What a Damp Meter Actually Measures
A damp meter does not directly measure damp. This is one of the biggest misconceptions. Instead, the device measures electrical resistance. Water and salt both conduct electricity, so when the meter detects lower resistance, it interprets that as a higher reading. This is why the numbers can vary even when there is no visible moisture present.
Different building materials conduct electricity differently, which means the same reading in two different walls may not mean the same thing at all. A reading of twenty in plasterboard is very different from a reading of twenty in a solid stone wall. Without understanding the material you are testing, the reading is almost meaningless.
Because of this, a meter should never be used in isolation. It cannot tell you whether the cause is rising damp, condensation, penetrating damp, plumbing leaks, or simply salts left behind from historical moisture. The number alone does not reveal the source of the problem.
Why Damp Meter Readings Can Be Misleading
Damp meters respond to salts as well as moisture. If a wall has had damp problems in the past, saltscan remain long after the wall has fully dried. These salts attract moisture from the air, which means the meter will continue to give high readings even when the underlying problem has been resolved.
Historic moisture can therefore give false positives. Likewise, a wall recently redecorated, skimmed, or plastered will naturally hold moisture while it cures. A meter used during this period will almost certainly show elevated readings. This does not mean the property has a damp issue.
Meters often give false readings when pipes, metal objects, foil-back insulation, or other hidden conductive materials are present. Since the tool cannot distinguish between moisture and metal conductivity, always interpret results alongside a visual inspection and knowledge of building’s construction.
How Surveyors Use Damp Meters Properly
When used correctly, a damp meter is a helpful screening tool. A surveyor will first look for visible signs of moisture, such as staining, mould, blistered paint, peeling wallpaper, or signs of salt crystallisation. Use the meter to support your observations, not replace them.
Surveyors compare readings across multiple locations rather than relying on a single measurement. They look for patterns, such as readings that increase toward the base of a wall, which might suggest rising damp, or higher readings near an external corner, which might point to penetrating damp or thermal bridging.
They also consider the wider context. A cold wall will naturally show higher relative moisture. Rooms with poor ventilation may have elevated readings even without a structural defect. Good surveyors use the meter to guide the investigation, but the final conclusion comes from experience, construction knowledge, and an understanding of how the building manages moisture.
Why Homeowners Should Be Cautious With DIY Damp Checks
Many homeowners buy a damp meter believing it will give a simple yes or no answer. Unfortunately, without a trained eye, readings are easily misinterpreted. This leads to unnecessary concern or, worse, unnecessary treatments that do not address the underlying cause.
Some contractors may also misuse damp meters to justify work that is not required. A high reading alone does not confirm a damp problem, and it certainly does not diagnose the cause. Expensive damp proofing treatments can be recommended based solely on meter readings, even when the issue is nothing more than condensation or salts from historic moisture.
If you suspect damp, a professional assessment is always the safest option. A qualified surveyor can identify the true source of the issue and recommend appropriate, cost-effective solutions.
Conclusion
A damp meter is a useful tool, but it is not a diagnostic instrument. It measures electrical resistance, not moisture, and its readings are influenced by salts, materials, temperature, and hidden objects within the wall. Used alone, the numbers can be misleading.
Understanding what the meter is really telling you is essential. For accurate diagnosis and reliable advice, a proper survey is the best way to ensure that any moisture issues are understood and treated correctly. If you are concerned about damp in your property, or simply want reassurance, Harrison Clarke is here to help with clear, informed guidance based on experience and building expertise.
For more expert advice on surveying and property matters, check out our range of informative videos on our website or YouTube channel. Harrison Clarke Chartered Surveyors is here to guide you every step of the way!
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