Monitoring moisture condition

All surveyors will have standard moisture meters with just the pin probe facility. Many will have the facility for use in capacitance or pin probe mode, which can be extremely useful in monitoring dampness, especially if the readings captured on a screen are numbers. Better still if the screen has a backlight.

• The key thing is to be methodical, but also to interpret the readings correctly.
• Log the exact position of all readings, so later visits to take a reading will be to the same place.
• Prepare a monitoring table on which to log your readings.
• You may find it useful to monitor dampness by setting out a grid of readings on a sketch. It can be a slow and laborious activity, but you can reap rich rewards.

You need to know the limitations of electronic moisture meter readings. For more detail see Surveying equipment and tests.

The conductance moisture meter can potentially tell you:

• whether position A is damper or drier near the surface than position B at time X; and
• whether A is damper or drier at time X or time Y.

Beware: an electronic moisture meter offers quite a good indication of the percentage moisture content of timber, but only near the surface using standard pin probes for conductance readings.

Some resistance meters (the other name for a conductance meter) offer a percentage moisture content in other material – such as concrete or screed. Meters that are calibrated for concrete will be of less use in a cement/sand screed as concrete is a material with different physical properties. Concrete itself varies considerably, from the porous ‘no-fines’ mixes to the denser more structurally efficient grades. Concrete might potentially contain say 5% of its dry weight as water, but a cementitious screed perhaps 15%.

A moisture meter can only give you a percentage moisture content in:

• a material for which it has been specifically calibrated by the manufacturer;
• another material that by coincidence has similar properties of electrical resistance; and
• material for which you have prepared a calibration graph;

and always providing that:

• the subject material does not contain unexpected carbon, salt or metal to magnify readings;
• the readings are not taken in the proximity of metals, e.g. a capacitance reading to wall plaster near a cast-iron fireplace surround or electrical cable;
• the batteries are of suitable type and charged; and
• the moisture meter is in good working order and used correctly.

The moisture meter will not be able to offer even a relative reading over the materials moisture content range if the calibration graph has a flat or near-flat section, i.e. an L-shaped calibration test graph.

Your readings of dampness and all the notes you take on building condition at date A could be referred back to on a later visit – date B – when you might have been asked to check the building to assess whether there has been a condition change. In other words, any survey could be a first monitoring visit.

Figure 1: A moisture meter reading of 13.2% is recorded in quite a thick skirting board

Figure 2: A capacitance reading of 1,000 means it is extremely likely this floor screed is significantly damp. You could check the timber edge gripper using the pin probes – a reading of 17% or more will probably mean you are at the scene of a damp problem. Make sure you have noted exactly where you checked the floor – e.g. ‘rear room floor screed, rear wall, 150mm from skirting, 1450mm from NE corner’. Mark this position too on a floor plan sketch. If checking under a floor underlay, sniff the underlay for mould

Figure 3: The same skirting section is removed, and you can at once see its dark discoloration – and mould. The pins are pronged, and a much higher reading behind the skirting of 24.4% fills our screen. A clear lesson that moisture meter readings taken on the room face of thick skirting boards can be misleading