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Hot Topic Highlight – Dampness in Buildings Part 3

Updated: Oct 28, 2023

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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 3 of a three part blog series, focussing specifically on testing to diagnose dampness and other causes of dampness.

This is Part 3 of a three part blog series, focussing specifically on types of damp and how to diagnose them accurately. You can read part 1 and part 2 by clicking on the links.

What tests can be used to diagnose dampness?

Below are a number of tests that can be used to help diagnose dampness. However, they are not capable of being carried out during a normal survey.

Concrete Moisture Test

One of the simplest, most economical test methods for determining whether there may be moisture in concrete is to duct tape a 450mm square piece of plastic onto the exposed concrete and leave it for 16 hours.

Condensed moisture accumulation under the plastic at 16 hours can indicate a problem.

Calcium Chloride Test

Another test is where a calcium chloride disk is placed under a sealed plastic sheet and left to collect moisture vapor. After 24 hours, the disk is retrieved, weighed and compared to the disk's pretest weight. This weight difference indicates how much moisture vapor has emerged from the slab in 24 hours.

Relative Humidity Testing

Relative Humidity Testing (ASTM F2170) of the slab is done via special moisture probes embedded in the concrete substrate. The most advanced and comprehensive of the three methods, this test measures the presence and quantity of moisture throughout the depth of the slab.

Once you know whether there is a moisture control problem, you can then advise the client to consult a professional flooring contractor who specialises in moisture mitigation.

What is penetrating damp?

Penetrating damp is when moisture enters from outside the property in a horizontal direction. It can occur at any level and at almost any location in a property.

Gravity then causes the downward movement of the damp into other areas, sometimes called falling damp. It is identifiable visually by isolated patches of dampness.

The causes of penetrating damp might be:

  • Leaking water supplies or waste pipes

  • Fretted mortar joints

  • Defective brickwork

  • Failure of tile grouts in showers and other wet areas

  • Poorly functioning membranes in wet areas

  • Cracked render

  • Poor flashings

  • A defect in the adjacent property outside the owner’s control

  • Air conditioning or hot water system overflows leading to small-localised patches of dampness

  • Failure of cavity trays, especially in earlier buildings when roofing felt was used

One of the major causes of penetrating damp in pre-second world war houses is linked to the use of cement strap pointing, which became popular in the late 1960's and 1970's.

A solid brick or stone wall, built with lime mortar, needs to breathe and it does this by losing its moisture content through the lime mortar joints. This is lost when a cement mortar is used. The wall immediately starts to get wet due to the trapped water and the only way it can get out is via the brick or stone or it is driven internally.

What is condensation?

Condensation is water vapour in the air in a house condensing on a cold surface. It can form on any surface, and it may not be noticed until mould growth or rotting of material occurs.

In the UK, it is mainly a winter problem, particularly where warm moist air is generated in living areas. This then moves to the colder parts of the building. It normally appears on windows in the early morning and evaporates harmlessly as the house warms.

A wall may be cold and attract condensation for several reasons:

  • Walls face North or East

  • A wall may only be 112mm thick, especially where an old external toilet or a coal house has been demolished or incorporated into the main house

  • The room may be unheated

  • Leaking gutters or pipes may make part of a wall cooler

  • Cold bridging, which is more likely to occur in flats built in the 1960’s and 1970’s

What conditions are required for condensation to occur?

The moisture in the air comes from a number of sources within a house. For example, water vapour is produced in relatively large quantities from normal day to day activities. A 5-person household puts about 10kg of water into the air every day (without considering any heating).

Houses have become more effectively sealed by the introduction of double glazing, draught excluders, fitted carpets (which prevent air movement up through suspended wooden floors) and the removal of open fireplaces with the introduction of central heating. This keeps any moisture produced within the house and provides optimal conditions for condensation to occur.

Ventilation is only effective if consistent throughout the whole envelope of the house. Often the conversion of houses into flats results in the inability or difficulty of obtaining a through air flow to aid ventilation due to the layout of the rooms.

Modern lifestyles also mean that many houses remain unoccupied and unheated throughout the greater part of the day, allowing the fabric of the building to cool down. The moisture producing activities are then concentrated into relatively short periods (morning and evening) when the structure is relatively cold and while the building is still warming up.

Heating is only on for short periods, effectively only warming the air and not the fabric of the building, so there is little or no radiant heat to keep the air temperature up.


We hope you enjoyed reading this three part blog series on dampness in buildings. The diagnosis of dampness is a complex area of practice and only experience and gaining further knowledge will help to make your diagnoses more robust and based on sound testing and further investigations.

How can we help?

Stay tuned for our next blog post to help build a better you.

N.b. Nothing in this article constitutes legal, professional or financial advice.


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