Damp (structural)

For other uses, see Damp (disambiguation).

Detail showing some of the causes of damp penetration

Structural dampness refers to the presence of unwanted moisture in the structure of a building, either the result of intrusion from outside or condensation from within the structure. A high proportion of damp problems in buildings are caused by the "big three," condensation, rain penetration, and rising damp, although other causes of dampness such as pipe leakage and construction moisture should not be overlooked.

Symptoms

Dampness tends to cause secondary damage to a building. The unwanted moisture enables the growth of various fungi in wood, causing rot. Plaster and paint 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.

A 2009 World Health Organisation report entitled "Children Living in Homes With Problems of Damp" stated that:

"Excess moisture leads – on almost all indoor materials – to growth of microbes such as moulds, fungi and bacteria, which subsequently emit spores, cells, fragments and volatile organic compounds into the indoor air. Moreover, dampness initiates chemical and/or biological degradation of materials, which also causes pollution of the indoor air. Exposure to microbial contaminants is clinically associated with respiratory symptoms, allergies, asthma and immunological reactions. Dampness has therefore been suggested to be a strong and consistent indicator of risk for asthma and respiratory symptoms such as cough and wheeze."

Identification

A wide range of instruments and techniques can be used to investigate the causes of moisture in building materials. When used correctly, they can provide a valuable aid to investigation. The competence and experience of the person undertaking the damp investigations is of greater importance than the kit he or she carries.

Processes for diagnosing rising Damp in buildings is set out in BRE Digest 245.

All of the above should be considered during any assessment for damp related defects in buildings.

Prevention and treatment

Most forms of dampness can be prevented by thoughtful building design and careful construction. In the UK, well built modern houses include a synthetic damp-proof course (DPC), about 15 cm above ground level, to act as a barrier through which water cannot pass. Slate or "engineering bricks" with a low porosity were often used for the first few courses above ground level, and these can in theory help minimise the problem.

There are many approaches to the treatment of dampness in existing buildings. Key to the selection of an appropriate treatment is a correct diagnosis of the types of dampness affecting a building. Details of possible treatments for specific types of dampness are covered in the sections below.

The cause of the dampness must first be eliminated, by providing better drainage or fixing leaking pipes. BRE Digest 245 describes several methods of treating rising damp, including the use of land-drains and the insertion of physical and chemical damp-proof courses. Then, any affected plaster or mortar must be removed, and the wall treated, before replacing the plaster and repainting.

The major damp treatment authorities within the U.K. are The British Wood Preserving and Damp-proofing Association (BWPDA) and The Property Care Association.

Condensation

Condensation comes from water vapour within the building. Common sources may include 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 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 (i.e. out of circulating air patterns).

In the United Kingdom, condensation problems are particularly common between October and March - to the extent that this period is often referred to as the "condensation season."

Identification of condensation

If it is suspected that the problem is condensation, then a room should be sealed off with a dehumidifier left running for the recommended time and then further instrument tests made. If the dampness has disappeared, then condensation is very likely the problem.

Alternatively Humiditect cards or dataloggers (measuring air humidity, air temperature, and surface temperature) can be used as tools for diagnosing a condensation problem.

Treatment

Typical remedies for condensation include increasing background heat and ventilation, improving the insulation of cold surfaces and reducing moisture generation (e.g. by avoiding the drying of clothes indoors).

Rain Penetration

Rain Penetration (also known as "penetrating damp") is a common form of dampness in buildings. It can occur through walls, roofs, or through openings (e.g. window reveals).

Water will often penetrate the outer envelope of a building and appear inside. Common defects include.

• Roof defects such as faulty flashing, cracked or missing slates or tiles.
• Faults in the brickwork or masonry 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.

Walls

Rain penetration is most often associated with single-skin walls, but can also occur through cavity walls - e.g. by tracking across wall ties.

Rising dampness

Rising damp describes the action of ground moisture rising up a masonry wall by capillary action. It could be identified by a characteristic "tide mark" on the lower section of affected walls, but this could be caused by damp ingression rather than rising damp. This tide mark is caused by soluble salts (particularly nitrates and chlorides) contained in the groundwater. Due to the effects of evaporation these salts accumulate at the "peak" of the rising damp.

