Identifying dampness in buildings​

These principles are intended to be evidence based and a living source of guidance to increase accuracy of diagnosis, remedial treatment, and public awareness of dampness in property.

It is intended that these principles should build on RICS, PCA, English Heritage etc. joint methodology 2019 (“JM-19”): “Investigation of Moisture and its Effects in Traditional Buildings”.

Despite its ambitions, there is no evidence that the joint methodology has improved diagnosis of dampness in buildings.

These principles were developed in UK, where misdiagnosis appears to a particular problem. However, the problems of dampness in buildings are global.

1. Damp detection

The purpose of a damp survey is to identify the sources of water affecting internal walls, list the remedies necessary to stop the flow of water and follow-on actions.

Symptoms of damp are useful for identify the source of water, but treatment should focus on the cause or causes of dampness not the symptoms.

Rising damp, mould, rot and condensation are symptoms of damp, not the root cause.

JM-19 recognises the need to focus on causes not symptoms. However, RICS and PCA members continue to diagnose rising damp in similar numbers to before JM-19.

2. Sources of water

Damp in buildings originates from 4 distinct sources of water:

A. Rainwater – known as penetrating damp.

B. Mainswater – which includes wastewater, accidental spillage, water used in construction etc.

C. Groundwater – the water under the water-table (source of water in rising damp).

D. Vapour – water held in the air in the form of a gas. Water condenses below the dew point.

JM-19 might disagree with:

  • Recognition that groundwater is the water under the water table, not simply water in soil, which could come from broken rainwater goods.
  • Recognition that hygroscopic salts cause deliquescence, which is a form of condensation, albeit at normal ambient humidity levels. Hygroscopic salts have multiple sources, only one of which is groundwater.

3. Holistic approach

A source of moisture found in one part of a building may have an impact on another part.

All parts of a building should be checked, such as floors, walls, internal and external, ceilings, roof space and subfloor or cellars, and on he neighbour’s side.

Surveyors who with experience of searching for unusual forms of damp are far better placed to diagnose damp correctly, then damp proofers who focus on damp at the base of a wall.

Examples include:

  • Hygroscopic salts on upper floor chimney breasts.
  • Water penetrating through micro cracks in impermeable paint.
  • Water penetrating through render cracks.

JM-19 might disagree with:

  • xxxx

4. Moisture is distributed in a porous wall through absorption

Absorption distributes water in all directions, mainly downwards through gravity, but if the lower pores are blocked because:

  • lower pores are saturated with water or
  • there is an impermeable membrane,

then wall can pile upwards causing rising damp like symptoms. That is dampness at the base of a wall.

All sources of water will cause a moisture to rise in a porous wall, above the contact point. However, the rise is normally only limited to about 100mm, unless there is barrier stopping water from descending.

Groundwater is unique in that soil under the water table is saturated, saturate soil acts as a barrier to downward absorption.

Water above an impermeable layer, for example condensation above a damp proof course, or a leak on a solid floor, with rise up a wall by about 1M. This is not groundwater and should not be treated with injections and impermeable plaster.

JM-19 does not recognise the importance of and distinction between the rise of water from groundwater and the rise of water from a leak on a solid floor or condensation above the damp proof course.

5. Evidence

The identification of the source of moisture must be based on evidence. See methods.

Dampness on a wall, possibly determined by a damp meter, or visually is evidence of damp, but it is NOT evidence of a source of moisture.

Unless as a source of water is positively identified, with evidence, then this lack of evidence should be stated, with a follow-up procedure for monitoring and low cost, non-invasive remediation, such as improved ventlation.

JM-19 does not require a diagnosis to be accompanied with evidence of source of water.

  • Groundwater can be excluded by reference to local water table data.
  • The risk of groundwater can be established by drilling 1M below the ground floor*.

4. Lack of evidence

The primary reasons for these methods and principles is to reduce, ideally eliminate the number of surveys for which there is no evidence of a cause.

Finding the root cause of dampness can be complex and time consuming. Lack of evidence of the source has previously resulted in default rising damp diagnosis (despite it being rare and a symptom not a cause).

This is bad practice.

Current default which needs changing

Surveyors are often expected to give a diagnosis, even if they are not sure themselves. This is especially true when surveying for a buyer.

The cost of remedying dampness depends largely on the diagnosis, with rising damp treatment and roofing being amongst the most costly.

Homebuyer surveyors are understandably concerned about avoiding litigation for missing a defect. Their motivation is to:

  • Pass the risk of incorrect diagnosis onto someone else.
  • Have the cost of remediating the defect be high enough to cover the cost, whether or not the final diagnosis is correct.
  • Have some form of guarantee, although of questionable value.
  • RICS surveyors appear unconcerned by the vested interests of damp proofing contractors.

For these reasons RICS homebuyer surveyors are motivated for damp proofing contractors to quote for rising damp treatment whenever dampness is found at the base of a wall. This despite rising damp being exceptionally rare.  

If there is no evidence, then this should always be stated

Excess vapour is the default if there is no other evidence. All standard test should be performed and if there is insufficient time to complete a survey, then a return should be arranged, ideally to see the other side of a wall, such as on the neighbour’s side.

Vapour will affect all properties to some degree and so should always be considered as a likely cause of dampness, even when the temperature is high and internal relative humidity low.

Condensation may have taken place months beforehand, in winter or during a previous occupation. A damp wall typically takes 10 months to dry out. 

Condensation from high sub-floor humidity, caused by any source, such as leak or penetrating damp, made worse by poor sub-floor ventilation is liable to cause condensation to form above the damp proof course.

Interstitial condensation, that is condensation within building material or component often looks like penetrating damp.

JM-19 agrees that condensation is the most common cause of dampness in walls.

Frequently Asked Questions.

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