Damp in historic buildings Diagnosis by luckboy


More Info

Damp in historic buildings
Damp /s one of the primary causes of decay in historic buildings. Effects of damp Damp affects the physical properties and endurance of materials and can create ideal conditions for fungal attack and beetle infestation. The thermal performance of walls is reduced causing greater heat loss. Traditional building construction Materials and methods used in the control of damp in modern buildings often have an adverse effect when used on historic buildings. Modern buildings: External walls are usually constructed of dense impermeable materials and rely on some form of physical barrier to prevent moisture penetration. External walls have an air cavity to isolate the inside masonry from the weather face, whilst solid floors are protected by a water resistant flexible membrane, (fig 1) Historic buildings: Traditional construction materials such as stone, brick, timber or wattle and daub are all porous and absorbent to varying degrees. Water can be sucked up from the ground or soak into the wall from rainfall. This moisture in the fabric evaporates from porous joints and finishes if the building is well ventilated. (fig 2) • A case of damp may have more than one cause. • A long term assessment may be necessary as dampness can be seasonal. • Leaks from concealed plumbing can be mistaken for water penetration. The source of damp must be located before considering specific treatment. Building defects are the most common source and professional advice is recommended. Expert advice The distribution and quantity of moisture and the presence and nature of salts are as important in the diagnosis of damp as its location. Experts in the diagnosis of damp can take measurements of damp and analyse small samples taken from walls to supplement visual diagnosis. Their advice on methods of treatment

Treatment of damp • Keep up regular maintenance of the external fabric. Defects must be remedied using compatible materials. (Factsheet 'Principles 01') • Control rising damp in walls and floors. • Maintain or increase levels of ventilation to aid evaporation. • Reduce water vapour and increase surface temperatures.

Treatment based upon incorrect diagnosis can result in damaging remedial work and further decay from the untreated damp. Causes of damp There are four main causes of damp in historic buildings: • Penetration of water. • Rising damp. • Condensation. • Hygroscopic salts (usually resulting from rising damp). Locating the source Tracing the course of moisture through the structure of an historic building can be complex, for instance: • The source may be remote from the symptom.

or repair will be based on all available information and an understanding of the building's construction. The advice of a specialist contractor may be biased towards a particular

Defence Works Services Design and Maintenance Guide

Investigating Damp
A visual inspection together with measurements of the damp should prevent incorrect diagnosis. Further monitoring on a seasonal basis may be necessary. Visual inspection of the fabric A systematic inspection of the roof,

roof space, walls and rainwater system should be carried out. Ground levels around the building should be checked, (fig 4) Visually assess the extent and properties of the damp and its effect on materials. Dampness due to rainwater penetration differs from rising damp.

Measuring the damp Instruments should be used to determine the amount of moisture present and its salt content. Relative humidity can also be measured. The efficiency of the instruments varies and the results need to be treated with caution and supported by visual diagnosis.

Causes of penetration
Roof and wall defects are the most common source of rainwater penetration. External faults Lack of routine maintenance is a major cause of building faults resulting in rain penetration. (Factsheet 'Principles 01') 1. Debris, leaves and moss in parapet 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. opening of mortar joints. 9. Damaged rainwater gutters and downpipes. 10. Hairline cracks in render. 11. Missing render. 12. Vegetable growth and debris in gulleys. 13. High ground level.

General failure of materials of the external fabric will also cause water penetration, for instance: • Structural movement eg opening of mortar joints. • .Decay or failure of materials. • Incorrect earlier repairs such as impervious external coatings and cement rich mortar pointing.

Internal symptoms Well defined patches of dampness will be most apparent after heavy rains. The damp will dry out during periods of little or no rainfall. Stains caused by running water may be seen in the roof space on chimney breasts, abutment walls and under the roof finish. The water may damage finishes below ceiling level.

fig 4

Water penetration through external fabric
Defence Works Services Design and Maintenance Guide

Roof level
Simple roof leaks are often left unrepaired because the fault is hidden, remote or inaccessible.

Access Major structural decay can develop undetected within a roof space if access is restricted or unavailable. Parapet and centre gutters are high maintenance areas normally hidden from view. Leaks can be detected early if access is made available for routine inspection. Common faults (fig 4) Water penetration through roofs is a major cause of structural decay in historic buildings. It can be easily prevented by repairing faults. Tile and similar roof finishes • Refix displaced tiles and renew those broken in a matching tile. (See Factsheet Tehnical 3.03') • Renew mortar joints in ridge tiles where damaged, and rebed any found to be loose. Ensure correct weathering details.

Abutments and parapets • Renew fractured or missing lead flashings and soakers at abutments, and parapets, (fig 5) • Renew cracked or missing mortar fillet at chimney stacks. Replacing the fillet with lead flashing is preferred where possible. • Repoint mortar joints on top surfaces of coping stones at parapets. A lead dpc below the coping may be required if water penetration is serious. Parapet gutters • Remove debris, leaves and moss and clear overflows. • Make patch repairs to fractured or damaged lead gutters. Do not line lead gutters with asphalt or bitumen. Refer to Factsheets on Roofs for further information.

