Rising damp is a condition caused by ground moisture rising up a masonry wall by capillary action
. It occurs where there is no damp-proof course
(DPC) or where the DPC has been damaged or bridged. Where present, moisture can rise up the wall to a height of a metre and it will often leave a characteristic horizontal 'tide mark'.
Rising damp tends to cause secondary damage to a building. The unwanted moisture enables the growth of various fungi
in wood, causing rot. Plaster
deteriorate and wallpaper
loosens. Stains, from the water, salts and from mold
, mar surfaces. Externally, mortar may crumble and salt stains may appear on the walls. Steel & iron fasteners rust
. It may also cause respiratory illness in occupants. In extreme cases, mortar or plaster may fall away from the affected wall.
Well built modern houses include a synthetic damp-proof course
(DPC), about 15cm above ground level, to act as a barrier through which water cannot pass. Therefore the problem is mainly related to older structures, where no water proof barrier was utilised, or where ill designed building modifications have been performed. However, slate or "engineering bricks" with a low porosity were often used for the first few courses above ground level, and these can minimise the problem.
Effective treatment is a specialized job and is expensive and labour intensive. The cause of the damp must first be eliminated, by installing a physical or chemical DPC. Then, any affected plaster or mortar must be removed, and the wall treated with a solution of acetic acid
, before replacing the plaster and repainting. The major Rising damp treatment authorities within the U.K. are The British Wood Preserving and Damp-proofing Association (BWPDA) and The Property Care Association.
Before any remedial work is started, it is important to determine that the symptoms seen in the structure are in fact rising damp and not penetrating damp
. It is common for any or all of these defects to be present present simulataneously.
Identification of rising damp
Inexpensive instruments are available which measure the extent and amount of moisture in building materials. They consist of a conductivity meter
and a couple of spikes that are pressed into the material to be tested. In general, they can only be used inside the building. They give an excellent indication of the extent of the problem even when there is not yet any visible damage, but no clue as to the cause. Hence further clues must be sought.
Once the extent of the problem has been determined by the instrument, further investgations have to be made,
Is there a damp course present and what is it's nature? The damp proof course, if there is one, should be clearly visble on the outside wall and it should be at least 150mm above the ground level and immediately below any floor joists. It's edges should be visible, ie not covered by cement. If there is a damp proof course and it is a plastic or bitumous type barrier them the chances of it's failure are slight although "bridging" sometimes occurs due to the application of cement rendering
If it consists of slate or engineering bricks, then failure is possible. Slate damp proof courses often have the appearance of the thick horizontal cement joint. A double layer of slates is common with the joints staggered. Failure occurs when the slates break through building movement or bridging takes place.
Engineering bricks (two or three courses)often have a different appearance to the other brickwork. Failure can be due to wide and/or porous vertices, or the bricks being of inferior quality
Even when there is no damp proof course there may be no significant rising damp (depending on the difference between internal and external floor levels, and also ground conditions.)
Condensation comes from human activities in the building, cooking bathing etc. The moisture in the air condenses on cold surfaces. Buildings with poorly insulated walls are very prone to this problem. It often causes damage similar to rising damp in a building and often appears in similar places. This is because it occurs in the "dead air" pockets that accumulate in both horizontal and vertical corners (ie out of circulating air patterns). One very important clue is that the damage due to rising damp tends to horizontal corners close to the ground whereas that due to condensation occurs at all levels.
Another important clue is the possible lack of ventilation devices in the building (eg fans in bathrooms and toilets and cooker hoods.)
If it is suspected that the problem is condensation, then a room should be sealed off with a dehumidifer
left running for the recommended time and then further instrument tests made. If the dampness has disappeared, then condensation is very likely the problem. Insulation of cold surfaces and/or the elimination of water vapour at source are the answers.
Other possible problems
Water can penetrate the building due to structural defects and appear inside. Likely causes are:-
Roof defects such as faulty flashing
, cracked or missing slates or tiles.
Faults in the brickwork or masonary such as missing or cracked pointing
. Porous bricks or stones.
Missing or defective mastic around windows and doors.
Blocked weep holes
Missing or defective trays in cavity walls.
Cement lodged on cavity wall ties
forming a bridge allowing moisture to cross between inner and outer leafs
Solid, ie non-cavity walls)
Damp proof course been buried by later building activities.
All of the above should be considered/repaired before remedial work on the damp proof course is undertaken.