Damp in historic buildings Diagnosis

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					                                                                                               HISTORIC BUILDINGS

Damp in historic buildings
Damp /s one of the primary causes           Traditional building construction
 of decay in historic buildings.            Materials and methods used in the
                                            control of damp in modern buildings
Effects of damp
                                            often have an adverse effect when
Damp affects the physical properties        used on historic buildings.
and endurance of materials and can
                                            Modern buildings: External walls are
create ideal conditions for fungal at-
                                            usually constructed of dense imper-
tack and beetle infestation.
                                            meable materials and rely on some
The thermal performance of walls is         form of physical barrier to prevent
reduced causing greater heat loss.          moisture penetration. External walls
                                            have an air cavity to isolate the in-
                                            side masonry from the weather face,        Treatment of damp
                                            whilst solid floors are protected by a
                                                                                       • Keep up regular maintenance of
                                            water resistant flexible membrane, (fig
                                                                                          the external fabric. Defects must
                                            1)                                            be remedied using compatible ma-
                                            Historic buildings: Traditional con-          terials. (Factsheet 'Principles 01')
                                            struction materials such as stone,
                                            brick, timber or wattle and daub are       • Control rising damp in walls and
                                            all porous and absorbent to varying          floors.
                                            degrees. Water can be sucked up            • Maintain or increase levels of ven-
                                            from the ground or soak into the wall        tilation to aid evaporation.
                                            from rainfall. This moisture in the fab-   • Reduce water vapour and in-
                                            ric evaporates from porous joints and        crease surface temperatures.
                                            finishes if the building is well venti-
                                            lated. (fig 2)

Diagnosis                                   • A case of damp may have more
                                                than one cause.
                                                                                       or repair will be based on all available
                                                                                       information and an understanding of
 Treatment based upon incorrect di-         • A long term assessment may be            the building's construction.
 agnosis can result in damaging re-             necessary as dampness can be           The advice of a specialist contractor
 medial work and further decay from             seasonal.                              may be biased towards a particular
 the untreated damp.
                                            • Leaks from concealed plumbing
                                                can be mistaken for water penetra-
Causes of damp                                  tion.
There are four main causes of damp          The source of damp must be located
in historic buildings:                      before considering specific treatment.
• Penetration of water.                     Building defects are the most com-
                                            mon source and professional advice
• Rising damp.
                                            is recommended.
• Condensation.
                                            Expert advice
• Hygroscopic salts (usually result-
                                            The distribution and quantity of mois-
    ing from rising damp).
                                            ture and the presence and nature of
Locating the source                         salts are as important in the diagno-
Tracing the course of moisture through      sis of damp as its location.
the structure of an historic building can   Experts in the diagnosis of damp can
be complex, for instance:                   take measurements of damp and
• The source may be remote from             analyse small samples taken from
    the symptom.                            walls to supplement visual diagnosis.
                                            Their advice on methods of treatment
                                                                              Defence Works Services Design and Maintenance Guide
Investigating Damp                         roof space, walls and rainwater sys-       Measuring the damp
                                           tem should be carried out. Ground          Instruments should be used to deter-
A visual inspection together with          levels around the building should be       mine the amount of moisture present
measurements of the damp should            checked, (fig 4)                           and its salt content. Relative humid-
prevent incorrect diagnosis. Further
                                           Visually assess the extent and prop-       ity can also be measured.
monitoring on a seasonal basis may
                                           erties of the damp and its effect on       The efficiency of the instruments var-
be necessary.                              materials. Dampness due to rainwa-         ies and the results need to be treated
Visual inspection of the fabric            ter penetration differs from rising        with caution and supported by visual
A systematic inspection of the roof,       damp.                                      diagnosis.

