Surveying equipment and tests

The 2 pins of the typical surveyor's meter are either in a plug at the end of a lead, or fitted directly to the body of the machine. (The pins may be unscrewed so that you can take readings using just the 2 studs to avoid making pin holes in decorative finishes.) The device is simple to use. When the pins are pressed into the material under investigation, a voltage is applied between the pins and resistance is measured, shown by a needle moving, lights changing or numbers appearing on a screen. Some meters show flashing lights or make bleeping or buzzing sounds. The higher the reading, the more moisture there is likely to be in the zone of the material under investigation unless, of course, the material contains metal, salts or carbonaceous materials.

Figure 1: This meter has a digital display for easy reading. The skirting reads 17.8 (just within the 'risk' zone), and this part of the building should now be surveyed quite carefully. Note that the skirting is quite thick – it could be even damper behind

The moisture meter's pins can also act as a useful probing tool, because they will be easier or harder to push in as the plaster type changes. This might alert you to areas where plasters have been repaired, or renewed; for example in conjunction with a retrofit dpc installation.

Deep wall probes Figure 2: These deep probes are reading 100, an indication that there could be significant dampness within the wall's thickness Deep probes are insulated extensions to the standard moisture meter pins. They are useful to assess potential moisture conditions deeper into a wall where, for example, condensation-led mould appears on a surface. In this case you need to find out whether there is moisture deeper in the wall's thickness, which could point to penetrating dampness as a contributory factor. The probes provide relative readings at various depths in the wall, but may be influenced by salts, metals or carbonaceous materials. To use deep probes you need to drill small holes 6mm in diameter into the wall, spaced between 50–75mm apart, and typically to a depth of 75mm in brickwork. (This is a destructive operation in a nicely decorated room and might not be allowed by house sellers in a pre-purchase survey. See Drilling for samples for more advice on drilling.) During drilling mortar and brick dust is brought out by the drill bit, and simply viewing and feeling the dust can give you some indication of the moisture conditions within the wall. The probes must be pressed firmly against the backs of the holes to give a reliable relative reading, and you can 'stage drill' to obtain readings at various depths. Deep probes offer good value for money and give the moisture meter an extra dimension.

Example A first-floor wall showed 20 R/R (resistance) at the surface of plasters which, for the type of old lime-based plaster under investigation, equated to slightly damp conditions. Deep probes were used, and a relative reading of 16 was obtained at 75mm depth in the brickwork. Such a reading clearly indicates that the wall at depth is dry. This test eliminated penetrating dampness but made it necessary to investigate further for any condensation problem at the surface.

Most of the electrical resistance instruments in common usage are calibrated for timber. Readings are usually presented as percentage moisture content (% mc). In timber there is a risk of rot developing above 20% mc. Dry rot typically occurs in the range 20–35% mc, and wet rot at 30–60% mc. Attacks from wood-boring insects tend to occur when timber is damp.

In British Standard BS EN 942 table B.1 sets out expectations for external and internal joinery as follows:

• external joinery – 12–19% mc;
• internal joinery (unheated buildings) – 12–16% mc;
• internal joinery (12°C to 21°C) – 9–13% mc;
• internal joinery (heating 21°C or higher) – 6–10% mc.

Readings of 17–20% mc might indicate a potential dampness problem. There is a fine line between safe and unsafe timber moisture condition. This is complicated by the fact that the accuracy of moisture meter readings is only guaranteed within a certain range (8–28% mc). Readings beyond this range are 'relative' only (see What is the moisture meter trying to tell us? (Burkinshaw 2002).

CIRIA advises that resistance-type meters are generally accurate to +/-2% in timber, but that precision measurement is difficult as preservatives, species (often unknown), consistency and temperature can all affect the reading. So the best accuracy is about +/-0.5%. In other words, you have to remember that multiple readings along and across the grain help you obtain a good representative value of moisture content in the area of timber under investigation.

