Monitoring moisture condition

Figure 1: What you might need to drill out a sample and carbide test it. The metal detector at the bottom right really is a key piece of equipment, and helps steer you away from risky drill sites. But assess also where the pipes and cables may run

If you do not feel confident taking a drilled sample, arrange further investigation by others. Do not drill where a basement is tanked, or where you might damage guaranteed plasters or sensitive finishes that cannot be made good. Be particularly careful when contemplating drilling a solid floor – and carry decorator’s filler (and a dust pan to clear up with) in your equipment bag.

The type of drill shown may need to be supplemented by a more powerful corded drill with extension cable.

If you only manage to drill out a small sample (less than 3 grams), you need to weigh the sample using sensitive scales and oven dry the sample to determine actual moisture content.

Where cables or pipes may be present, it is a little risky to drill a hole in a wall or floor. However, it is so often a great way to find out exactly how damp or dry materials are at depth. The method of obtaining a sample and the carbide testing are explained in Surveying equipment and tests.

You can only drill a particular hole out once, so you cannot carry out a repeat moisture content test in exactly the same place – just nearby. Therefore, repeated monitoring checks are not so easy if you plan to extract material by drilling. The extraction of material for carbide testing does not always involve drilling – samples can be gouged out, cut out, broken out or even coaxed out.

The author has often been called upon to check out the drying out of buildings after a flood. Taking a carbide meter reading in such a survey is an invaluable activity – you can finally convince a client or contractor that the subject wall or floor is still wet (or acceptably dry). This test is really the ‘gold standard’ of site tests.

Use the drill hole to obtain an ERH reading by inserting a thermo-hygrometer.

Do not be misled by those who attempt to over-complicate the interpretation of carbide meter readings. All you need to be aware of is that in damp air (and it always will be damp in Britain), porous materials will always contain a very small amount of air-sourced moisture. So when you find a sample of material from a wall or floor to be very damp (say 5% TMC from a carbide test) a little of that moisture would probably be there anyway from the air, e.g. hygroscopic moisture: this might be just 0.5% (or 1/10th of the TMC).

Carbide tests are useful to determine whether a wall has been penetrated from an external source and as a double check at key stages in the monitoring, rather than being the ongoing monitoring method.