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Taking the rise out of damp

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What is rising damp? This article explores the issues and recommends that surveys be taken more seriously

In 1999, the BBC2 programme Raising the Roof caused annoyance within the remedial industry by suggesting that rising damp did not exist, and that it was a concept invented by the industry in order to sell damp-proof courses. Damp problems, it suggested, were apparently always attributable to some other cause that could usually be remedied.

The British Wood Preserving and Damp-proofing Association responded by pointing out that regulations dating as far back as 1875 required homes to be built with a damp-proof course, and that current Building Regulations require a dpc in all new buildings and extensions. Legislation to combat a problem that does not exist?

Open pores A major source of confusion may be the meaning of the term rising damp.

The Building Research Establishment Digest 245 ('Rising damp in walls:

diagnosis and treatment') indicates that if a wall is constructed from a porous building material and stands in saturated soil, then water will tend to rise up within the structure by capillary action. The height it rises to will depend, to a large extent, on pore size and distribution - because small pores exert a stronger capillary attraction than large ones - and on the amount of surface evaporation.

If the soil is not saturated, then this capillary attraction will tend to be countered by the suction pressure exerted by the soil. This, it states, is approximately equivalent to the negative pressure exerted by a column of water extending from the base of the wall to the water table. If the water table rises, then the suction pressure decreases, and the moisture rises higher within the wall.

The presence, and extent, of rising damp, according to the BRE concept, will therefore depend on a range of parameters, which will include the pore structures of the wall and soil, and the height of the water table. Rising damp may be a problem in fine-pored Georgian brickwork on clay soil with a high water table, but it should not, for example, affect masonry with courser pores on opentextured sandy soil. If damp appears at the base of a wall in the second situation then it is likely to be caused by a drainage problem. Drainage problems may cause damp to rise in a wall, but this is not 'rising damp'.

Probing issues Unfortunately, many people believe that any water at the base of a wall indicates rising damp, and that any old building will be prone to rising damp unless a dpc is installed. The problem is not assisted by the incautious use of moisture meters by persons anxious to sell damp-proof courses. It should, however, now be apparent that groundwater does not automatically cause a problem with 'unprotected' walls.

The British Standard Code of Practice for the installation of chemical damp-proof courses (BS 6576: 1985) states in section 3.1 that it is essential to 'confirm whether the property has rising damp, or has suffered rising damp', before installing a dpc.

Section 3.2.2 of the same document states that surface measurements alone cannot confirm the presence of rising damp, although they can, when moisture meters are correctly used, confirm the presence of dampness. This is perhaps an oversimplification because a series of meter readings taken up a wall may suggest a rising damp profile, but they cannot show the severity of the problem.

A major difficulty is that 'moisture meters' are actually measuring the electrical properties of the material tested, and any salts that have been mobilised or carried into the test area by water movement strongly influence these electrical properties. Salts may produce high meter readings in a wall that is virtually dry.

One certainty is that wandering around a room prodding the wall at intervals with a meter probe will not diagnose rising damp, but this is the method usually employed by surveyors. It may, however, indicate areas that require closer investigation.

Have salt will travel BS 6576 refers the reader to BRE Digest 245 for proof of rising damp or quantitative measurements, and the method described in that digest will demonstrate the extent of any dampness that may be present, and its likely source. The method is based on the fact that soil contains salts that will be carried into the building material by any water travelling up through the upper soil layer (rising damp). These salts, predominantly chlorides and nitrates, will tend to accumulate in a band at the highest zone within the wall that the moisture reaches.

If their presence is demonstrated as a common phenomenon around the walls, then there is rising damp. If a salt profile is only found adjacent to a downpipe, then there is probably a long-term fault in the drain. If there are no soil salts in an old wall, then any moisture that is present has not travelled far through the soil, and may have come from faulty rainwater goods or surface run-off.

The term 'soil salts' is used because water movement may mobilise large amounts of salts from the building materials. Brickwork, for example, may be coated in blown plaster and sodium sulphate crystals, but this does not mean there is rising damp.Most of the sulphate probably came from the clay from which the bricks were made.

The method used to detect the presence of rising damp must therefore assess the moisture content of the wall, detect the presence of salts, and differentiate between salts that have come from the ground and salts that have come from the wall.

The analysis requires samples to be taken in columns up the wall.

Sampling at 250mm intervals with a normal masonry bit is convenient, and to a height of 1.5m with a control taken at 2m. The first 10mm of dust from each hole is discarded or analysed separately so that surface effects do not confound the results.

Samples are then placed into tightly lidded pots for laboratory analysis.

The initial moisture content (MC) assessment is made by weighing the sample and heating it in an oven until it is dry. Digest 245 recommends that this is done for about an hour at 100infinityC, but this temperature will drive off water of crystallisation from any hydrated salts that may be present and may give erroneous results. It should certainly never be used if plaster is to be tested. It is preferable to leave samples overnight at a temperature of 40infinityC.

Slippery when wet The samples are then reweighed so that the original moisture content is established from percentage weight loss, and stored for a few days at a humidity of 75 per cent. Some labs prefer to place the samples straight into the controlled humidity chamber after the initial weighing and then obtain one dry weight at the end of the procedure.

The point of the constant humidity chamber is that chlorides and nitrates from ground water will absorb moisture from the atmosphere, but 75 per cent relative humidity (RH) is not high enough to affect the sodium sulphate. It is also easy to obtain 75 per cent RH by storing the samples over a saturated solution of common salt, but substantial errors may occur if there is insufficient salt solution or the solution is not fully saturated. In both cases the humidity will be too high and water absorption by any sulphate present may confound the results. If the samples are now reweighed, then hygroscopic moisture contents (HMC) are obtained and these provide a measure of salt contamination from ground water - rising damp.

The table on the next page provides some typical results. It is considered that a dry reading in mortar or soft brick will be less than about three per cent moisture. Digest 245 recommends a HMC threshold of five per cent before remedial treatment is contemplated.

The sample one series shows moisture at the base of the wall (samples 1.6 and 1.7), but the consistently low hygroscopicity demonstrates this water has not travelled far through the soil, or has come from a fault which is too recent for salts to have accumulated.One might look for a blocked gully or, perhaps, a gap at the junction of a concrete path and the wall. If the high moisture contents continued up the wall then one would look for a faulty downpipe or gutter.

The series two moisture contents also show moisture at the base of the wall, but now the HMC readings indicate a different cause. The wall is wet to a height of about 1m (sample 2.4) and at this point sufficient hygroscopic salts have accumulated to produce an HMC of 7.2 per cent. There is little hygroscopicity above the band at 750mm to 1m and little below it. This is a typical rising damp profile. If significant hygroscopicity was found at every height, then the probable cause would be natural hygroscopicity in the building materials. Here, then, rising damp does exist, but it is uncommon.

Rising damp need not be the only reason for installing a damp-proof course. If water is being funnelled into a wall from a neighbour's concrete yard, for example, then lowering ground levels or constructing a dry area may not be an option. The important factor is that the situation has been properly investigated so that an appropriate solution can be found.

Dr Brian Ridout is the author of Timber Decay in Buildings: The Conservation Approach to Treatment, and director of Ridout Associates, Environmental Monitors and Timber Decay Analysts. Tel 01562 885135 or e-mail ridout-associates@lineone. net

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