Surveying equipment and tests

Moisture meters are used to identify those parts of the property that could be subject to a damp problem. Control readings in dry parts of the subject dwelling are useful because they put any higher readings in potential damp zones into perspective.

In the early stages of dampness diagnosis, the moisture meter is the key piece of equipment, but it must not be used on its own or without due care and attention to its findings. GE Sensing, a leading moisture meter
manufacturer in the UK, states clearly that:

'[it] would be foolish to carry out a survey without a moisture meter, but it would be no less foolish to rely on a moisture meter on its own for a complete diagnosis'.

Ideally, conduct a close visual examination of the property inside and out, and view and analyse any moisture readings in the context of site observations made prior to and after use of the instrument. There will be times when you must home in on a suspected damp zone after noting relevant symptoms of potential defects inside or outside the building, and high moisture meter readings will back up your original suspicion that a damp problem was likely. In other cases the recording of high moisture meter readings may initiate closer inspection inside and outside the building to help you understand why the high readings were recorded.

Any attempt to reach a diagnosis of dampness based primarily on moisture meter readings must be viewed with some suspicion, although there are a number of conclusions that may sensibly be reached if meter readings are used in conjunction with careful site observation. In a mortgage valuation, for example (where the survey may be limited to perhaps 30 minutes) if significant moisture meter readings are recorded the surveyor has a duty to spend some time attempting to find the reason for the high readings (see the legal case Roberts v J. Hampson & Co (1989) in Negligence in Valuations and Surveys). Merely noting 'high moisture meter readings' in a report, with little or no attempt to qualify them, and then passing on all responsibility for further investigation to others (such as remedial treatment contractors) is not good practice. There is a legal duty for a surveyor, even in a limited inspection such as a mortgage valuation, to follow a 'trail' of suspicion. The 'Dampus metallicus' – foiled again case study is a typical example of where such unsatisfactory practice may lead.

Also in the seminal case of Southwark LB v McIntosh (2001) 47 EG 145 the claimant’s expert in the original trial for damages against the Landlord under section 11 of the Landlord & Tenant Act 1985 appeared to prove that dampness and mould internally to a bathroom external wall was due to rainwater penetration from external defective rainwater goods. The expert witness surveyor was relying solely on the use of observation plus an electrical moisture meter. However the appellant court herd on appeal by the landlord that the expert surveyor had failed to prove a causal link between a failure of the structure exterior and/or services to the building and the mould growth on the external wall of the bathroom. It was held by the appeal judges that there was no breach of section 11 proven by the claimant’s expert and it was held that the mould to the walls had resulted from Ms McIntosh’s use and occupation of the dwelling. Invasive testing would be required to determine if the core of the external wall was damp in determining a causal link to an external defect and potential source of internal dampness and mould.

Until invasive testing is undertaken, it is difficult to confirm whether the main body of a wall is damp or air dry. High readings at wall plasters do not tell you if the wall itself is damp – a fact you need to know if you are to understand the symptoms presented. Damp plasters in conjunction with a dry wall often indicate a condensation problem caused by internal moisture, whereas damp plaster in conjunction with a wet wall could mean penetrating dampness that lowers the thermal resistivity of surfaces. Dry plaster, in conjunction with a wet wall, could occur where a moisture resistant plaster is used to mask dampness – 'damming the damp'.

The 2 types of moisture meter in most common use are resistance meters and capacitance meters. However, it is now possible to purchase multi-functional instruments that feature a resistance function, a capacitance function, a humidity sensor, an air thermometer, a surface thermometer, and deep probe facility in a single unit. Some digital meters may also be used as salts detectors.

Whichever instrument you plan to use, practice with the kit without commercial pressures, in your own home or in the office. Invisible damp describes a familiarisation exercise with a moisture meter.

Electrical resistance and capacitance meters must be used with caution. The resistance meter detects moisture because it has electrolytic properties, but other substances that are often present in the building fabric – salts, metals or carbonaceous materials – can conduct electricity too, and influence the meter readings. The capacitance meter is strongly influenced by metals, and measures the dielectric properties of a material.

When a resistance or capacitance meter indicates a low reading, then this would indicate that the wall is free of moisture, free of carbonaceous material, free of metallic material – in fact free of any material that could trigger the instruments to give a high or falsely high reading. This is how we find areas of wall that are dry to take our control samples for normal hygroscopic moisture content (HMC). You could use the term 'electrical dry meters' in these circumstances.