... come in all different shapes and sizes. Some are the traditional 'resisitive' ones, where you push sharp metal prongs into the material to be measured, others are the 'electrostatic' types, which create an electrical field in the material being measured and deduce the moisture content from that, and there are also ones that utilise radio frequencies. They all have their good and bad points but, of much more interest to property owners, are the people who use, or, in the main, misuse them.

 

I can't quite remember the exact law case but, in the 1960's or 70's, a judge in Wales opined that searching for moisture in a property, without the use of a moisture meter, was 'a bad thing'! Apparently, when a judge gives a judgement, the rest of the law industry take it as read and, unless it's appealed at a later date, it then becomes a precedent. So, ever since then, if you don't use a moisture meter when you're looking for dampness - you're a naughty person, and, if you get caught; well, it's the Tower for you! Fair enough! But unfortunately, a lot of people have taken that judgment literally, and a moisture meter is the only thing they use.
This applies particularly to a lot of chartered surveyors, who carry out building society surveys, and also to a lot of timber and damp contractors, who carry out goodness knows what kind of survey!

Let's go back to the basics: electronic [resistive] moisture meters were, and still are, designed to give a pretty good indication of the moisture content of timber. I'll say that again - timber! They're calibrated to do this and, in a range between 7%, up to about 30% moisture content, they're quite accurate and are relied upon by the timber industry in their day-to-day activities.

They can perform this task because the conductivity of timber is reasonably constant throughout the different species and is proportional to the moisture content. There are exceptions to the rules but these can be catered for by use of divergency tables. For those interested, a piece of Douglas Fir with moisture content of 7% will have a resistance of approximately 22,400 Megohms - yes Megohms! When it gets to 25% moisture content that comes down to around one half a meg.

But even one half a megohm is a pretty substantial resistance and even the slightest amount of moisture in a masonry wall will have a resistance a lot less than that. Consequently we can get full-scale deflection, in the shape of red lights and beeps even with only minor instances of moisture!

And that of course is the problem. No-one has worked out the conductivity/resistivity features of a masonry wall. You couldn't do; because there are variations in each one, caused by factors such as the molecular structure of the materials in the wall itself and even the salt content of the water used to build it!

Therefore: you cannot deduce the moisture content of a masonry wall by the sole use of a moisture meter! You can't even guess at it! Don't even try!

A moisture meter can indicate to you that moisture may be present in your wall but it can't tell you if that moisture is in sufficient quantities to require further consideration; and the lights and beeps are very often a Red Herring used by the unscrupulous to decieve customers into thinking they have significant problems.

There is one thing in its favour though: if the wall is actually dry, then it can tell you that quite truthfully; because if it gives a zero reading it means there aren't any water molecules available to allow conduction.

Don't run away with the idea that these meters are useless though; because used correctly they can give you a very good indication of what type of problem you have; and you can then deduce the scale of the problem by other means.

It's the associated deduction and observations that enable these machines to achieve the importance they have, and used correctly they are still the only way to carry out a quick and effective survey. But, and I stress again, they have to be used correctly; in the manner you'd perhaps use a road atlas - as a guide to where you're going. What you do when you get there is a different thing altogether.

Instead of just poking it into the base of a wall, a surveyor should be taking a lot of observations from different places in the walls and noting the pattern of readings produced. This can tell you what sort of a problem you actually have: rising dampness, penetrating dampness condensation or even metallic bits in the wallpaper itself! But, even if your meter displays patterns that are indicative of a certain kind of dampness profile; whether you do anything about it depends on the effects it's having on surrounding materials, such as wood or wall finishes.

That last statement may surprise some people. Surely, if something has moisture in it, it's damp and needs treatment? Not necessarily, most things have a degree of moisture in them. Look at an example: timbers, such as skirting boards etc in a centrally heated house can have moisture content up to 4 or 5 per cent less than those in a house without central heating. But that doesn't mean to say the timbers in an unheated house are in jeopardy, they just happen reside in a less dry atmosphere, and have therefore absorbed more moisture from the air. Unless their moisture content rises above 20%, they're not in any danger at all.

The individual moisture content of a host of materials: walls, skirtings, joists, etc, recorded by moisture meters, may be higher in some houses than in others; but, if it isn't causing any problems, and further investigations show it isn't likely to do so in the future, why indulge in expensive remedial treatments. Especially those that probably won't work anyway!

Similarly, there are lots of masonry walls that suffer from condensation problems, which can light all the lights and ring all the bells on the meter. But indications on a meter are often all there are; with nothing more to indicate that there's a dangerous degree of moisture present: no deterioration of wall surfaces; and, most importantly, no moisture content above danger point in any timbers in contact with it. As I say elsewhere on the site, there are thousands of houses in the UK that have had all sorts of painful treatments inflicted upon them merely as a result of high readings on a moisture meter!

Remember: 'Dampness' is present to a degree in lots of materials, and it isn't a problem until it causes problems: a high reading on a moisture meter doesn't alter that truism. Next time you see an 'expert' using one of these things, to the exclusion of common sense and visual observations, beware. Your house just might be in danger of drilling and filling for no better reason than a few over-excited ohms!