Building surveyors must be aware that moisture meters and other methods used to identify the presence of moisture in building materials do not differentiate between dampness from various sources. Therefore, it is essential to consider all potential causes of dampness before concluding. BS 6576:2005 +A1:2012 is the Code of Practice for diagnosing damp in walls.
Causes of Dampness
Dampness in buildings, excluding those recently built, usually stems from one of five natural sources: Rainwater penetrating damp, plumbing leaks, water from the air within the building, condensation, and water from the ground rising damp and salt damp when wall plaster gets contaminated with hydroponic salts.
Methods of Moisture Determination
Accurate diagnosis of the cause of dampness is assisted by the ability to map the distribution of moisture within a wall and ideally measure the extent of dampness. There are three fundamentally different ways of determining and/or measuring moisture in brickwork and other masonry:
1. Electrical Conductivity and Capacitance
2. Chemical – Carbide method
3. Gravimetric – Oven drying method
Electrical Methods
These methods are non-destructive and provide quick and easy readings. However, in walls of uncertain composition, the electric method may not give a direct percentage reading of moisture content. Readings are typically recorded on a relative scale to establish the moisture pattern. Electrical methods are useful tools but can lead to confusion and incorrect diagnosis, especially where ground salts are present or the substrate being tested is conducting.
Conductivity Meters: Two probes are used to complete an electrical circuit through the meter on the wall’s surface or embedded in the wall’s depth. The electrical resistance in the wall influences the circuit and is affected by the moisture content.
Capacitance Meters: The meter or separate head with conducting plates or concentric rings is placed on the surface being measured for moisture content. The fringe capacitance in the sensor, influenced by moisture content, is measured by the meter. Some instruments use visual or audible indicators for changes in moisture content.
Gravimetric or Oven Drying Method
This is a destructive method where drillings must be taken from the wall. It is time-consuming and requires laboratory facilities, making it unsuitable for on-site use. However, it is the most accurate method available and can differentiate between ground moisture and hygroscopic moisture.
Chemical Method
The carbide meter is a destructive method, but it can be used on-site and provides rapid results. Although it does not directly distinguish between ground moisture and hygroscopic moisture, comparisons and in-depth drilling can offer a good indication.