According to Jurin's Law the maximum height of rise is inversely proportional to the capillary radius. Taking a typical pore radius for building materials of 1 µm, Jurin's Law would give a maximum rise of about 15 m, however, due to the effects of evaporation, in practice the rise would be considerably lower.

A "physical model" of rising damp was developed by Christopher Hall and William D Hoff in their paper "Rising damp: capillary rise dynamics in walls". The model is based on experimentally well established properties of porous building materials and the physics of evaporation from building surfaces. They state that model can be used to predict the height to which damp will rise in a wall, based on factors such as wall thickness, the sorptivity of the wall structure and the effect of evaporation. Further work has confirmed experimentally the importance of mortar properties in determining the height to which damp will rise in walls. BRE Digest 245 lists several factors that can influence the height of the rise including rate of evaporation from the wall, pore sizes of the masonry, salt content of the materials and the soil, groundwater and degree of saturation, and use of heating within the property.

A number of people have expressed the view that rising damp is a myth. A former chairman of the construction arm of the Royal Institution of Chartered Surveyors (RICS), Stephen Boniface, has said that ‘true rising damp’ is a myth and chemically injected damp-proof courses (DPC) are ‘a complete waste of money’. Konrad Fisher's article "The Fraud of Rising Damp" points out that the historic city hall in Bamberg stands in the river Regnitz and its bridge remains dry without any chemical, mechanical or electronic damp-proof course. In fact, evidence would suggest that not all walls are capable of supporting rising damp, so this finding is not particularly surprising. These views are not reflected by the views the UK Building Research establishment (BRE). Indeed, Part C of the Building Regulations for England and Wales specifically calls for the inclusion of a damp-proof course in all new properties.

A review of data and publications commissioned by the Property Care Association and carried out by the University of Portsmouth concluded that "Rising damp is an age-old and ubiquitous problem." It also noted that "Records on observation and descriptions on this phenomenon date back to early times. It was identified as a public health issue in the second half of the 19th Century."

Diagnosis

The first step in assessing damp is to check for standing water. Removing water with good drainage will remove any form of dampness. Once done, and dampness remains, the next step is to look for the presence of a damp-proof course. If a damp-proof course is present, it is likely to be functioning, as the materials from which damp proof courses are manufactured tend to have a long lifespan. However, it should be acknowledged that there are cases where existing damp proof courses fail for one reason or another.

One method that is often used to determine if the source of dampness is rising damp (rather than other forms of dampness) is to look for the presence of salts - in particular a tell tale "salt band" or "tide mark" at the peak of the damp's rise. Although this is a useful indicator, it is not completely reliable as salts can enter the fabric of the wall in other ways - e.g. unwashed sea sand or gravel used in the construction of the wall.

If there is no damp-proof course and rising damp is suspected (tide mark, moisture confined to lower section of wall etc...) then a number of diagnostic techniques can be used to determine the source of damoness. BRE Digest 245 states that the most satisfactory approach is to obtain samples of mortar in the affected wall using a drill and then analysing these samples do determine their moisture and salt content. The fact that this technique is destructive to the wall finish often makes it unacceptable to homeowners. It is for this reason that electrical moisture meters are often used when surveying for rising damp. These instruments are unable to accurately measure the moisture content of masonry (they were developed for use on timber), however the patterns of readings that they provide can provide a useful indication of the source of dampness.

Treatment

In many cases, damp is caused by "bridging" of a damp-proof course that is otherwise working effectively. For example a flower bed next to an affected wall might result in soil being piled up against the wall above the level of the DPC. In this example, moisture from the ground would be able to ingress through the wall from the soil. Such a damp problem could be rectified by simply lowering the flower bed to below DPC level.

Where a rising damp problem is caused by a lack of a damp-proof course (common in buildings over approximately 100 years old) or by a failed damp-proof course (comparatively rare) there are a wide range of possible solutions available. These include:

• Replacement physical damp proof course
• Injection of a liquid or cream chemical damp proof course (DPC Injection)
• Porous tubes
• Electrical-osmotic systems
• Land drainage

