1 The cause of dampness
2 Tracing the cause
3 Rain penetration
4 Rising damp
6 Internal wall treatment
There are several ways in which the decay and damage of the fabric of an old building can generate excessive dampness. Waters carrying impurities may rise up from the soil into the building or driving rain or snow may penetrate through hairline cracks on rendered walls or through decayed brickwork. Excessive dampness encourages the growth of wood-rotting Fungi, the infestation of wood-boring insects and the corrosion of Metals. It can damage the contents of a building, the decoration and the furniture. Most importantly excessive dampness inside buildings can be harmful to the occupants’ health, and therefore must be prevented or treated.
1 The cause of dampness
Most traditional buildings absorb moisture and are porous to a certain degree. Roof timbers can on average maintain up to 12-16% water content, which generally is fine and not susceptible to fungal attack until its moisture content rises above 21%. This increase is generally caused by leaks, condensation or damp masonry.
Most old buildings have solid walls, no damp proof courses and no felt under tiles or slates. Driving Rain can therefore penetrate the fabric of the building and rising damp can also enter into the building.
Traditional building material such as stone, soft brick, lime-wash, lime based mortars and plasters absorb more water than modern day building materials but these traditional building materials have the advantage of allowing moisture to evaporate during drier conditions. Traditional living generated small quantities of water vapour.
Before central heating was commonplace, open fireplaces would heat buildings. The open fireplace would draw large quantities of air through windows and doors, evaporating moisture from porous internal walls and the draw of air would remove moisture from timbers.
Today’s requirement for comfort and for warmth has meant that we have seen a reduction in ventilation, and with modern materials and decoration can in turn trap moisture in old buildings.
Central heating when first started up after summer months can draw soluble salts to the wall surface which then crystallize causing efflorescence. Therefore the heating should be started up gradually, at lowest levels to allow the building to acclimatize.
Rain penetration and spillage
Rain penetration can be caused by incorrect design, bad workmanship, structural movement, badly executed repairs or lack of routine maintenance or the use of the wrong materials.
Roofs, chimneys and parapets are the most exposed part of the building and therefore can be the most susceptible to rain and snow penetration. In traditional buildings felt tends not to have been used under tiles or slates, and therefore if gaps occur or slates are missing, cracked or displaced, rain and snow can be driven into the roof-space. The abutment of a chimney or parapet to a roof is a vulnerable junction especially if a cement mortar has been used rather than a lead flashing. However, on occasions fillets of properly gauged lime mortar have been used successfully.
The free flow of water in gutters is essential to prevent leakage or overflow. All leaves, moss and dirt should regularly be removed from gutters. Leakage from parapet gutters is particularly serious, as the space beneath tends to be warm, unventilated and contains the bearings of the roof trusses. Penetration can create Dry Rot, which has been the cause of the destruction of many old building. All down pipes should be clear of obstructions as blockages can create backups and then spill out at the joints. If prolonged wetting occurs, this can lead to damp patches and growth of moss, which will then retain moisture.
Traditional Stone walls are generally porous and when set in a lime mortar of equal porosity slowly absorb the falling water, but if the wall has sufficient thickness then the wall will retain the water and draw it back to the surface to evaporate when the rains have stopped. If however the walls are not thick enough or the joints are weak or damaged then a lime based render would be used to protect and cover the walls. Modern renders made with cement are brittle and have a tendency to crack with the buildings natural movement or with variations in climate and therefore create cracks that allow rain fall to penetrate easily but find difficult to escape by evaporation. The build may therefore find its way into the internal face of a wall or even blow the external reader leading to further exposure and potential damage.
If a porous walling material such as Stone or brick is placed directly onto the damp soil, moisture will be drawn into it, this is called Capillary action. The absorbed moisture will rise into the wall to a height where there is a balance between the rate of evaporation and the rate it can be drawn up by the Capillary forces, the height will vary depending on the time of the year and the height of the water table of the soil.
Prior to the Public Health Act of 1875 damp-proof courses were not compulsory in buildings and therefore most old buildings will have damp in the walls to a certain degree.
