolding and temporary works.
tied' scaffolds will normally be provided for painting, pointing or other
maintenance work. They consist of two rows of standards (the vertical
supports) connected by ledgers and transoms (the horizontal elements).
'Independent' scaffolds are not quite what their name suggests. They are
termed 'independent' because they derive no vertical support from the
building and 'tied' because they must be tied to the building for horizontal
stability. Because of the need to avoid damage, tying scaffolding to the
facade of historic buildings can sometimes present difficult problems.
Sometimes, if the historic building is fragile, it will not be capable
of providing the horizontal restraint that the scaffolding needs and this
must be achieved in other ways, such as by providing external scaffold
buttresses or by tying the external scaffold to an internal birdcage framework
scaffolds', used for the construction of brick walls, have only one row
of standards which are usually erected some 900mm from the face of the
wall, with the boards carried on horizontal members known as 'putlogs'.
When used in new construction, the flattened ends of the putlogs are built
into the bed joints as work proceeds and then withdrawn on completion,
the resulting hole being pointed up. (100mm square holes used for timber
putlogs can sometimes be found in medieval work.) Putlog scaffolds should
not be used on historic building work as unnecessary damage is caused
by cutting holes for the bearing of the putlogs.
SHORING OR SUPPORT SCAFFOLDING
works are often needed either because there is a risk that a structure
might otherwise collapse or because it is necessary to remove some vital
supporting member for renewal or alteration. The loads to be carried by
shoring can be very great and the danger posed to passers by and the fabric
by an inadequate design should never be under estimated.
main difficulty with shoring historic buildings is to ensure that their
installation does not cause damage. Shoring must be designed by a structural
engineer or other competent person.
design of scaffolding should not, unless it is very straightforward, be
left to the scaffold erector. It is important that prior thought be given
to the location of scaffold foundations, where standards can and cannot
go and where boarded out decks are required to enable the work to proceed
with as little difficulty and risk as possible.
temporary works should be designed before the site staff begin erection
and the level of design and drawing of scaffolding and temporary works
must be commensurate with the scale of the works. A pencil sketch on a
sheet of paper may well suffice, indicating that at least someone has
thought about what is needed before work commences. A bigger job may well
demand calculations and proper drawings.
failure of even a single telescopic prop supporting a major element of
a building under repair could be fatal; therefore, as the dangers do not
necessarily relate to the size of the project, the architect or engineer
should examine the contractor's proposals for all scaffolding and shoring.
After all, the architect and engineer will have been dealing with the
building for some long time and are more likely to be aware of its weaknesses
than the contractor who, however experienced, may well have only seen
the building briefly before being expected to commence work. It must be
ensured that schemes are erected so as to conform with previously presented
must be exercised to ensure that the contractor's responsibility for temporary
works is eroded as little as possible. Contract documentation for works
to historic buildings should always include a section concerning scaffolding
and temporary works.
It is a statutory requirement that all working scaffolds are inspected
weekly by a suitably qualified person and that the results of these inspections
are recorded in the 'Scaffold Register', an official book of forms which
have to be completed weekly.
NECESSARY FEATURES AND COMMON PROBLEMS
following features and problems are all basic and mostly fall into the
category of 'common sense' rather than being highly technical requirements.
Foundations should always be on firm, level ground and should never be
undermined. Standards and props should be concentric on foundations. When
scaffolding is to remain erected for six months or more, railway sleepers
or similar sized treated timbers should be used as foundations.
buildings often have basements outside the periphery of the ground floor
which may well be incapable of supporting scaffolding, so thought needs
to be given to the means of transferring loads to the ground. One site
on which the author was involved had below ground storage tanks. A huge
lorry-mounted crane was to be used to erect a temporary roof and it was
vital to locate these tanks before the lorry arrived on site and found
excavations for foundations are required, there may be a need to provide
archaeological supervision of the digging operations.
it is necessary to erect temporary structures on floors or roofs it is
important to ensure that the supporting structure can safely bear the
weight or that precautions are taken to ensure that the extra loads will
be adequately supported.
historic buildings often have overhanging cornices and other projections,
correct setting out of the standards needs to be considered in the light
of what is directly overhead.