Causes of penetration                                                                 Internal symptoms
                                           General failure of materials of the ex-
Roof and wall defects are the most         ternal fabric will also cause water pen-   Well defined patches of dampness will
common source of rainwater pen-            etration, for instance:                    be most apparent after heavy rains.
etration.                                  • Structural movement eg opening           The damp will dry out during periods
                                               of mortar joints.                      of little or no rainfall.
External faults                            • .Decay or failure of materials.          Stains caused by running water may
Lack of routine maintenance is a ma-                                                  be seen in the roof space on chim-
                                           • Incorrect earlier repairs such as
jor cause of building faults resulting                                                ney breasts, abutment walls and un-
                                               impervious external coatings and
in rain penetration. (Factsheet 'Prin-                                                der the roof finish. The water may
                                               cement rich mortar pointing.
ciples 01')                                                                           damage finishes below ceiling level.

 1. Debris, leaves and moss in para-
 pet and centre gutters.

 2. Damaged lead or asphalt gutters.

 3. Mortar joints in ridge tiles.

 4. Displaced or damaged slates.

 5. Lead flashings at abutments,
 chimney stacks and parapets.

 6. Mortar joints on top surfaces of
 coping stones and parapets.

 7. Open joints in brickwork.

 8. Structural movement e.g. open-
 ing of mortar joints.

 9. Damaged rainwater gutters and

 10. Hairline cracks in render.

 11. Missing render.

 12. Vegetable growth and debris in

  13. High ground level.

  fig 4          Water penetration
                 through external fabric
                                                                             Defence Works Services Design and Maintenance Guide
Roof level                               Access                                        Abutments and parapets
                                         Major structural decay can develop         • Renew fractured or missing lead
Simple roof leaks are often left         undetected within a roof space if ac-         flashings and soakers at abut-
unrepaired because the fault is hid-     cess is restricted or unavailable. Para-      ments, and parapets, (fig 5)
den, remote or inaccessible.             pet and centre gutters are high main-      • Renew cracked or missing mortar
                                         tenance areas normally hidden from            fillet at chimney stacks. Replacing
                                         view. Leaks can be detected early if          the fillet with lead flashing is pre-
                                         access is made available for routine          ferred where possible.
                                                                                    • Repoint mortar joints on top sur-
                                         Common faults (fig 4)                         faces of coping stones at parapets.
                                         Water penetration through roofs is a          A lead dpc below the coping may
                                         major cause of structural decay in his-       be required if water penetration is
                                         toric buildings. It can be easily pre-        serious.
                                         vented by repairing faults.                   Parapet gutters
                                             Tile and similar roof finishes         • Remove debris, leaves and moss
                                         • Refix displaced tiles and renew             and clear overflows.
                                             those broken in a matching tile.       • Make patch repairs to fractured or
                                             (See Factsheet Tehnical 3.03')            damaged lead gutters. Do not line
                                         • Renew mortar joints in ridge tiles          lead gutters with asphalt or bitu-
                                             where damaged, and rebed any              men.
                                             found to be loose. Ensure correct      Refer to Factsheets on Roofs for fur-
                                             weathering details.                    ther information.

Penetration through walls
Defects in rainwater systems and             Use the identical section and ma-      • Repoint open joints in brickwork in
external wall finishes can cause in-'s       terial as the original.                  a mortar to match the original
ternal damp in solid walls.              • Dismantle gutters and downpipes            pointing where the mortar is absent
                                             where joints have failed and re-         or extremely loose, (fig 7)
Defective rainwater goods
                                             assemble with a yam and red lead       • Specialist advice should be sought
Leaks from damaged rainwater gut-            joint.                                   where joints have opened due to
ters and downpipes are normally ap-                                                   movement in the wall.
                                         • Repair cast iron hopper heads
parent on the surface of the adjacent
                                             and all lead rainwater goods if de-    • Hairline cracks in render should be
wall. (fig 6) Evidence may be ob-
                                             fective, do not replace them. Splits     widened sufficiently to repair.
scured where for instance square
                                             in lead downpipes should be              Render for the repair should be of
downpipes are flush with the wall sur-
                                             patched not soldered.                    a similar mix to the original.
face or where exposed gutters are
recessed into elaborate fascias.         • Remove all vegetable growth and          DO NOT use waterproof membranes
                                             debris from the discharge point of       such as bitumens, silicone water
• Replace cracked or damaged cast
                                             downpipes at gulleys.                    repellants or oil paint to remedy
    iron gutter and downpipe sections.
                                         • Realign gutters if necessary to
                                         External Conditions
                                         Some walls can be more susceptible
                                         to water penetration. The degree of
                                         penetration will depend partly on the
                                         • Thickness of wall.
                                         • Degree of exposure.
                                         • Porosity of material.
                                         • Type of construction.
                                         Defective external wall
                                         Rainwater will penetrate walls more
                                         readily where mortar joints have been
                                         washed out or where original render
                                         has cracked or blown.