Traditionally where Flagstones or bricks were directly placed directly onto the soil to create floors the moisture that would rise through would be carried away with ventilation. Therefore any impervious covering such as a vinyl floor will trap in the moisture and become soaking wet underneath. The same with walls, any attempt to seal the walls with moisture barriers or impermeable paints or dense plaster renders will only act to push the damp further up the wall.
Rising Damp contains salts carried up from the soil or found in the walling materials. Some salts are hygroscopic i.e. they absorb moisture from the air and will maintain dampness after the rising damp has been eliminated.
In some cases damp can also penetrate the wall at the point where there is an abutment of a garden wall or boundary wall to the external wall of the building.
Moisture is present in the atmosphere in the form of water vapour that is invisible. The amount is limited but it increases with temperature. As air is warmed it will absorb more moisture but if air is cooled to a certain point below dew-point the air will revert to water. If any surface within a building is below dew-point it will cool the air next to it, and will cause condensation. Condensation is dependent upon the temperature of surfaces and the humidity of the surrounding air.
Many old buildings are difficult or expensive to heat due to the internal cold surfaces through dampness or poor insulation. If ventilation is reduced in an attempt to conserve warmth, the air humidity inside will increase by the evaporation of rising damp or domestic activities such as cooking, washing up, drying clothes and bathing. Any surface, which is below the dew-point, will become damp by condensation.
Condensation can occur in old buildings with thick walls such as churches if the building is heated intermittently and the air is warmed before the surfaces have risen above dew-point.
Chimney flues can become damp through condensation. Modern boilers and closed stoves draw considerably less air through than open fires. Warm rising gasses may condense on a part of the flue, which is exposed or has a poor thermal insulation. The result maybe, deposits being carried through the brickwork and leaving stains on the internal wallpaper or plaster
2. Tracing the cause of damp
“The incorrect diagnosis of a damp problem may lead to unnecessary and expensive ‘remedial’ work involving damage to building fabric.
Diagnosis may sometimes require more scientific approaches than sight, feel and smell of damp, but the value of these senses must not be underestimated.” According to the SPAB technical pamphlet 8,
Tests for dampness include:
- Electrical conductivity
- Capacitance Meters
- On-site Moisture testers
- Laboratory oven drying and moisture tests
- Assessments of salt content
No single test will give a full picture of the damp content within a building, and too often a reliance on electrical moisture meters to detect rising damp, can give a hit and miss result to testing. Such as a high level of salts in the plaster-work but maybe the underlying masonry may not be damp.
Results should be treated with caution and an ongoing monitoring of the situation would give a true picture of cause and effect.
Check Roofs, Parapets, gutters and down-pipes
Check exposed horizontal surfaces – areas that are likely to collect water.
Water leaks – these may run down rafters or the underside of the roof to appear inside the building some distance away.
Damp patch on ceilings – is this due to a roof leak, dripping condensation or both.
Roof space – ideally examine the roof-space during a period of wind and rain.
Rainwater disposal system – ideally examine during heavy rainfall. Systems may appear to deal with average rainfall but can fail under heavy or prolonged periods of rain.
External wall surfaces should be carefully examined.
Attention should be paid to the pointing. Is it traditional lime mortar or a modern cementitious mortar that may allow for water penetration through hairline cracks.
Parapet Joints – these can be very vulnerable especially if not covered with sheet lead.
Check, are there any water supply pipes that may have been embedded in walls and may have failed.
Rain penetration – these may produce defined damp patches on South or west facing walls and may evaporate after a few days then reappear following a period of heavy rain.
Rising damp – Usually extend from 300mm to 900mm above floor level and will show a sharp change from wet to dry or a ‘tidemark’ stain on wallpaper. Contaminating salts may be seen as a white deposits or feathery crystals. Mould growth would usually suggest condensation.
Dampness due to condensation – Usually found in patches. Beads of moisture on hard shiny surfaces or growths of mould on wallpaper, plaster or paintwork. Usually intermittent and independent of rainy weather.