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

Defence Works Services Design and Maintenance Guide

Condensation results from a complex relationship between the moisture content and temperature of the air and that of the structure. It cannot be satisfactorily controlled whilst other problems of damp remain untreated. Why does condensation occur? Moisture is present in the atmosphere in the form of water vapour. Warm air can hold more than cool air. When the air is cooled the vapour will revert to liquid at a certain temperature called dew-point, (fig 8) If any surface inside a building is below dew-point temperature it will cool the adjacent air causing condensation on that surface, (fig 9) The temperature at which dew-point occurs varies depending on the amount of moisture in the air.

Symptoms Cool porous surfaces: Condensation will be absorbed. • Moulds, normally black in colour will occur on the surface of paint or wallpaper (fig 10). Rising damp however may cause similar growth. • Moisture readings will be uniformly high, unlike those for rising damp which decrease with height. Cool impervious surfaces: Condensation appears as tiny droplets of water on cool non-porous surfaces. In extreme cases it forms vertical streamlets. • It can occur for instance on oil paint, metal surfaces and wall tiles.

Remedies The following measures will help reduce condensation: Reducing water vapour: • Maintain high levels of ventilation to the building, especially in kitchens, bathrooms, stores, cellars and roof voids. • Treat damp in external walls and solid floors caused by rising damp or rainwater penetration. • Reduce human activities which raise the moisture content of air eg clothes drying, cooking etc. Increasing surface temperatures: Surfaces warmer than dew-point will not attract condensation. • Provide continuous low level radiant heating to maintain warm surfaces. Intermittent heating in old buildings can actually aggravate condensation. • Insulate roof voids and install a continuous vapour barrier.

Ventilation is the key to a healthy historic building. It reduces dampness, combats condensation and prevents timber decay. The need for ventilation A well maintained historic building will gradually suffer the problems associated with damp when starved of all ventilation. Natural ventilation is vital: • In vacant buildings. Continuous low level heating may also be required. • To all areas and rooms, especially those with high humidity.

• In cellars and roof spaces. Extra ventilation is necessary where felt has been installed, (fig 11) • Behind timber panelling or dry lining to walls where there may be a risk of damp. • At chimney flues when they have been blocked up. • In stores adjacent to external walls. • Below raised ground floors. • Behind furniture where it is close to the wall in areas of high humidity. Provide an air gap.

Defence Works Services Design and Maintenance Guide

A damp proof course (dpc) which acts as a barrier against rising damp was rare before the mid nineteenth century. Rising damp therefore has affected many historic buildings. Symptoms of rising damp Ground water rises by capillary action and affects mainly masonry walls and solid floors. Its extent depends on the amount of water in the ground, the wall's capacity to absorb it, and evaporation levels. Its symptoms differ from those of penetrating rain: • Dampness is always present, even during dry weather but not always easily detectable. Damp will retreat in prolonged dry weather.

• A characteristic 'tide mark' of crystalline salts is often apparent at the top of the damp. • Salts expand forcing plaster to exfoliate and spall, (fig 12)

• When measured dampness reduces with height. Is rising damp worsening? The effects of rising damp may have worsened as conditions in and around the building have changed. • External ground levels may have risen, pushing dampness further up the wall. • An impervious floor, floor finish or damp proof membrane may have been installed. This increases rising damp in walls lacking a dpc. • Ventilation may have been restricted to avoid heat loss. • Non-porous surfaces may have been applied or pointing carried out in cement rich mortar, driving moisture up the wall.

Can a dpc be avoided? •
Do not immediately assume that the installation of a dpc is the bestoronly solution to rising damp. Concentrate initially on the control rather than the elimination of rising damp. If the dampness can be reduced to where further deterioration is prevented, a dpc could be avoided. Reduce the moisture content of the structure and excessive ground water, and increase internal evaporation: • • •


Examine underground drains for leaks and repair. Remedy all faults causing penetration of rainwater. Take measures to reduce condensation. Increase levels of natural ventilation throughout the building (see 'Ventilation' - this Factsheet) Use vapour permeable emulsion rather than impervious paints or other impervious finishes.

Secondary measures The treatment above may be sufficient to reduce the level of damp. The following measures should also be undertaken: • Timber such as skirtings and door linings in contact with the remaining damp wall must be isolated to prevent fungal attack. • Treat any salts which appear on the surface after drying out. (See 'Internal finishes' - this Factsheet) ter in the walls. It is therefore only effective if used in conjunction with a dpc but this must not be the sole justification for installing one.Less excavation is required than if relaying on

Solid floor finishes
Rising damp may affect some valuable historic floors. The type of treatment should be tailored to avoid disruption of the floor finish. Valuable floor finishes Porous floor finishes such as stone, brick and some clay tiles which have been laid directly onto ground may suffer from rising damp. • Remove impervious floor coverings such as vinyl sheet. • Do not apply waterproof sealers. A beeswax and turpentine polish may be suitable. • Open the joints in a brick or stone floor by removing hard pointing. • Create a dry area (see next page) or batter the external ground to a channel to direct water away from the building.