When ties pass through sash windows, one sash can be raised to allow the
tube to pass through, the resulting gap sealed and the sashes screwed
to each other to prevent unauthorised entry. Casement windows are more
difficult. If they carry leaded lights it may be possible to remove one
small pane but casements with a single glazed sheet may need to be taken
off their hinges and stored safely. Regrettably some scaffolders just
smash a window (which may contain historic glass) to put their ties in
to masonry: Where fixings are made to stone or brickwork it is necessary
to check that the masonry is adequate beforehand. Such a fixing to a facade
could dislodge a stone or an area of brick thus endangering the safety
of the scaffold and damaging the historic fabric. All fixings made to
the wall of an historic structure must be of stainless steel. Listed building
consent may be needed before permanent drilled-in fixings are installed.
Decayed, warped or split boards must never been used as they create tripping
hazards. Boards that have become slippery or damaged should be discarded
and precautions should be taken to hold boards down in high winds. Excessive
loading on platforms should be avoided unless the scaffolding has been
specifically designed to carry heavy loads. If dismantled masonry is to
be stored on a scaffold platform the scaffold designer should be told
of this before the design is commenced.
to building interfaces: However well constructed, scaffolding is always
likely to move slightly and a tube end rubbing on a wall face can easily
cause permanent scarring. All points of contact or near contact between
scaffolding and historic buildings should be protected in some way. All
tube ends that either touch a wall or are within 25mm of it should have
plastic end caps. All standards should sit on timber sole plates to spread
the load and floors beneath should be protected with polythene sheet,
old carpet or similar materials to prevent damage. All scaffolding should
be galvanised to avoid the risk of rust staining.
The wind loading created by sheeting, which is sometimes provided for
weather protection, can be very high and special consideration needs to
be given to the spacing of scaffold to building ties.
props: These may need bracing if they are over two metres high or
if they carry heavy loads. They must be plumb and be properly founded.
It is common to find a missing support pin being replaced by a short piece
of reinforcing bar or something even less satisfactory such as a big nail.
Only the manufacturer's high tensile steel pin should be used.
roofs and temporary buildings: In relation to their area or volume
temporary roofs and buildings are, by nature, light structures. As a consequence
their need for lateral stability and resistance to wind uplift is a major
but often ignored requirement. It is usually advisable to seek the help
of a structural engineer in the erection of such structures. The contractor
should always be required to provide a drawing of his proposals and in
any but the smallest of cases, supporting calculations.
All scaffolding structures which are at risk from lightning strikes should
be properly earthed.
access to the building: Scaffolding can make buildings more vulnerable
to intruders; ladders should be locked away at night and extra security
precautions may be wise.
Efforts should be made to ensure that the workforce is aware of the value
of the historic fabric. It is well worth considering giving the scaffolders,
and indeed other craftspersons, a 'conducted tour' of the building so
that they can begin to understand its importance and not assume that is
'just another old building'. On one English Heritage site the house administrator
took the scaffolders around the building before they began work. This
seemed to engender some genuine enthusiasm and concern for the building
and paid dividends in the extra care which they took.
FORETHOUGHT AND ATTENTION TO DETAIL
and temporary works are not always given the consideration that they deserve.
Consequently there is risk of damage to the historic fabric either in
relatively minor ways such as scarring of surface finishes or in more
serious ways such as partial collapse. There is the additional risk caused
to members of the workforce or to passers-by.
both that produced by the architect or engineer and that produced by the
contractor needs to be commensurate with the scale of the job, bearing
in mind that failure of even a small element can cause serious problems.
Even if only a single telescopic prop is proposed it is important that
some proper estimate of the weight to be carried is made and reference
made to literature to ensure that the prop proposed can carry the weight
and engineers involved in historic buildings work should have a clear
understanding of the requirements of scaffolding and temporary works and
be aware of the consequences if something goes wrong. The safety and success
of scaffolding and temporary works in the historic building field relies
heavily on two things; forethought and attention to detail.
an historic building there will be no second chance.
BS 1139: Metal Scaffolding
Part 1 specification for tubes for use in scaffolding
Part 2 specification for couplers and fittings for use in tubular
Part 3 specification for prefabricated access and working towers
BS 2482: Specification for timber scaffold boards
BS 4074: Specification for metal props and struts
BS 5973:1993 Code of practice for access and working scaffolds and
special scaffold structures in steel
BS 5975:1982 Code of practice for falsework
and Safety Executive (HMSO):
Guidance note GS15: General access scaffolds
Guidance note GS42: Tower scaffolds
Construction summary sheet SS10: Tower scaffolds
Basic scaffolding check guide, National Association of Scaffolding
CJ Wiltshire, Access Scaffolding, Thomas Telford Ltd
Ronald E Brand, Falsework and Access Scaffolds in Tubular Steel, McGraw Hill