Check wardrobes and kitchen cabinets built against external walls for signs of damp through condensation.
3. Controlling rain penetration
Roof tiles or slates may become cracked or move as a result of rotten fixings or movement in the building. Individual roof tiles or slates can be replaced, but during the life of any roof it will be required to be stripped and recovered.
Under felting is useful but may be inappropriate for some old roofs, particularly under thatch. If it is used, it is essential that the roof space be provided with additional ventilation. Counter battens will be required for certain types of stone tiles and slate.
Modern mastics or resins should not be used in an attempt to seal a roof or secure the covering as these have been known to fail quickly and damage the covering to such an extent as to preclude the reuse of the roof covering in the traditional manner.
Systems developed over recent years include rubber, polyester, urethane and polyisocyanate foams sprayed onto the underside of slates and tiles, may restrict the ventilation of the roof timbers, which could restrict evaporation of moisture and encourage decay.
A competent builder who is experienced in this type of conservation work should carry out the repair of an old roof. Contact Taliesin Conservation to discuss your project.
Maintenance of gutters and downpipes and protection of walls.
Gutters and downpipes should be cleaned and inspected at regular intervals. If there are any damaged cornices, string courses or other projections designed to throw water away from the building they will need to be repaired or reinstated.
Some very exposed walls suffer from rain penetration because of insufficient wall thickness or due to decay. A traditional solution for this problem was to slate or tile the side of a building. This would provide protection and still allow for evaporation. Alternatively a wall may be protected by a coat of lime based render or roughcast and finished with a limewash.
Timber framed buildings
Rain penetration in old timber framed buildings usually occurs at the joints between the timber and the panel infill. Traditionally panels are made of either lath or wattle and daub (a mixture of lime, cow dung etc pressed in between wattles and staves) these are efficient if kept in good repair. Old panels can be repaired appropriately or if beyond repair replaced with the addition of thermal insulation.
Cracks in Oak framing should not be filled with cement mortar as this will retain moisture and lead to decay.
4. Controlling rising damp.
Is a damp proof course necessary?
Rising damp is a common cause of deterioration in stone, brick and earth walls. It can cause dry and wet rot. It often damages internal finishes but on occasions the degree of moisture is to minor to be of consequence.
- Is the problem caused by the neglect of gutters or downpipes?
- Is the problem caused by condensation?
- Are any timbers at risk from damp and fungal attack?
- Can it be isolated?
- Can it be remedy without damaging the character of the building?
- Will the remedy be more damaging than the defect?
Solutions may be to;
- Help the building to ‘Breathe’
- Encourage the moisture to evaporate quickly and easily rather than restrain the rising damp.
- Re-point masonry
- Replace a dense render with a lime mortar to enable the rising damp to dry out.
If a cement rich mortar is used it is likely to create or exacerbate damp problems.
If a damp proof membrane is placed under a solid ground floor, this may drive damp up the walls, where if there is not a damp proof course could cause more damage and increase the problem of rising damp.
Ground works / improved drainage
Removing plants, trees or shrubs adjacent to walls or reduce ground level by at least 150mm to form a ‘dry area’ can reduce rising damp.
A drainage trench around a wall can help but this would not be practical where buildings have shallow wall foundations. The structural consequence should be considered.
There may be Archaeological implications of excavating close to walls of old buildings and therefore The Council for British Archaeology should be notified.
Insertion of a damp proof barrier
Most old buildings were built without a damp proof course, but sometimes it may be possible to insert a continuous physical damp proof course around the whole building. This can be done in stages, removing sections inserting a thin strip of damp-proof material such as copper or lead cored felt. This can only be done if the wall is built as a continuous horizontal. If the wall is irregular stone or a random rubble wall, other solutions are available. Care should also be taken that any method adopted will not damage the structure of the walls.
Other solutions include;
Chemical Impregnation. – A chemical solution that impregnates the wall forming a water repellent film within the masonry and joints.
Ceramic Tubes. – Hollow open ended tubes that absorb damp and then evaporates from the inner surface. According to SPAB their effectiveness is a matter of debate.
