All surveyors and contractors use a ‘damp meter’. These are claimed to ‘measure’ damp walls and timbers and are a commonly used device. Unfortunately, they are mis-used and it is important to point out that they don’t actually measure anything at all. They are a resistance meter. To accurately measure the moisture content of a brick wall, for example, you would need to drill holes and do a gravimetric test or carbide test. Neither of these science-based techniques is a viable option for a pre-purchase survey though.
In the photos shown here you can see a small piece of lime render from a wall. It is a three-coat lime, haired, and has both a lime wash and a modern ‘plastic’ paint surface. Prior to the photo being taken it had been kept in a warm indoor environment and it is dry and dusty to the touch. It is most definitely not wet or even slightly damp. However, as you can see, the Protimeter clearly shows a relatively high reading. If this were in a house and on a wall, this is the exact scenario where someone might claim there is a damp problem which requires a modern remedial treatment. Quite clearly that is wrong.
The device is actually reacting to something within the lime, probably a salt, which is influencing the resistance meter.
It is very important that homeowners are aware of the mis-selling of remedial damp-proofing which is based on inaccurate ‘measurements’, comments about a failed or missing dpc and will lead to expensive quotes for unnecessary alterations. It is highly unlikely that Listed Building Consent would be granted for remedial damp-proofing.
Having an awarenesses of the function and limitations of the use of a resistance meter is essential.