BRE Digest 245 suggests that with the exception of replacement physical DPCs, only methods of treatment with third party accreditation (E.g. British Board of Agreement Certificate) should be considered. It then goes on to state that the only method of currently satisfying this requirement is DPC injection and that "this is the only method which BRE considers suitable where insertion of a physical DPC is not possible." The Royal Institute of Chartered Surveryors (RICS) publication "Remedying Damp" is more cautious about reliance on third party accredititation, casting doubt upon the validity of the test methods employed, arguing that trials are usually conducted using "specially built masonry panels - which do not match up in many respects to walls found in real properties," and that "if a DPC were proved to not work in a specially built masonry panel, this would be the more significant result." The MOAT No 39 test employed by the British Board of Agreement (BBA) in the UK is dismissed as "quite a clever test idea but in the author's opinion not actually replicating a real wall." Furthermore, the point is made that "BBA testing is paid for by manufacturers, and the results are not thought to be publicly available."

Replastering

Replastering will often be carried out as part of a rising damp treatment. Where plaster has become severely damaged by ground salts there is little argument about the need to replaster. However there is considerable debate about:

1. The extent of replastering required
2. The use of hard sand:cement renders to replaster as part of a rising damp treatment

BS6576:2005 states that "the function of the new plaster is to prevent hygroscopic salts that might be present in the wall from migrating through to its surface, while still allowing the wall to dry." However, writing in the RICS publication "Remedying Damp", Ralph Burkinshaw claims that, "the plaster is really there for two main reasons." He accepts the need for replastering when significant amounts of ground salts have built up in the existing plaster, however he then goes on to say that replastering is often carried out to make up for an unreliable chemical DPC. He also suggests that damp-proofers have an incentive to carry out more replastering than is strictly necessary as it allows them to finish the job without having to wait for walls to dry out, resulting in faster payment.

Although the sand:cement renders typically installed as part of a rising damp treatment are very effective at holding back damp and ground salts, they have a number of disadvantages. These include an incompatibility with the soft bricks and mortars encountered in older buildings and a lack of insulation properties compared with more traditional plasters, resulting in an increased risk of condensation. Replastering is also one of the most expensive parts of a rising damp treatment.

Porous renders to German WTA specification 2-2-91 can be used as an alternative to dense sand-cement renders. These have a minimum porosity of 40% of total volume. Salts crystalise in these pores rather than on the plaster surface, avoiding decorative spoiling. Such plasters offer a better solution than dense sand-cement renders when used on moderately salt-contaminated walls as their porous nature gives them insulation properties, resulting in a warmer surface temperature and making condensation problems less likely to occur. However, when used on heavily salt contaminated walls they may need to be replaced frequently as they lose effectiveness once all the pores have become filled with crystallised salt.

Rising Dampness References by Victorians

The issue of rising damp has been a concern since Victorian times. Indeed the Public Heath Act of 1875 introduced the requirement for a damp-proof course in walls to prevent rising damp. An entry in the British medical Journal from 1872 describes the phenomonen as follows:

"Even if the rising damp be arrested by what is technically called an impervious damp-proof course, it will be frequently found that this is built in the wall too near the ground line, so that the heavy rain besplatters the ground and splashes above it. As time rolls on the surface of the ground also becomes elevated, and this damp course is soon lost to sight. Attempts have been made to remedy this evil of porous bricks by the substitution of the hard blue bricks of Staffordshire; and then it may often be noticed that the wet has only struck, sailor-like, across the mortar-joints and chequered the inside walls like a tartan plaid."

The architect, Thomas Worthington, described rising damp in his 1892 essay, "The Dwellings of the Poor: And Weekly Wage-Earners in and Around Towns":

"It should be bourne in mind that damp walls absorb much more heat than dry ones and that they are frequent agents in causing rheumatism, kidney disease and colds. Rising damp from the ground may be prevented by most simple means. Six inches of good Portland cement concrete should cover the whole site of the dwelling, and concrete never less than nine inches thick should underlie all walls. A damp course should disconnect the whole of the foundations from the superstructure. This preventative may consist of a double layer of thick slates bedded in cement, or of patent perforated stone-ware blocks or of three-quarters of an inch of best asphalt."

External links

• Coleman, Graham. Articles about dampness in buildings
• World Health Organisation Damp and mould: health risks, prevention and remedial actions
• Howell, Jeff. The rising damp myth
• Property Care Association. UK trade association for damp-proofing professionals
• Waite, Richard (2009-06-26). Rising damp is a myth, says [Stephen Boniface, former Royal Institution of Chartered Surveyors] chief