Damp survey

We all naturally understand water, but tracing damp back to its source can be fiendishly complex and can take time for further investigation.

Currently neither RICS nor PCA teach surveyors to test damp:
– holistically (cellar, sub-floor, each room, loft, neighbours etc – checking ventilation, humidity, leaks, etc)
– using a root cause approach (that is tracing damp to source, and the cause of the source)
– based on evidence (such as rising damp replicated in the lab, and on-site evidence of high ground water and damaged DPC).
– risk against online resources, such as risk of groundwater from the British Geological Survey.

Damp wall

Reasons for damp misdiagnosis

These are the primary reasons for widespread misunderstanding and mistreatment of damp in UK – in my opinion. Around 70% of properties survey at damp surveys have previously been damp proofed, normally injecting chemicals to provide a damp proof course. NB business would be tough for independent damp surveyors if a) rising damp treatment worked, b) RICS surveyors were correctly taught.

1) Categorising damp

RICS / PCA focus on symptoms, by categorising damp as; penetrating damp, mains leaks, rising damp and condensation. 
If you take a source based approach to categorising damp, you are more likely to find the source and therefore stop the damp, without need to hide it.

  • Rainwater,
  • Mains water,
  • Groundwater,
  • Vapour.

For ease, water from waste, cleaning, spillage, building water etc, I categorise as mains water.
Hygroscopic salts I sub-categorise under vapour as they effectively condense vapour and are in dynamic equilibrium, so don’t add water over the medium term. Hygroscopic salts are not a symptom of rising damp. In practice these salts are more likely to move through replastering, intense condensation or rain.

Hygroscopic salts, incorrectly said by PCA to be evidence of rising damp. But slope & height suggests otherwise.

2) Defining rising damp and groundwater

  • Rising damp is defined as “upwards flow of moisture through a permeable wall structure, the moisture being derived from groundwater”.
  • RICS and PCA agree with this definition, but don’t explain the importance of defining groundwater.
  • Groundwater is subterranean water at or below the water table. 
    [interesting fact: There is 100 times more groundwater than water in all the world’s rivers and lakes (“Groundwater in the environment” Paul Younger)].
  • Groundwater is pumped out of many/most big cities, so groundwater (and therefore rising damp), is rare in big cities – certainly London where I survey.

The risk and presence of groundwater should be relatively easy to determine with:

  1. reference to local water table-data,
  2. groundwater normally has nitrate salts which can be detected, whereas condensation and rainwater don’t have nitrate salts (unless rain has spent a long time in soil).
  3. you can drill into soil under or near a property (carefully away from drains and electrics) and drop in a tube to measure groundwater fluctuation over time.

3) Process of elimination

  • RICS (and by implication PCA) lecturers teach surveyors to diagnose rising damp at the base of a wall, if no other source of water is positively identified.
  • While a process of elimination is understandable, rising damp should not be the default.
  • Any default should be highlighted as a “possible source”, without certainly or invasive treatment before trying non-invasive treatments and monitoring, as correctly recommended by RICS, Historic England and PCA in damp survey guidance.
  • I have taken many PCA/RICS courses and other than inconclusive photos, I have never studied a wall with rising damp.
  • I’ve tried replicating it, but like many, can’t get a rise above a couple of courses.

In my opinion, the rising damp default approach and lack of reproducible evidence, go to the heart of the problem. 

  • Surveyors have limited time and data, often only visit once and can’t always see above, below or the other side of a wall.
  • So, leaving aside motivation of some surveyors to diagnose rising damp, even the best intentioned are tempted to put a damp wall down to rising damp if nothing else can be found.
  • Reputation/ litigation from past diagnoses, makes it hard to change RICS/PCA attitudes.

4) Understanding rising damp

  • Water spreads out in an absorbent material, through the force of attraction (polarity) commonly described as “capillary” action. 
  • The rise of water is limited by the force of gravity.  Were it were not for gravity, the rise and fall would be equal.
  • Water in saturated pores will fall while connected pores below remain less saturated.
  • The greater the pore saturation, the stronger the gravitational effect [weight of water]. 
  • The reason that rising damp is define as the rise from groundwater is because the pores in groundwater are saturated below the water table. 
  • In the case of groundwater, the force of gravity is neutralised by a reservoir of water on bedrock.

By contrast, rainwater in unsaturated soil is free to gravitate down, so the force of upward force of attraction is temporary and limited:

  1. rainwater will eventually percolate down to the water-table and become groundwater, and
  2. rainwater should be drained away from a house, 
  3. whereas it is normally impractical for an individual home owner to remove groundwater.

5) Capillary action

There is a misunderstanding in the general public about capillaries. Most people see capillaries standing upwards, but the force of attraction is in all directions and drawn down by gravity.

Understanding that capillary is in all directions.

I took this photo to demonstrate that water will tend to spread out through a capillary, more than up.

Damp wall

I use this image to demonstrate the comparative rise and fall from a central point of ingress (the tear in the kitchen paper near the top is sitting in vessel of water). The rise is limited to about 8cm, but the fall is only limited by the quantity of water and speed of evaporation.

Damp wall

I use this image to demonstrate how water spreads out, here the base of the kitchen paper is acting like a DPC. The dip towards the edges is caused by evaporation, otherwise it would be horizontal.

6)  Damp Proof CourseOne misconception is where water appears to increase in height from the base of a wall, so people assume that water is rising (whereas water is typically falling and piling up).I think you explain this well. Most water either falls or is absorbed horizontally, but because it can’t fall further, it spreads out and piles up (thus appearing to rise).DPCs make matters worse as they block the downward capillarity effect.
7) High sub-floor humidityI regularly find properties with high sub-floor humidity causing rising-damp-like symptoms from condensation forming on bricks above the damp proof course.Normally the root cause is a blocked gulley or drain, made worse by insufficient sub-floor ventilation, possibly following a rear extension.Before the British Wood Preserving Association (BWPA) joined the British Chemical Dampcourse Association to eventually become the PCA, it differentiated sub-floor condensation from rising damp.While I don’t agree with their stats, it’s interesting that in their 1984 annual report, they ranked sub-floor condensation and rising damp as more or less equally prevalent, well below penetrating damp.You never hear PCA (nor RICS) talk about sub-floor condensation (nor do PCA discuss diagnosing leaks – a topic for another time).
One of my favourite properties is a 15th century house that was built next to a small brook (presumably for potable water).A few centuries ago the house was extended over the small brook, which was allowed to continue flowing through the cellar.

There is a more or less constant flow of groundwater, providing life for a colony of frogs. A sump pump now stops water flooding over the stone floor.

Sufficient ventilation ensures that damp evaporates from wall and does not rise to the ground floor above.

The brook is not a problem because the walls are about 2M high, and there’s plenty of sub-floor ventilation, as you can see.I’m a fan of sub-floor venta as a safety mechanism (I’m also a fan of monitoring sub-floor humidity – a topic for another time).

8) Profiling

PCA talk a bit about profiling damp on their courses, but it never seems to be mentioned or used as a tool for identifying damp in PCA damp surveyor reports (likewise RICS).For example rising damp should be more or less horizontal like floodwater, with a slight dip in corners and openings due to evaporation. There should be no discolouration nor mould (as nitrates in soil inhibits mould growth).

Whereas condensation tends to rise up openings and corners. Of course a wall with an impermeable coating, such as slurry, can obscure the profile.


I’m neither academic nor a purist. I’m a pragmatist.
Chemical treatments would be great if:

  1. The root cause is genuinely rising damp,
  2. Injections were the best solution (without damage).

I have no financial interest in any recommended remedial actions.]

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