Electro-Osmotic systems. – This is dependent on placing electrodes of different metals one into the wall and the other into the ground then passing a current through them. This method has mainly gone out of practice in the UK due to the little evidence of their success.
Water-proof renders and Bitumastic coatings. –Internal plaster is striped and replaced with a water-proof render up to and well above the tide mark. This tends to be a temporary solution as it can breakdown quickly and tends to force the damp up higher into the building, which recreates the same damp problem in the rooms above. This method also tends to encourage condensation.
Dry linings. – Timber panelling is the earliest example of this method, however due to moisture levels fungal decay is common behind the panels. If space allows and there are no historic features within the building a new entirely separate inner wall of 50mm blocks (or thicker) can be constructed on its own damp proof course and separated by a 50mm cavity. The inner wall is then plastered in the normal way.
5. Controlling Condensation
Condensation with a building can be reduced by reducing air humidity or maintain the surfaces above dew-point temperature. High levels of humidity are caused by rising damp, excessive water vapour and poor ventilation.
A certain amount of water vapour is expected, and created within the modern household, with clothes drying indoors, modern heating systems and regular bathing. Each should consider its output and remove the condensation by ventilating the space, by opening windows or having tumble dryers vent to the outside air.
Heating systems should be run at low temperatures if run intermittently, and lime wash walls are preferable in order to absorb moisture. Ceilings below cold roof spaces should be insulated and a vapour barrier introduced. All waterpipes should be thoroughly lagged.
If there are rooms that have had fireplaces sealed up grills or ventilation blocks should be added to increase ventilation.
Dehumidifiers can help remove excessive dampness from buildings, but if constantly used is a poor substitute for effective heating and adequate ventilation.
Internal wall treatments
Damp walls may take up to a year to dry out properly following the removal of the cause of damp. Rising damp often leaves hygroscopic salts on the surface which absorb moisture and crystalize. These may also be the result of sea sand being used in the mortar, or from Coke or Ash being heaped up against the wall or the storage of artificial fertilisers in farm buildings.
Salt deposits on plain faced walls are best brushed away and then the wall washed with clean water. Plastered walls are more problematic. If the plaster is removed, the wall must dry out prior to the new plaster being applied otherwise the new plaster can become contaminated.
Behind Plastered walls
Damp treatment companies will usually insist that up to 1m of the internal plaster be removed. Where in most buildings built before the 18th Century have wall paintings or simple wall decorations applied to the walls. If you believe that your building may have these wallpaintings then specialist advice should be sought before proceeding with any treatment.
Paints and surface treatments
Emulsions or Oil paints should not be used to decorate old building walls as the tend to fail quickly and prevent the evaporation of moisture from within the wall. Lime wash is the preferred solution as it is more tolerant to damp.
Internal joinery and panelling.
Damp treatment companies would usually remove Skirting boards, dado rails, architraves and other wooden fittings from the area where they are treating. These are often thrown away. It is important that a specialist carpenter is employed and care taken to remove and number the timber fittings in accordance with a keyed drawing so that they can be checked for damage and treated if required for fungal or insect attack and repaired and replaced on completion of the work. If there are panels in a room and there is a question regarding damp proofing then specialist architectural advice should be sought.
- The Society for the protection of Ancient Buildings – Technical pamphlet 8. The Control of Damp in Old Buildings. By Andrew R Thomas AA Dipl RIBA. Gilbert Williams FRIBA PPInst RA FRSA and Nicola Ashurst March MBEnv (Build Cons)
- The cover drawing was based on Ailwyn Best’s as used in the SPAB Technical pamphlet 8
- Information sheet1 – Basic Limewash by Jane Schofield. SPAB 1986
- Information Sheet 4 “The need for old buildings to breathe” by P.Hughes. SPAB 1986
- Technical pamphlet 11 “Panel Infilling to Timber-framed buildings” by K.Reid
- Information sheet 9 “An introduction to building Limes” by M.Wingate
- Technical pamphlet 5
- Pointing Stone and Brick Walling by G.Williams