                                                                            Defence Works Services Design and Maintenance Guide
Condensation                                                                         Remedies
                                                                                     The following measures will help re-
Condensation results from a complex                                                  duce condensation:
relationship between the moisture                                                    Reducing water vapour:
content and temperature of the air
and that of the structure. It cannot be                                              • Maintain high levels of ventilation
satisfactorily controlled whilst other                                                  to the building, especially in kitch-
problems of damp remain untreated.                                                      ens, bathrooms, stores, cellars
                                                                                        and roof voids.
Why does condensation occur?                                                         • Treat damp in external walls and
Moisture is present in the atmosphere                                                   solid floors caused by rising damp
in the form of water vapour. Warm air                                                   or rainwater penetration.
can hold more than cool air.                                                         • Reduce human activities which
When the air is cooled the vapour will                                                  raise the moisture content of air eg
revert to liquid at a certain tempera-                                                  clothes drying, cooking etc.
ture called dew-point, (fig 8)                                                       Increasing surface temperatures:
If any surface inside a building is be-                                              Surfaces warmer than dew-point will
low dew-point temperature it will cool                                               not attract condensation.
the adjacent air causing condensation       Symptoms                                 • Provide continuous low level radi-
on that surface, (fig 9)                    Cool porous surfaces: Condensa-             ant heating to maintain warm sur-
The temperature at which dew-point          tion will be absorbed.                      faces. Intermittent heating in old
occurs varies depending on the              • Moulds, normally black in colour          buildings can actually aggravate
amount of moisture in the air.                  will occur on the surface of paint      condensation.
                                                or wallpaper (fig 10). Rising        • Insulate roof voids and install a
                                                damp however may cause simi-            continuous vapour barrier.
                                                lar growth.
                                            • Moisture readings will be uni-
                                                formly high, unlike those for ris-
                                                ing damp which decrease with
                                            Cool impervious surfaces: Con-
                                            densation appears as tiny droplets
                                            of water on cool non-porous sur-
                                            faces. In extreme cases it forms ver-
                                            tical streamlets.
                                            • It can occur for instance on oil
                                                paint, metal surfaces and wall

Ventilation                                                                          • In cellars and roof spaces. Extra
                                                                                       ventilation is necessary where felt
 Ventilation is the key to a healthy his-                                              has been installed, (fig 11)
 toric building. It reduces dampness,                                                • Behind timber panelling or dry lin-
 combats condensation and prevents                                                     ing to walls where there may be a
 timber decay.                                                                         risk of damp.
                                                                                     • At chimney flues when they have
The need for ventilation
                                                                                       been blocked up.
A well maintained historic building will
                                                                                     • In stores adjacent to external walls.
gradually suffer the problems associ-
ated with damp when starved of all                                                   • Below raised ground floors.
ventilation.                                                                         • Behind furniture where it is close
Natural ventilation is vital:                                                          to the wall in areas of high humid-
                                                                                        ity. Provide an air gap.
• In vacant buildings. Continuous low
   level heating may also be required.
• To all areas and rooms, especially
   those with high humidity.