Lifting the floor finish • Levels of damp may be unacceptable and some types of floor such as stone slabs can be lifted and re-laid onto a modified base without damage. The methods are disruptive and should be viewed as a last resort. • Take care in lifting the floor as disruption to the finish is inevitable. • An archaeologist must be consulted to advise on the procedure for excavation. Relaying the floor on hardcore Water levels will be lowered by relaying the floor on sand blinding over hand packed hardcore. The treatment may however cause movement in the structure, (fig 13) Relaying the floor on a damp proof membrane This concentrates more ground wa-

Defence Works Services Design and Maintenance Guide

Damp proofing
The installation of a dpc should be considered a last resort. Less disruptive measures may reduce dampness to acceptable levels. Alternatives to a dpc Improve ground drainage and evaporation around the building before installing a dpc. These measures may be the only option if a dpc is unsuitable. Ground levels and finishes • Lower the ground level around the building to 150mm below the internal floor level. • Remove impervious surfaces from around the building. Leave at least a 50mm gap to aid evaporation. • Create a fall away from the building to the surrounding ground to prevent ponding. External Land Drainage A shallow trench with a French drain near to the building will reduce ground water. Backfill with porous material. A similar trench adjacent to the building will encourage drying out at the base of the wall. The backfill should be laid in a geotextile lining, (fig 14) • An archaeologist must be involved at all stages. CAUTION: These measures could affect the stability of the foundations.

Types of dpc An efficient dpc should lower the moisture content of a wall, reducing internal damp.Many systems are available, those most appropriate for historic buildings are described here. Physical dpc: In some regions a band of bricks were removed and replaced with engineering bricks which form a barrier. Dpc's within mortar bed joints involve making a saw cut in sections across the full width of the wall to open the joint. Materials used for the dpc include copper, lead cored felt and heavy duty polythene with carbon black core. (fig 15) • It can only be installed in coursed masonry where horizontal joints are sufficiently wide to avoid damage to the masonry. • There is a risk of settlement in the joint although proprietary

packing methods using grouted polythene envelopes or polyester resin can reduce settlement. • The dpc must be the full width of the wall and continuous. Chemical systems: The wall is impregnated with chemicals through holes drilled at 150mm centres horizontally. The silicone solutions which are used either block or line the pores in the masonry and establish a water repellent zone across the wall. When the solution is cured the holes are grouted up. • It can be used on uncoursed masonry. • There is little structural disturbance but holes will damage masonry. • Any voids in a rubble filled wall must be grouted beforehand or the system is ineffective. The wall can then be replastered in a mix similar to the original. Valuable wall finishes Under no circumstances should they be removed. A clay poultice can be applied to small areas to remove the salts or a sacrificial band of new plaster applied below them. Specialist advice should be sought. Timber mouldings Ensure that a carpenter carefully removes all timber skirtings, dados, architraves etc before treatment begins. They must be numbered to a key plan, examined and treated for fungal and insect attack before reinstatement.
Thomas, A.R. (et al) The control of damp in old buildings. SPAB

Internal finishes
Development of salts Walls will begin to dry out after treatment for rising damp and salts may form on the surface internally from the contaminated masonry. Retention of plaster finishes In historic buildings, the removal of salt contaminated plaster must not automatically be part of the dpc installation. Emphasis must be placed on conserving the original plaster. If the degree of salt contamination in the plaster is tolerable and has no adverse effect on the wall the salts can be brushed off as they appear. The deSelected Bibliography Ashurst, J & N Practical building conservation vol. 2 English Heritage 1991 BRE BRE Digest 245. Rising Damp in Walls

velopment of salts in these circumstances needs to be monitored for worsening conditions. Hygroscopic salts Hygroscopic salts draw moisture from the air and make the wall damp. Removal of the plaster may be avoided if the moisture content in the area is reduced. The contaminated plaster may have to be removed if large quantities of salt are still present. Specialist contractors recommend 1 .Om in height but this may vary. If practicable, allow the masonry to dry out over 4-6 months and regularly brush off salts.
Feilden, B.M. Conservation of Historic Buildings. Butterworth Fidler, J (Ed) Rising damp and damp basements Traditional Homes

Printed in the UK for HMSO 1/96 Dd 008504957D2204B C6 5600 CCN 56294

Acknowledgement L.S.E. Preservation Ltd. Defence Works Services Design and Maintenance Guide

To top