So, the meter measures conductivity. The more moisture, the greater the conductivity.

By taking measurements across a wall, they'll see a pattern emerge. If, for example, it's just concentrated in one finite area in the middle of the wall, and measurements tail off as you move away from that spot in all directions, then it's highly likely be penetrating damp.

If, on the other hand, measurements are highest at the bottom of the wall, and reduce as you go up the wall, then it's most likely to be rising damp.

They aren't going to be looking as to whether someone has been spraying the inside of the wall with water. Because they aren't looking for the equivalent of sabotage or vandalism.

Thank you for coming back. It is now clear to me - much appreciated because with practical things my knowledge is limited. Pity the school system never gave a full education.

There are 'moisture meters' which are calibrated and intended for use on timber: these will almost always indicate that a wall is 'damp', which is why they are a favourite tool of some surveyors who then advise various kinds of costly remedy (e.g. the installation of ceramic tubes in the wall, though these have been proved ineffective years ago.).

So when the meters probes are pressed into wood, the meter will express a quantitative reading. In effect if 12% is displayed on the meter’s display, it’s right thereabouts. It’s 12% Wood Moisture Content.

However, stick the meter is anything else; plaster, brick, stone or mortar and the reading is only qualitative – if the meter says 12% – you can discount that completely; the true moisture content of the material may be higher or lower.

This is because the meter cannot be calibrated to account for the variable nature of masonry.

Exact moisture percentage can be measured by drilling the plaster, weighing the sample, drying it by heating and then re-weighing.

NB, your surveyor may have a meter which does measure accurately, if technology has advanced in recent years.

If you approach the surveyor and ask for more details I am sure he would be glad to explain. What has been said already is correct. If it is a solid wall construction then penetrating damp can be cured by repointing/rendering. Sometimes a severe rain storm with wind can blow against the wall and make it quite damp for some time until it evaporates. Usually this means the pointing/brickwork is porous and needs attention.

If it is a cavity wall the surveyor may possess a boroscope. He can drill through the outer leaf in the damp area, insert the boroscope and should be able to see the problem. Usually wall ties conveying moisture across the cavity to the inner leaf. Then some brickwork can be removed, the problem rectified and the bricks cemented back into place. If done carefully you would hardly notice after a few weeks. This is most likely the cause if it is a plain flat wall with no penetrations, downpipes or water services concealed in it.

Cheers Concrete

Thanks. Does it make much difference of it is cement or lime mortar used for pointing ?

Lime mortar would be an unusual choice for standard brick construction. It is mainly used on restoration projects where older buildings are more likely to suffer movement. I would not recommend it on an exterior brick leaf of fairly modern construction and it would probably be rejected by Building Control too. Standard pointing mortar is the ideal material. Very strong and weather resistant and can me mixed with water resisting additives to help repel water. Cheers Concrete

Sorry. Should have said it's Victorian stone building.

"...the water could 'track' through the fabric of the building until it reaches the internal surface some distance away and becomes visible."

That last bit is important - the external point of water penetration could be a long way from where you're aware of it.

These posts are very valuable for me. It is a complicated situation outside what you would look for on a forum. Historical, legal and personal issues. I appreciate that posters have given me knowledge on this particular aspects. Many thanks all.

"We had similar liners installed in our current 1960's house which was built with concrete roof gutters."

Oh, that reminds me of a house I once was involved with - a major renovation. Concrete gutters - the spawn of the devil. What a stupid (and thankfully short-lived) feature of fifties and sixties construction. Some of them were even partially supported by top-floor window frames, so replacing windows could run into serious problems.

There are specialist firms that remove them (expensive, involving scaffolding and reconstruction of the lower roof), but extruded liners (cheaper, but not cheap) are the best solution.

Edited by FP on 08/12/2017 at 14:35

Thanks again, you're a star. Before long I might start to know what I'm talking about.