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Rising Damp

Back in the 19th century, the first attempt was made to legislate the insertion of a physical damp proof course in new housing construction a statutory requirement. Despite this however, the presence of a damp proof course was not always assured in all buildings. Many of the original damp proof courses, being of poured or sheet bitumen, have become brittle with age. As a result, and because settlement of the buildings has occurred, many have failed. Moreover, numerous existing and potentially effective damp proof courses have been bypassed by adjacent building alterations or by changes in ground levels. These are just some of the reasons why rising dampness is still a problem today.

The maximum expected height for rising damp to occur is 1200mm, but this may be influenced by the amount of moisture in the soil and by the height of the water table. Climatic factors internally as well as externally can affect the rate of evaporation off the wall. If high humidities are prevail the dampness may rise even further. Any obstruction to the evaporation by a dense render or matchboarding dado, for instance, would tend to drive the water further up the wall. Conversely, if heating levels are increased this will assist evaporation and reduce the height to which dampness will rise.

The most characteristic salts from the soil found in rising dampness are nitrates and, to a lesser extent, chlorides. Nitrates of course can emanate from fertilizer but are also a natural consistuent of the soil itself. They are unlikely to be found in masonry unless they have originated in the ground. The only exceptions arise if soil has accumulated in roof valley gutters, for example, or where defective soil pipes from WC’s have allowed nitrate-containing water to penetrate the masonry. Chlorides are found naturally in masonry to a very small extent, so their presence can also be regarded as characteristic rising damp. The presence of chlorides however is less definitive than that of nitrates.

The salts have a significant effect near surfaces where rising damp moisture is evaporating. In these areas, the salts accumulate and eventually may block the pores and capilliaries through which the water is evaporating. This pore-blocking effect may cause more dampness to rise even further up the wall. Calcium sulphate is the most likely candidate salt for this effect because it is only sparing water soluable and comes out of solution first.

Both nitrate and chloride salts of calcium, potassium and sodium are much more soluable than the comparable sulphate salts, so these are more likely to remain in solution longer in the rising damp moisture. In fact, because they are less likely to crystalise out on drying, it is the soluble salts, especially sulphates, that form the characteristic efferfecesence that is associated with dampness in brickwork. Another and perhaps even more significant effect of these salts is a result of their hygroscopic action. Hydroscopic substances have the ability to attract and hold water molecules from the surrounding environment. Hygroscopicity will commence at various humidities from 32% for calcium chloride (common salt) to 97% for potassium sulphate (sometimes called sulphate of potash). It is to be noted that chlorides and nitrates are particularly hygroscopic. This means that wherever these salts accumulate they add considerably to the moisture level in the wall.

Causes of Rising Damp

It is a natural process for moisture to rise from the ground in a porous medium such as a masonry wall. This has been recognized for many years and the need for a damp proof course has been a stipulation in public health legislation for well over a century. In building regulations it is a requirement that any wall shall not transmit moisture from the ground to the inside of the building, or to any material in its construction liable to be adversely affected. The appropriate provision to satisfy this regulation states that the wall should have a damp proof course 150mm above the outside ground level. In addition, it is necessary to protect vulnerable timbers from dampness and any sub floor voids must be adequately ventilated.

Many buildings have their DPC’s rendered ineffective by bridging. This can take many forms. In particular, high external ground levels caused by paths or garden flower beds built up against the wall are common causes: hence the reason for the 150mm height of the dpc from ground level. Abutting walls are likely to be troublesome unless they are constructed with a dpc and have another one at the top of the coping. Alternatively, abutting walls should be isolated from the main wall by a vertical dpc. External renders and plinths are often installed in an attempt to reduce dampness, but if they bridge the dpc they are liable, in time, to crack off the underlying brickwork or masonry sufficiently to allow moisture to rise in the crevice formed behind.

Internally, bridging is often caused by a floor screed being laid either without a membrane or including a membrane that is not overlapped with the dpc in the wall. Internal plaster may be applied to a wall in such a way as to bridge the dpc. In cavity walls, mortar droppings in the cavity can cause the dpc to be bridged. Cavity wall insulation can exaggerate the effects of such cavity bridging.


The major visible effects of rising dampness are, of course, caused by the deposition of salts on wall surfaces where evaporation of water is taking place. These hygroscopic salts attract moisture from the atmosphere, and towards the apex of the rising damp this hygroscopic moisture is the major factor in the total dampness situation. It will continue to be a problem even after the rising dampness has been eliminated.

Therefore, for any rising damp curative treatment to be satisfactory, the problem of salt contaminated plaster must be addressed, and this normally means removal. Ideally this would be delayed for as long as possible after the installation of the retrofit dpc so that any residual salts that accumulate at drying surfaces during the first drying out period could be eliminated at the same time. It is usually estimated that masonry will dry out at a rate of 25mm per month, so a 225mm (9inch) solid brick wall would be expected to dry out by about nine months after installation of the new dpc. A nine month delay in replastering would therefore be preferred in that situation. In practice, of course, it is only very infrequently that such delay can be encountered. Rapid drying out methods listed in BRE digest 163 could be used to accelerate this process.

The purpose of the plaster is twofold: (a) to replace the existing contaminated plaster, and (b) to provide a barrier to prevent any residual dampness and hygroscopic salts from reappearing on the newly plastered surfaces. The new plaster must act as an effective barrier, and this is achieved by correct formulation as well as by proper application. The extent of replastering must be sufficient to prevent any residual moisture and contaminating salts in the wall from bypassing the new barrier. Normally a distance of 300mm above the last sign of rising damp is specified. At dpc level itself, the new plaster should not cause bridging. Usually this could be achieved by stopping the plaster short of the floor and arranging a gap of about 25mm, which can, in turn, be concealed by a skirting board.

A vital requirement of the new plaster is that it must not contain gypsum (calcium sulphate), because this will quickly deteriorate if it gets wet, either directly or by contact with hygroscopic salt moisture. Therefore plasters used for replastering are usually cement based. One of the most successful formulations is to use a 3:1 sand cement mix, provided coarse graded (salt free) sand is employed. Sands graded to BS1199: 1986 Type A or Type M in BS822: 1983 are suitable. This has proved to be a most effective barrier even where positive hydrostatic pressure is present, and the protection can be enhanced by incorporating waterproofing admixtures (or additives) such as stearates, oleates, or styrene butadiene. These admixtures act as air entrainers as well as improving workability. They reduce the amount of water needed for mixing and consequently reduce drying and shrinkage cracking.

How to Spot

Not sure what the signs of rising damp are? Find out more information.


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