Exploring dampness

'Rising damp' receives much more attention in surveying texts than the subject merits. What is often reported as rising damp is most likely caused by penetrating dampness. In fact, rising dampness is a significant factor in only a very small proportion of dampness problems.

Some writers claim that 'rising damp' is a myth. Although rising damp can be difficult to confirm, this claim does not hold true. Moisture does travel upwards through a porous material - through the pore structure, or via small fissures or cracks, or as water vapour – against the force of gravity. (Place a section of 'dry' fletton brick in a shallow saucer of water, and the brick will take up moisture readily.) Remember that, during any movement of moisture upwards through a porous material, there will always be an element of lateral spread.

However, we certainly need to know more about dampness at the base of walls. There has been a lack of careful surveying of this part of the building over the years, perhaps because surveyors have often passed the surveying over to so-called damp specialists who have themselves not used the full range of surveying equipment available.

Rising dampness can be difficult to diagnose. Symptoms of a dampness problem seen on wall finishes, such as the 'horizontal tidemark' often associated with rising damp, are indeed no more than visual symptoms of dampness on wall finishes.

The height of the evidence is not an indication of 'rising damp'. Attempts are often made to calculate the height dampness may rise to in masonry, with reference to the estimated pore size of bricks. Such a theoretical calculation will only offer a poor estimate because 'real' brickwork is made up of combinations of materials - bricks of variable porosity, mortars of variable mix, the occasional built-in timber, plasters, renders, paints and other finishes - and the moisture travelling through the bricks will be of variable make-up. Several other factors affect the height water can reach:

• the height of the water table;
• surface and subsoil drainage;
• the rate of evaporation; 
• the presence of a damp-proof course (dpc);
• dpc 'bridging';
• the wall finishes (particularly if dense renders are present);
• wall thickness;
• the amount of foundation wall surface area underground;
• the presence of salts, etc;
• the presence of moisture entering the wall above ground from other sources; and
• the application of non-breathable materials, i.e. harder Portland cement renders, water proof additives mixed with cement renders, some paints and sealers etc.

Studies conducted by Prof. Guido Biscontine at the Foscari University in Venice revealed that the use of water proofing additives and non-breathable materials applied to the external walls of venetian buildings can create the conditions to push the capillarity vertically as high as the eaves of a building in defiance of gravity by creating a narrower capillary between the non-breathable outer surface and the masonry walls. See Mike Parrett's Guide to Building Pathology film series for more information.

Rising can happen anywhere in a wall where moisture moves upwards, but for the purposes of building surveys of residential property it is defined here as 'moisture that is soaking up (typically) a wall or floor from below the ground, i.e. finished external ground level, or internal oversite level.'

If an effective dpc is present then rising damp should not be a problem. Traditionally built houses are able to deal with rising damp because they usually incorporate considerable ventilation opportunities such as suspended ventilated ground floors that enable walls to breathe where they emerge from the ground. The dpc is a second line of defence. (The dpc is not always a discreet physical barrier; it may be courses of dense masonry bedded in cement mortar.)

Buildings without a dpc situated on very wet sites may be subject to some rising damp. But it is useful to note that many old buildings surveyed and managed by Lewisham Council 1992-2003, for example, cope perfectly well without a dpc and in those dwellings with a dpc suffering from dampness, the causes were found to be due to multifarious problems relating to dampness penetration, i.e. defective rainwater goods, bridging of the dpc, blocked cavity wall voids. None of the cases related to a failure of the physical dpc.

Figure 1: Key moisture sources at the intersection of an existing solid wall and a new concrete floor

Problems are likely to arise because houses originally built with a dpc have been 'modified'. Modifications can result in raised external ground levels, which cause increased penetrating dampness just below or above the dpc. Floor ventilation may be reduced or totally blocked. Worse still, raised timber ground floors may have been thoughtlessly replaced with ground-supported concrete, upsetting the building's moisture equilibrium at the base of walls. Alterations also mean that concrete floors trap water spillage in kitchens or bathrooms, and the water may collect under foam-backed carpet to rot skirting boards. A traditional timber floor would enable spilt water to percolate through to the ventilated subfloor void, which acts as a 'sink'. If modifications have compromised the effectiveness of an existing dpc, the cost of bringing the building back to original or near original condition could be prohibitive.

Buildings faced with very wet conditions - for example, from seasonal flooding - are particularly vulnerable if they do not possess an effective dpc. Sometimes 'remedies' only exacerbate the problem.

There is often the need to consider additional external attenuation schemes to reduce the level of the local water table. In more serious cases of flooding (as has been seen with increasing regularity in different parts of the UK and around the world), there is a greater need for major infrastructure town and regional attenuation schemes to mitigate some of the extreme forces of nature.

In the late 19th century it was advocated to separate buildings by 3 foot in an attempt to lower the water table and to provide greater ventilation between buildings to dissipate disease, mainly in those time cholera and typhus. Nowadays useful guidance can be found in the CIRIA publication Sustainable Urban Drainage Systems (SuDS) and from the Environment Agency web site and National Rivers Authority concerning mitigating the effects of flooding and identifying flood risk zones.