Moisture problems are a common concern in buildings, and identifying their source and extent is crucial to preventing long‑term damage. Moisture meters are popular tools among surveyors and building professionals, and while these instruments can be extremely helpful in diagnosing damp issues in timber, caution is advised when using them in other materials—particularly plaster at the base of walls. As it’s the brand we use most frequently, this article will focus on Protimeter devices while also addressing broader considerations when using moisture meters in building diagnostics.

Why Do False Positives Occur in Plaster and Other Non‑Wood Materials?

Protimeter moisture meters work by measuring electrical conductivity. Since wood has well‑understood electrical properties, these devices can be calibrated for it quite accurately. Plaster, masonry, and other materials, however, have very different characteristics. Factors like metallic particles, density variations, or natural salts in these materials can skew the readings, leading to false positives.

Older buildings or areas affected by rising damp often have plaster contaminated with hygroscopic salts. These salts absorb moisture from the air, causing the meter to register high levels even when the material itself isn’t damp. The “problem” may be salt deposits rather than active water ingress.

What Protimeter Says About Their Devices

• They are calibrated for wood. Using them on plaster or masonry requires correction factors; without them, results can be misleading.
• Interpretation matters. Moisture meters are a first‑step screening tool and should be followed by confirmatory tests.

Insights from British Standard 5250

British Standard 5250: Management of moisture in buildings – Code of practice urges caution when using moisture meters because they can yield false positives on non‑timber substrates or where salts are present. The standard recommends complementary testing—such as carbide (CM) or gravimetric analysis—to pin down the true extent and source of damp.

In short, treat moisture meters as screening tools. Relying on them alone—especially on plaster—can lead to misdiagnosis and unnecessary remedial work.

Good Practice for Using Protimeters

A balanced approach—combining additional tests with a careful assessment of the building’s overall condition—yields the most reliable damp diagnosis.

1. Cross‑check with other methods. Use hygrometer tests, CM sampling, or specialised probes for plaster/masonry to verify readings.
2. Don’t rely solely on the meter. Carry out a thorough visual inspection and consider building context: age, ventilation, construction type, and possible moisture sources.
3. Call in the experts. For older or heritage buildings—with salt contamination or atypical materials—specialist interpretation is invaluable.

Conclusion

Protimeter moisture meters are an excellent starting point for damp investigations, but both the manufacturer and BS 5250 stress careful use. On plaster or masonry, salts and differing conductivity can distort readings, so always back up meter results with further tests and a wider survey. By recognising these limitations and following best practice, surveyors and property owners can spot genuine moisture problems more reliably and avoid unnecessary or ineffective remedial work—ultimately protecting both the building and its occupants.