                                                                             Defence Works Services Design and Maintenance Guide
Introduction                                • A characteristic 'tide mark' of crys-    • When measured dampness re-
A damp proof course (dpc) which               talline salts is often apparent at           duces with height.
acts as a barrier against rising damp         the top of the damp.                     Is rising damp worsening?
was rare before the mid nineteenth          • Salts expand forcing plaster to ex-      The effects of rising damp may have
century. Rising damp therefore has            foliate and spall, (fig 12)              worsened as conditions in and around
affected many historic buildings.                                                      the building have changed.
Symptoms of rising damp                                                                • External ground levels may have
                                                                                           risen, pushing dampness further
Ground water rises by capillary action
                                                                                           up the wall.
and affects mainly masonry walls and
solid floors. Its extent depends on the                                                • An impervious floor, floor finish or
amount of water in the ground, the                                                         damp proof membrane may have
wall's capacity to absorb it, and evapo-                                                   been installed. This increases ris-
ration levels.                                                                             ing damp in walls lacking a dpc.
Its symptoms differ from those of pen-                                                 • Ventilation may have been re-
etrating rain:                                                                           stricted to avoid heat loss.
• Dampness is always present, even                                                     • Non-porous surfaces may have
    during dry weather but not always                                                    been applied or pointing carried
    easily detectable. Damp will retreat                                                 out in cement rich mortar, driving
    in prolonged dry weather.                                                            moisture up the wall.

Can a dpc be avoided? •                         Examine underground drains for         Secondary measures
 Do not immediately assume that the             leaks and repair.                      The treatment above may be suffi-
 installation of a dpc is the bestoronly    •   Remedy all faults causing pen-         cient to reduce the level of damp. The
 solution to rising damp.                       etration of rainwater.                 following measures should also be
                                            •   Take measures to reduce conden-        undertaken:
Concentrate initially on the control            sation.                                • Timber such as skirtings and door
rather than the elimination of rising                                                      linings in contact with the remain-
damp. If the dampness can be re-            •   Increase levels of natural ventila-
                                                tion throughout the building (see          ing damp wall must be isolated to
duced to where further deterioration                                                       prevent fungal attack.
is prevented, a dpc could be avoided.           'Ventilation' - this Factsheet)
                                            •   Use vapour permeable emulsion          • Treat any salts which appear on
Reduce the moisture content of the                                                         the surface after drying out. (See
structure and excessive ground wa-              rather than impervious paints or
                                                other impervious finishes.                 'Internal finishes' - this Factsheet)
ter, and increase internal evaporation:

Solid floor finishes                        Lifting the floor finish                   ter in the walls. It is therefore only ef-
                                                                                       fective if used in conjunction with a
                                            • Levels of damp may be unaccept-
 Rising damp may affect some valu-              able and some types of floor such      dpc but this must not be the sole jus-
 able historic floors. The type of treat-       as stone slabs can be lifted and       tification for installing one.Less exca-
 ment should be tailored to avoid dis-          re-laid onto a modified base with-     vation is required than if relaying on
 ruption of the floor finish.                   out damage. The methods are dis-
Valuable floor finishes                         ruptive and should be viewed as
                                                a last resort.
Porous floor finishes such as stone,
                                            • Take care in lifting the floor as dis-
brick and some clay tiles which have
been laid directly onto ground may              ruption to the finish is inevitable.
suffer from rising damp.                    • An archaeologist must be con-
• Remove impervious floor cover-                sulted to advise on the procedure
                                                for excavation.
    ings such as vinyl sheet.
• Do not apply waterproof sealers.          Relaying the floor on hardcore
    A beeswax and turpentine polish         Water levels will be lowered by re-
    may be suitable.                        laying the floor on sand blinding over
• Open the joints in a brick or stone       hand packed hardcore. The treatment
    floor by removing hard pointing.        may however cause movement in the
                                            structure, (fig 13)
• Create a dry area (see next page)
    or batter the external ground to a      Relaying the floor on a damp proof
    channel to direct water away from       membrane
    the building.                           This concentrates more ground wa-

