Identifying dampness in buildings​

These methods are a guide to improve the accurate diagnosis of the root cause of damp.

The list is not exhaustive. Please propose your own method.

Methods are intended to build on RICS, PCA, English Heritage etc. joint methodology 2019 (“JM-19”): “Investigation of Moisture and its Effects in Traditional Buildings”. Differences are highlighted below.

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

These methods can be applied to properties thoughout the world.


1. Profile of damp patch

Damp in buildings originates from 4 distinct sources of water:

A. All sources of moisture tend to form or pile up at the base of a wall. The presence of damp at the base of a wall is not sufficient to identify a source of water.

B. Condensation forms on the coldest section of wall, which tends to be on the base of a wall, in window and door reveals, around metal and up corners with external faces.

C. Leaks on solid floors, flood water or groundwater produces an even horizontal profile of damp to about 600mm but can be higher when humidity is high.

D. A horizontal band of dribble marks is a sign of condensation.

E. Mainswater leaks and rainwater typically caused one or two discrete points of dampness.

The highest point or centre of damp tends to be closest to the point of ingress.

F. Changes to a building, such as past damp proofing treatment involving impermeable plaster or plasterboard can hide damp in a wall. However, the interface between the new and old plaster is vulnerable to damp, especially condensation.

JM-19 might disagree with:

  • xxx

3. Colour

Water passing through building material, such as red brick or timber, will tend to pick up colour and cause a stain after a few months. The presence or lack of colour can be used as an indication of the source of water. Typically, brown stains are caused by mains water leaks or rainwater, but interstitial condensation can also cause brown stains.

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 treatment and monitoring.

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*.


3. Mould

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface.

Mould is inhibited by nitrates found in groundwater. The presence of mould at the bottom of a wall eliminates rising damp as the root cause and points toward condensation from unvented excess vapour.

JM-19 xxx

4. Crystallised surface salts

Crystallised surface salts are normally calcium sulphate (AKA gypsum). Surface salts are caused by a damp wall becoming moist then drying out. Surface salts are commonly associated with constant fluctuation between condensation and evaporation.

JM-19 xxx


5. Hygroscopic salts

  • Hygroscopic salts mainly come from the historic burning of wood and coal, but equine urine and road de-icers can also contain hygroscopic salts.
  • Groundwater also contains hygroscopic salts, but the presence of hygroscopic salts is not itself an indication of groundwater, without corroborating evidence.
  • Hygroscopic salts tend to form horizontal or vertical bands of dampness visible when the air is humid, around old chimney breasts or walls near to areas once close to horses.
  • Hygroscopic salts are not a source of damp. Moisture condenses on the surface in dynamic equilibrium.

JM-19 xxx

7. Below ground

Damp proof courses (“DPC”) and sub-floor ventilation are both precautionary, in case of dampness in the sub-floor. The bridging of a damp proof course, lack of DPC or restricted sub-floor ventilation need not cause damp but can increase the risk of damp and rot.
Water entering the sub-floor void is liable to cause rot or condensation on the DPC.

JM-19 xxx


8. Detection tools, techniques and monitoring

JM-19 xxx

9. Rising damp

Damp proofers are incentivised to recommend damp proofing treatment, whatever the cause. No one can blame damp proofers and builders for recommending building work. It is how they make a profit. 

However, some phrases in their damp reports can appear to be purposefully misleading such as; 

we were asked to determine if there is rising damp, we found damp and recommend damp proofing treatment. 

It says damp was found, but does not identify a source. 

This is bad practice and materially misleading, as our, in our opinion, the following forms of bad practice:

  • Rising damp, that is the upwards absorption of groundwater, is often diagnosed based solely on a high damp meter reading at the base of a wall.
  • In many parts of the country, a majority of damp properties have previously been damp proofed against rising damp.
  • Rising damp treatment may hide the symptoms, but often exacerbate the risks caused by moisture, such as rot and mould.
  • Rising damp is often the default diagnosis because a surveyor cannot find another source of damp, rather than there being any evidence of groundwater.
  • Groundwater can be definitively dismissed by drilling down below ground level under of near to a property, carefully avoiding pipes and cables.
  • The risk of groundwater below a property can be assessed by reference to the British Geological Survey. The risk of groundwater flooding can be assessed with reference to Government groundwater situation reports.
  • According to a presenter on a PCA webinar on floodwater last year, there are only about 2,000 properties in the UK at risk of groundwater flooding.

JM-19 xxx


10. Unusual causes of damp

JM-19 xxx

Frequently Asked Questions.

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