The timber industry has carried out many tests on moisture meters, and TRADA, the industry body, advises that:

'For general use, in timber above 20% moisture content, a meter should give a reading within +/-2%; in timber below 15% the moisture content reading should be within +/-1%'.

So you should check the specification and any available testing data of the moisture meter to assess its accuracy and suitability for purpose. Some manufacturers offer a correction table for timber measurements, so that you can slightly adjust the percentage recorded for the particular timber species you are checking. In practice, it is unlikely that many surveyors actually use such a correction chart, and joinery is often painted making identification of tree species difficult.

The moisture meters used by surveyors are sensitive to moisture content, whether they are used to test timber or masonry, but masonry materials such as concrete, brick, mortar or plaster are extremely variable in their physical and chemical properties – particularly density. If the instrument were to offer a % mc reading in all eventualities, it would need to be carefully calibrated for a huge range of materials. Even if we possessed such a device, it would still be difficult to interpret readings because it is not always possible, in the context of a house survey, to closely identify the masonry material under investigation. When testing masonry, the surveyor's meter (calibrated for timber) is mainly used to identify changes in (potential) moisture content on a relative basis only, and the meter does not offer us a % mc figure.

For example, readings from an electrical resistance meter taken from a timber skirting and plastered walling immediately adjacent, should be noted as:

Skirting 16% mc, wall adj R/R 22 (where R/R stands for 'relative reading').

Unfortunately, most meters used for house surveying have just 1 scale, and in many instruments the percentage symbol is still there on the screen or dial to confuse us all. It must be ignored. Any reference by a resistance meter to % mc in materials other than timber is incorrect. To make matters more confusing, some manufacturers give readings on that one scale a different name – wood moisture equivalent (WME) readings (see box below). For the sake of clarity, we refer to these readings as 'relative readings' or R/R, and that is how such readings are referred to henceforth in this text.

Moisture meters give a more reliable indication of relative moisture content at the lower end of the scale, as they tend to register 100 R/R when materials are potentially very damp. If, for example, a reading of 28 R/R was recorded to a softish plaster, you might find that just pushing the pins further into the material would cause the instrument to record progressively higher readings, and often escalating to 100 R/R. Note also that some instruments do not record any reading below around 8%, so the instrument registers a nil reading below the 8–28% range, due to the way the instrument is calibrated or the difficulty it has in detecting low moisture content.

Wood moisture equivalent Protimeter divides up the moisture meter scale as follows: green zone (below 17%, 'DRY') – no risks to building yellow zone (between 17–20%, 'AT RISK') – moisture level probably a little higher than we would expect in a well-constructed and properly-used building. If such readings persist or increase action may be needed to reduce the threat of material degradation. red zone (20% or more, 'WET') – a serious moisture condition may exist. Further investigation needed. Wood moisture equivalent (WME), a principle developed by Protimeter, relates to the relationship that may exist between different materials in close contact with each other in a state of moisture equilibrium. Protimeter predicts that materials in close contact, such as masonry, plaster and timber should show the same reading on the meter's scale, in spite of differences in the physical characteristics of these materials. The reasoning behind this idea follows from the way in which different materials may obtain the same equilibrium relative humidity (ERH) even if their individual moisture contents may vary. However, tests run by the author (unpublished) produced conflicting evidence: We kept samples of brick, plaster and timber in close contact in the controlled humidity chamber (at 75% RH) in the physics laboratory of South Bank University for over 4 months. Changes in weight and moisture meter readings of each sample were graphed over that time. Each material began at an 'office air-dry' state. During the initial 28 days, moisture meter readings did clearly converge, but after that readings for all 3 materials showed a gradual parallel reduction until they became constant, when the weight of each material had stabilised. After 4 months readings changed as follows: timber from 14.6% to 12.9%; brick from nil reading to R/R 9.7; plaster from R/R 10.1 to R/R 11.5; and PFA lightweight block from R/R 15.5 to R/R 19.1 (the PFA block recorded a reading of R/R 12.2 even when oven-dry, because it contained carbonaceous material). We also analysed moisture meter readings in the dry zones of 3 properties (18th, 19th and 20th century examples), where readings were taken to skirting and adjacent plaster for comparison. The readings were found to be different for timber and plaster in equilibrium. Despite these results, the value of using a moisture meter in either timber or masonry materials cannot be underestimated, as this text goes on to demonstrate, if and only if, you come to appreciate exactly what the readings in various materials mean. We term readings in material other than timber 'relative readings'.

