TECHNICAL NOTE No. 52 - Cleaning Masonry Buildings ( pdf

TECHNICAL NOTE No. 52
June 1992
Updated January 2006
Cleaning Masonry Buildings
(Brick, Stone and External Renders)
Introduction
In most instances the possibilities of cleaning a building are first considered for
aesthetic reasons. If after a proper appraisal of the situation there is any doubt as to
the usefulness of cleaning then the decision should be to leave the building alone.
Cleaning of a building can result in hastened decay for newly exposed surfaces and
should only be undertaken after careful consideration.
Before embarking on a contract for cleaning there are a number of questions to be
answered:
i. Does the surface or the pointing need repair?
Most cleaning methods involve water. If excessive amounts are allowed to enter
through fractures or decayed joints it may later be the cause of outbreaks of fungal
decay in embedded timbers and/or the corrosion and expansion of iron cramps and
lintels. These faults must be attended to before any cleaning is attempted;
ii. Know your material. Different repair and cleaning techniques will be appropriate for
different materials. For example, many limestones may benefit from cleaning on a 10-
15 year cycle reducing the build up of harmful crusts. Most sandstones will not derive
much benefit from cleaning and the risks associated may hasten their decay. While
water washing will have little effect removing dirt from sandstones, it can be quite
effective on limestone
iii. What is the nature of the material which is to be removed?
The material may be organic in the form of algae, lichens or mosses or it may be
chemical discoloration and the build up of chemical deposits.
Organic growth is the most common cause of surface discoloration met with in
Northern Ireland. It can be removed by the application of biocides to kill the growth
which is then brushed off dry, followed by the application of a chemical dressing to
inhibit recolonization. Biocides are strictly regulated under Health and Safety
Legislation and their chemical effect on the material of the wall should be carefully
considered before any decision is taken.
Organic growths are not normally damaging to the masonry itself but they do excrete
acids which attack metal work (flashings and lower metal clad roofs) and the matrix of
asbestos cement products.
Where staining and surface salt deposits are the symptom of faults elsewhere following
long term saturation of the masonry, the faults must be first found and corrected.
Deposits of this nature are not usually harmful except in appearance, however deposit

from atmospheric pollution can be a very real and great danger. Thankfully this is rare
in Northern Ireland, but where it does exist it must be removed.
Cleaning Methods
Always have a sample panel as a trial carried out and approved before embarking
upon a cleaning scheme. Cleaning operatives should be skilled and named for the job.
They should be able to demonstrate this skill as part of the trial as standards can vary
considerably.
The methods available for cleaning chemical deposits are:
(A) Water Washing – This is used principally for limestone or marble. First the
deposits are soaked and softened, then in some techniques loosened by brushing. Do
not use steel wire brushes because the detached wires will be left behind and corrode
causing stains. Bristle, phosphor bronze and brass wire can be acceptable. Finally the
surface is washed down. There are a number of specific techniques from water
sprays, mist sprays, pulse cleaning, water lances and many types of brushes.
The amount of water used must be kept to a practical minimum. Local areas of dirt may
be persistent, in which case a pressure water lance or a different cleaning method may
be used to complete the work. Do not simply continue to saturate the fabric until the dirt
moves. Sometimes on larger schemes, temporary guttering fixed to scaffolding is used
to direct water away from floors below.
(B) Abrasive – this is used principally for sandstones but also for some limestones and
marble. These methods are noisy and create dust or slurry, and may be considered too
great a nuisance. Care is required on the part of the operatives not to over clean,
destroying the surface and detailing. Properly handled, these methods are fast and
effective. Wet abrasive techniques clean with the least damage to the surface. They
use less water than pure washing with water and so reduce the dangers of fungal and
other later decay, but they are more difficult to operate than dry methods and so are
not generally popular with contractors. The size and type of abrasive and the pressure
at which it is applied can vary greatly. Choice should be based upon the hardness of
the masonry and of the dirt to be removed. Sand is the cheapest abrasive but the most
dangerous to health. Copper or Iron Slag, nut shells, glass beads and talc are all used
in different systems. All dry blasting techniques should be followed with a washing
down of the building to remove deposits.
(C) Chemical –
i Alkaline or caustic strippers are only designed to be used on limestones or ceramics.
Their disadvantage is the danger of depositing salts in solution which later discolour
the surface. Their use should be considered only as a last resort. They must be strictly
supervised and staining and damage are common. The advantage is that they do not
attack polished and glazed surfaces.
Ii Acid cleaners are principally used for sandstones and granites. Of the acid cleaners,
hydrofluoric acid either in straight dilution or in combination with other chemicals (as in
Neolith 625ss – Agrément certified) is the only material not to deposit salts in solution
and has been the most popular for this reason over the years. It does however, attack
polishes and glazes and these must be masked. It acts by dissolving the silica binder
which holds dirt onto the surface of sandstones. If the chemical is left on too long the
binder can be deposited as a white mark on the stone which can be very difficult to
remove. Iron deposits in some sandstones can also be mobilised leaving brown
streaks. Other acids such as sodium hydroxide or hydrochloric acid are used but

ecause of the risk of salt mobilisation they have a specific use such as paint or
cement removal or they form part of a proprietary poultice.
Only use a proprietary chemical in cleaning buildings and adhere strictly to the
manufacturer’s instructions. Repeated applications are much better than too long a
dwell time on the stone. Maximum recommended periods are from twenty minutes to
one hour though ten to twelve minutes dwell times can often provide optimum cleaning.
Ensure the area to be cleaned is pre wetted with water to avoid absorption. Application
by brush or spray should be even and planned. The chemical should be thoroughly
washed off at the correct time. Scaffolding should also be thoroughly washed.
It is good practice when using any chemical product on stonework to obtain an analysis
of the masonry. Different chemicals will react differently on different stones. These
effects will be long term and knowledge at the start will help to ensure that an
appropriate decision has been made.
(D) Mechanical – this method includes the use of power tools, scrapers, etc. It is
generally labour intensive and is most suitable for plain flat surfaces rather than detail.
This technique inevitably removes some of the surface and is notoriously difficult to
control. Circular saw marks or depressions often result. The best quality work will be
finished off by hand. Care needs to be taken that removal of the surface will not
accelerate decay.
Post-Cleaning Treatment
After cleaning is complete the drains and gulleys must all be checked and left clean
and in working order. Ensure full records are left for further reference.
Further Reading
Ashurst J.,Dimes F.G., Conservation of Building and Decorative Stone, Butterworth
Heinemann, Oxford, 1999.
BS 8221 Stone Cleaning
TAN9 Stone Cleaning Of Granite Buildings, Historic Scotland 1997
Stonecleaning in Scotland, Research Summary, Historic Scotland,1992.
TAN18 The treatment of Graffiti on Historic Surfaces, Historic Scotland 1999
TAN 10 Biological Growths on Sandstone Buildings: Control and Treatment, Historic
Scotland 1998
Environment and Heritage Service
Built Heritage
Protecting Historic Buildings
Waterman House
5-33 Hill Street
BELFAST
BT1 2LA
Tel: 028 9054 3145