                                                                              Defence Works Services Design and Maintenance Guide
Damp proofing                                            Types of dpc
                                                         An efficient dpc should lower the
The installation of a dpc should be                      moisture content of a wall, reducing
considered a last resort. Less disrup-                   internal damp.Many systems are
tive measures may reduce dampness                        available, those most appropriate for
to acceptable levels.                                    historic buildings are described here.
Alternatives to a dpc                                    Physical dpc: In some regions a
                                                         band of bricks were removed and re-
Improve ground drainage and evapo-                       placed with engineering bricks
ration around the building before install-               which form a barrier.
ing a dpc. These measures may be the
only option if a dpc is unsuitable.                      Dpc's within mortar bed joints in-
                                                         volve making a saw cut in sections
Ground levels and finishes                               across the full width of the wall to
• Lower the ground level around the                      open the joint. Materials used for the
    building to 150mm below the inter-                                                                     packing methods using grouted
                                                         dpc include copper, lead cored felt
    nal floor level.                                                                                       polythene envelopes or polyester
                                                         and heavy duty polythene with car-
                                                                                                           resin can reduce settlement.
• Remove impervious surfaces from                        bon black core. (fig 15)
    around the building. Leave at least                                                                • The dpc must be the full width of
                                                         • It can only be installed in coursed
    a 50mm gap to aid evaporation.                                                                         the wall and continuous.
                                                             masonry where horizontal joints
• Create a fall away from the building                       are sufficiently wide to avoid            Chemical systems: The wall is im-
    to the surrounding ground to pre-                        damage to the masonry.                    pregnated with chemicals through
    vent ponding.                                                                                      holes drilled at 150mm centres hori-
                                                         • There is a risk of settlement in
                                                                                                       zontally. The silicone solutions which
External Land Drainage                                       the joint although proprietary
                                                                                                       are used either block or line the pores
A shallow trench with a French drain                                                                   in the masonry and establish a water
near to the building will reduce ground                                                                repellent zone across the wall. When
water. Backfill with porous material.                                                                  the solution is cured the holes are
A similar trench adjacent to the build-                                                                grouted up.
ing will encourage drying out at the                                                                   • It can be used on uncoursed ma-
base of the wall. The backfill should be                                                                   sonry.
laid in a geotextile lining, (fig 14)                                                                  • There is little structural disturbance
• An archaeologist must be involved                                                                        but holes will damage masonry.
    at all stages.                                                                                     • Any voids in a rubble filled wall
CAUTION: These measures could af-                                                                          must be grouted beforehand or the
fect the stability of the foundations.                                                                     system is ineffective.

Internal finishes                                        velopment of salts in these circum-           The wall can then be replastered in a
                                                                                                       mix similar to the original.
Development of salts                                     stances needs to be monitored for
                                                         worsening conditions.                         Valuable wall finishes
Walls will begin to dry out after treat-
ment for rising damp and salts may                       Hygroscopic salts                             Under no circumstances should they
form on the surface internally from the                  Hygroscopic salts draw moisture               be removed. A clay poultice can be
contaminated masonry.                                    from the air and make the wall                applied to small areas to remove the
                                                         damp. Removal of the plaster may              salts or a sacrificial band of new plas-
Retention of plaster finishes                                                                          ter applied below them. Specialist ad-
                                                         be avoided if the moisture content
In historic buildings, the removal of salt                                                             vice should be sought.
                                                         in the area is reduced.
contaminated plaster must not auto-
                                                         The contaminated plaster may have             Timber mouldings
matically be part of the dpc installation.
Emphasis must be placed on conserv-                      to be removed if large quantities of          Ensure that a carpenter carefully re-
ing the original plaster.                                salt are still present. Specialist con-       moves all timber skirtings, dados, ar-
                                                         tractors recommend 1 .Om in height            chitraves etc before treatment begins.
If the degree of salt contamination in                                                                 They must be numbered to a key plan,
                                                         but this may vary. If practicable, al-
the plaster is tolerable and has no ad-                                                                examined and treated for fungal and
                                                         low the masonry to dry out over 4-6
verse effect on the wall the salts can                                                                 insect attack before reinstatement.
                                                         months and regularly brush off salts.
be brushed off as they appear. The de-
Selected Bibliography
Ashurst, J & N                                           Feilden, B.M.                                  Thomas, A.R. (et al)
Practical building conservation vol. 2                   Conservation of Historic Buildings.            The control of damp in old buildings.
English Heritage 1991                                     Butterworth                                   SPAB
BRE                                                      Fidler, J (Ed)
BRE Digest 245. Rising Damp in Walls                     Rising damp and damp basements                 Acknowledgement
                                                         Traditional Homes                              L.S.E. Preservation Ltd.
Printed in the UK for HMSO 1/96 Dd 008504957D2204B C6 5600 CCN 56294                           Defence Works Services Design and Maintenance Guide

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