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What is today's blog about?
In this week’s blog, Brian Robinson MRICS, Building Surveying consultant at Property Elite, discusses dampness in buildings. This is essential reading for any candidates with Building Pathology or Inspection as technical competencies.
This is Part 2 of a three part blog series, focussing specifically on types of damp and how to diagnose them accurately. You can read the first part here.
Is rising damp a myth or a reality?
In 2008, Jeff Howell, in his publication, ‘The Rising Damp Myth’ declared that generations of surveyors were wrong when it came to rising damp in buildings.
It was claimed that Chartered Surveyors, as a profession, simply did not understand damp meter results.
While the phenomenon was a matter of long-held collective wisdom, there was little in the way of supporting research – leading to the contention that rising damp was simply a myth.
RICS, in their Property Journal on 23rd November 2020, published an article drawing attention to a published thesis by Leslie Sellers entitled Rising Damp Evaluation and Treatment: A Quasi-Experimental Case Study, for which he was awarded a doctorate in the built environment (DBEnv).
This article concluded that ‘rising damp is a real phenomenon that warrants treatment and the contemporary method of damp-proofing, installed as it would be on a real construction site, does provide effective control’.
So, surveyors can now be assured that there is research confirming that rising damp is a real phenomenon.
How does rising damp occur?
Rising damp is the movement of moisture upward through permeable building materials by capillary action. It ismore common in old (pre-1900) buildings than new ones, but rarer than often supposed.
It is a significant factor in only a very small percentage of reported ‘rising damp cases’. An analysis found that only 5% of the cases reported as rising damp were actually rising damp.
The rest were due to blocked cavities, bridged wall ties, bridged DPCs or a lack of DPC/DPM lap.
How high can rising damp reach?
Traditionally, it has been understood that rising damp can go no higher than 1.0m but this height is a theoretical calculation and cannot be taken as an accurate guide. Research has identified several examples where rising damp heights of 5-6 meters have been documented, such as in the basilica of San Marco in Venice.
The 1.0m theoretical height, therefore, does not consider real life factors that occur such as:
Bricks of variable porosity
Mortars of variable mix
Occasional built-in timbers
Plasters, renders, paints, and other finishes
Moisture travelling though bricks will be of a variable make up
Several other factors that affect the height are:
Height of the water table, which may be rising in areas were heavy industry is no longer present
Surface and subsoil drainage
Rate of evaporation
Lack of a DPC
DPC bridging
Wall finishes, especially if dense renders are present
Wall thickness
Amount of foundation wall surface underground
Presence of salts
Presence of moisture entering the wall above ground from another source
What else contributes to the misdiagnosis of rising damp?
Poor surveying techniques or understanding of the limitations of the equipment used often contribute to the problem of misdiagnosis.
In the book, ‘Diagnosing Damp’ the authors (Mike Parrett and Ralph Burkinshaw) identify there are 4 stages in the investigation of a damp problem:
Stage 1: Visual inspection of the property
Stage 2: Inspection with the use of a moisture meter
Stage 3: A more detailed inspection
Stage 4: Involving opening up and sampling, which may result in damage to surface finishes
Most initial surveys on a house or commercial property will only be able to carry out the first two of these stages. Yet, often a diagnosis of rising damp is made based on this limited survey.
The well-known tide mark shown in BRE Digest 245 can also be caused by the cavity being bridged by mortar in the cavity.
Furthermore, the readings from damp meters taken in plaster alone can be misleading. This is because the pins on a moisture meter do not go all the way through the plaster so the reading only indicates how wet the plaster may be, not how wet the wall is. The readings are also suspect because of the possible influence of hygroscopic salts, condensation, and ambient humidity.
However, quite often, surveyors are giving a diagnosis of rising damp without considering that the cavity being bridged by mortar in a cavity wall, mortar on cavity wall ties, or the plaster on the wall being taken down below the DPC or there being no DPC/DPM lap, allowing water to rise in the gap between the concrete slab and the wall.
The most common source of moisture in the base of the walls of buildings is from defective ground and surface drainage. This is present to some degree in almost every building in the country, due to a combination of such factors as rising ground levels, the failure of ground drainage systems, and the increased use of concrete or finishes around buildings without consideration of drainage slopes.
The accumulation of 'moisture reservoirs' in the foundations may also arise as the result of chronic plumbing leaks or floods from catastrophic plumbing or drainage defects.
Condensation can also create damp conditions at the foot of walls, but this is often not considered or understood.
Rising damp in concrete floors
Rising damp in concrete floors is much less common than in walls, and it is often confused with excess residual construction moisture, a common problem in new buildings.
Whilst modern concrete floors have a DPM beneath the concrete to prevent water coming through, older floors do not. The form of the DPM in older buildings, especially residential, may not be recognised by modern surveyors.
Floors laid between 1945 and 1966 may or may not have a DPM, whilst those laid before 1940 will almost certainly not have one.
In many houses built in the 1950’s, the only barrier to rising damp was the bitumen bedding material under the wood blocks or thermoplastic tiles. It is common to find, during a refurbishment, that the wood blocks or the vinyl tiles have been removed and the bitumen damaged by contractors with no appropriate knowledge, thereby reducing the effectiveness of the DPM leading to rising damp.
A moisture barrier installed in the 1960’s and 70’s may not be up to current standards and may not perform as well as contemporary solutions. In some cases, a simple polythene sheet may be all that separates the slab from ground moisture. Over time, a substandard membrane can break down or become perforated, reducing, or eliminating its ability to control moisture intrusion.
Quite often, concrete slabs laid as a replacement floor in a Victorin kitchen will not have a DPM at all.
The use of a damp meter for measuring if damp is rising through a concrete slab is very limited. A humid or wet environment will likely cause false or inaccurate readings of a concrete slab’s moisture when taken at surface level.
As concrete’s electrical properties are varied and do not change just because of the presence of moisture, it makes moisture meters useful for guidance measurements only.
What will be in Part 3 of this three part blog series?
In a future blog, part 3 of this series, we will focus specifically on testing to diagnose dampness and other causes of dampness.
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N.b. Nothing in this article constitutes legal, professional or financial advice.