Principles of Damp Surveys

The principles of damp in properties

Root cause

Damp treatment is most effective and risk free, if the cause is traced back to the source of moisture and stopped. We use a process of elimination. One cause can mask another cause.  It rarely takes long to stop water at source, at least temporarily, such as unblocking a gutter or opening a window. There may be additional smarter more durable options.

To see what can go wrong when the root cause is not identified and stopped.

Sources of damp fall into four categories

  1. Rain water; often seen as penetrating damp. This typically results from a damaged or blocked gutter, hopper, down pipe, gully, damaged roof or raised garden.
    • Ingress (or penetrating damp) is common in basements. Thermal stress to external render can resulting shrinkage and cracks for water to penetrate.
    • A check of rainwater goods front and back, is the starting point of any damp survey.
  2. Water vapour resulting in condensation or interstitial condensation (within a building material) and mould.
    • 85% of all reported dampness in properties is caused by condensation.
    • Condensation often accompanies all other forms of dampness,
  3. Water leak; typically, mains water, radiator, waste water or shower leak.
  4. Ground water; known as rising damp, this is caused by a high-water table and is very rare.
    • Condensation or defective rainwater goods are commonly misdiagnosed as being caused by rising damp, through commercial motivation.
    • All other forms of damp must be thoroughly investigated, and monitored over time, before considering chemical rising damp treatment.
    • It is important to see both sides of a damp wall, such as the neighbour’s side of a party wall. Note a Party Wall etc. Act 1996 agreement will be needed, so they will be involved.
    • Both sides of a wall must be damp and the line of dampness must be horizontal, for there to be rising damp.
    • The risk of misdiagnosis is that the dampness becomes hidden, possibly trapped and causes greater damage elsewhere.
rain is the main source of water and sometimes water vapour - a principle
Rain is the main source of water. Broken rainwater goods is a frequent cause of damp.

Properties are not built damp

Properties are not built with damp defects. It is the changes to the properties that cause dampness, be poorly designed extensions, inappropriate damp proofing treatment, central heating, double glazing or life styles changes such as daily showers.

Movement of water

Water moves in three ways;

  • Flowing liquid, always downwards whether it’s rain water or a leak.
  • Vapour will disperse water through air from high vapour pressure (quantity) to low.
  • Absorption of water. Porous building material will absorb moisture in all directions equally in zero gravity, but mainly downwards with gravity.

Eliminating the effects of vapour

Condensation is found in almost every property to some degree. It is caused by insufficient extraction of vapour at source and is made worse by insufficient heat or insulation. 

Air at any given temperature has the capacity to hold water vapour. The percentage of vapour compared to the capacity is known as relative humidity or %RH. As temperature increases so capacity increases, conversely as temperature decreases so air holds less vapour until a temperature when condensation starts to form. This is called the dew point. Unless vapour is vented out of a property, when the temperature drops at night, condensation will form on a cold surface inside or within the building material.

Often damp from other source can be eliminated through ventilation. So improving and monitoring ventilation is vital tool. If you carefully read the documents supplied by damp proofing companies, such as Kenwood, Rentokil and Aquapol, they will typically recommend improved ventilation, along with their recommended treatment.

It is likely to be the inexpensive ventilation that solves the damp problem. The expensive damp proofing treatment probably makes no difference. In some cases it makes matters worse. In other cases it masks the problem allowing the source of moisture to cause rot.

Thermal bridging

Moisture condenses on cold surfaces first. Some surfaces can lose heat more rapidly than other surfaces, such as a single pane of glass, a metal RSJ supporting an extension, plasterers metal beading, chimneys, damp brickwork, solid floors or uninsulated ceilings. 

Timber defects

The greatest risk to any property results from rot, especially dry rot damaging below ground floor timber. Rot only grows where water is present. The risk of rot diminishes if airflow evaporates away water. Misdiagnosed and incorrectly treated damp increases the risk of rot.

Woodworm damage is common in old timber, but very rarely active as old timber is not normally sufficiently nutritious. Damp timber is more likely to be affected by woodworm than dry timber. Unlike rot, woodworm can continue after the source of damp has been stopped. Active woodworm is easy to identify and treat. 


Mould only forms when relative humidity exceeds 85%RH for 6 to 8 hours depending on the species. Mould point is the temperature below which there is a risk of mould growing. Mould is not dangerous in the residential setting but a sign that there is insufficient ventilation.

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