Using a resistance meter

You need to be methodical in your use of this instrument to get the best results from it. If you find readings in masonry or timber above the perceived threshold, investigate the potential problem further.

Like many surveying instruments, the resistance meter is not difficult to operate, but requires considerable experience in order to interpret the readings obtained. In materials other than timber, for example, you need a certain familiarity with the typical relative readings you might expect for the material being investigated as discussed above. You need experience, and sometimes imagination, to analyse patterns of readings; and insight to know whether readings might have been influenced by salts, metals or carbonaceous materials (see picture below).

Figure 3: The moisture meter reads 35.5 R/R in the clinker building block, which is known to be 'air dry'; the carbon in the material is exaggerating the true moisture content

If you find high readings, check that salts are not present using a salts detector, and check that the meter is not being swayed by metals or carbonaceous material rather than moisture. A simple metal detector may be useful here, and in any case is always worth taking to site to locate hidden or buried pipes and cables. (Do not rely on cheaper DIY metal detectors – refer to your survey instrument supplier for reliable products.)

Manufacturers recommend 'triangulation' in dampness investigations where you bring together various pieces of evidence to make a meaningful diagnosis. Here are some additional points to note:

• Seek permission to use pin probes. If necessary, you can remove the moisture meter's pins and press the studs against a surface, but this may produce less accurate readings. Any restrictions in use must be stated in your report, and the implications clearly stated. (A capacitance meter may be useful if permission is not granted.)
• Check calibration before use. Most resistance meters can be tested against a calibration checking device.
• Sketch the floor plan, showing information regarding drain location, downpipes and gullies, water mains, etc.
• Try to find low benchmark or 'control' readings in walls and skirtings in a 'dry zone', i.e. to find the lowest reading you can for each material. These readings provide you with a marker for the house.
• Establish an efficient work pattern. You may be faced with a huge house and a tiny resistance meter, so a sensible approach where time is used efficiently is needed.
• Push the pins into surfaces using consistent force or you will not get the benefit of reliable relative readings.
• Use the probes regularly at skirtings and often just above the skirtings. If appropriate, take readings at intervals laterally along a wall and vertically up the height of the wall. Work to a pattern in each room or space. This speeds up surveying and reduces the risk of missing potential problems. Take readings around external openings, or where you have noted potential penetrating damp positions from your initial walk round. Take readings where materials appear irregular in surface finish, or marked or stained or damaged in any way.
• Where you find a 'damp zone', make a more systematic investigation by placing a tape measure along the floor, so that you can log readings in walls or skirting at regular intervals. CIRIA advises that measurements by resistance meter should be taken at metre intervals on skirting boards and more frequently if there appears to be a damp problem.

CIRIA also recommends the plotting of vertical readings. The methodical plotting of moisture meter readings on a grid is also described and illustrated in the Invisible damp case study.

You will not be able to use the pin probes of a moisture meter on a tiled surface, and instead have the following options:

• Inform the client that moisture meter readings could not be obtained where surfaces are tiled.
• Record capacitance meter readings instead (with caution; see 'Dampus metallicus' - foiled again).
• Recommend further investigation if necessary. (This might include removing strategic tiles for sample drilling and testing for moisture content by carbide meter, with the tiles then carefully re-fixed.)
• Electrical resistance and capacitance meters are very useful as DRY meters in conventional masonry walls with internal plaster and render. If these meters record a low reading indicating that the wall is dry, this is more reliable as there is unlikely to be any conductive material present to trigger a high (false) reading or any dampness. Therefore they can be used as dry meters!