Surveying equipment and tests

Sampling is the extraction of (usually) masonry material from within the wall, usually by cold drilling (to between 10mm and 80mm depth). The sample can be tested to determine its total moisture content. (Testing using a carbide meter can be done either on or offsite.) Other tests (e.g. for salts) can also be carried out on the extracted material. You can also use the drill holes for taking equilibrium relative humidity (ERH) readings.

As well as uncovering details about the wall's moisture content at depth, drilling for samples provides an accurate record of moisture content on the actual day of sampling, so you have an opportunity to revisit the property at a later date to monitor moisture levels, if needed. If you plan to monitor moisture conditions in the wall over time, you might enlarge the drill hole to 16mm (the size of the standard plastic plugs) and insert sleeved humidity sensors inside.

If you are not allowed to use resistance meter pin probes, then the owner/occupier probably will not want you drilling holes with an electric drill. In cases where drilling is allowed, you must hope to be able to drill in the places where you can learn the most.

Many more house owners would probably allow you to drill for masonry samples if they fully understood the reasons for sampling masonry within a wall's thickness. But there are some instances where hole drilling might cause damage to a plaster system, for example if there is basement tanking or other waterproof plastering, where guarantee conditions could be compromised. There are also problems associated with drilling concrete floors (there is a risk of puncturing a floor dpm) in view of the length of the humidity sensor in relation to the thickness of a floor screed. In addition, electric cables and other pipes are often routed in screeds. Vertical and lateral sampling of walls is usually far less of a risk, except when there is a vertical dpc, dpm or specialist re-plastering. (In most cases it is easier to predict the location of dpcs, pipes, and cables by careful site reconnaissance before the drilling begins.)

You can choose where to sample: you can drill bricks, mortars, you can take plaster samples, or you can do all three. There is a lively ongoing debate about the relative merits of sampling bricks or mortars. There is no universal strategy: you must make the best decision for each building you face. But a clear advantage of drilling and testing bricks is that they are generally more reliable to test. Mortar varies virtually with each mix due to the way it is often hand-mixed on sites. But if you record a low moisture content in a dense brick, you ought to test the mortars too, because moisture may be soaking through the masonry via the joints. BRE Digest 245 states that

'the moisture content of the two components in the wall [mortar and bricks or blocks] will generally be different and it is the higher figure that must be considered'.

In hard stone or flint walls you have no option but to test mortars because the drill bit will not get very far into such hard materials. But bear in mind that the walls of old buildings, which may have been altered and repaired during the course of their lives, could contain mixtures of materials – for example, the odd rogue softer or harder brick could be a different type to those in other parts of the wall (figure 1).

Figure 1: Beware: the wall could be an unbelievable concoction of materials. Always cut a square of plaster so that you can identify what lies below

Before you begin to drill you need to know the size of sample required. The more tests you plan to carry out on each sample, the larger the samples you need. Careful practitioners also use a sharp bolster to cut out a square of plaster (at least 65 × 65mm) before drilling to confirm the substrate. Take wallpaper or plaster scrapings too and sampling positions logged for onsite salts analysis.

Sampling involves using a sharp 10mm masonry bit to drill to a pre-determined depth depending on wall thickness. For a 225mm thick wall a 75mm depth of sampling is sensible. You need a sample beyond the condensation zone. In most cases you will require a 3/4-full film case of material. To obtain a sample of this volume it is sometimes necessary to double drill the hole – first using a 10mm drill bit and second a larger 16mm bit. If a sufficient sample is drilled out, there is plenty of material remaining after the onsite carbide test for further testing back at the office, or occasionally for sending to labs for further analysis. The sample is usually already in powder form because of the drilling, but sometimes it might need to be ground down further with a mortar and pestle to produce material of suitable particle size for reliable carbide testing.

• Fine powders begin to dry out quickly, so there must be no delay in pouring samples into resealable plastic cases with labels already attached.
• During drilling, check the temperature of the drill bit regularly by careful touching it with hand or finger. If the drill bit becomes hot to the touch you cannot rely on the result of the carbide test because moisture may have been driven out of the sample by the hot drill bit.
• The first 10mm of drillings should be discarded because it will be in the condensation zone. Also, proceed with caution if drilling and testing in humid rooms, because the dust sample will take in moisture from the damp air. If possible, consider organising the testing in an adjacent drier room.

Buried hearth, buried pipes, buried truth also describes drilling and sampling techniques using the carbide meter.

Drilling for samples can be time consuming. If you were to drill 3 holes in a wall at various heights at 1 key position or 'station', and you needed to take samples of brick, mortar and plaster you would need 9 sample vessels. If you wanted to test each sample for salts content as well as moisture content you would have to undertake 18 tests for 1 wall position. This is quite daunting, and is probably the reason why specialist consultants either charge high fees or test just the bricks or the mortar.

You can obtain an insight into the wall's condition by just drilling 1 hole in the property. However, you would find out much more by drilling at intervals laterally in damp zones to pinpoint dampness in much the same way as you would check wall surfaces with a moisture meter to establish a pattern of readings. Where there is a large area of dampness extending upwards, consider drilling at intervals vertically, typically starting just above the skirting.

Coleman's Guide to identification of dampness in buildings (2nd edition, 1990) recommended that a control sample be taken close to ceiling height. Take detailed notes on the materials sampled and exactly where each sample was drilled. For example:

64 High Street, 2/2/02, Flank Wall, 2m from rear, 'A' on sketch plan, height 225mm, brick.

You can speed up the actual time spent on site by placing the brick, mortar or plaster samples in sealed containers so that you can test them in a more relaxed and careful manner offsite. However, whenever possible test some samples for moisture content using a carbide meter while onsite, so that you can pursue your investigations with the reliable information on moisture content from the carbide meter. It is then possible to make any follow-up site checks while you are still at the property.

After drilling, you may need to organise or carry out the repairing of drill holes, including re-fixing ceramic tiles, if you have drilled a bathroom wall.

The above data is subject to ongoing trials and research.