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Thursday, 21 April 2016

Beware the "Damp Meter"

This is the tool of choice for damp proofing companies
The Protimeter is pretty much ubiquitous in the damp and building surveying world. The meter is used to show owners how damp their homes are. This is also invariably 'rising' damp that 'will require' an injected DPC. However, is this tool an actual damp meter?

The short answer is NO!

These meters measure electrical resistance between the two prongs. That is not necessarily the same as the amount of water between them. Originally they were developed for timber to check how dry it was. In timber the only real thing that will conduct electricity is water and hence they were quite useful. However, this is not the case in masonry.

So what else could they be measuring?

Well, there is another factor that can give a high readings on a protimeter:

Carbon content. As you may be aware some old mortars were made using dust from industrial waste. This is especially true in areas like Cardiff where we have black mortar. This mortar is high in carbon and hence conducts electricity well. So high readings may well be to do with the mortar, not moisture.

So, if you have a high reading in a house without old mortar / plaster, then a high reading is probably indicative of dampness. However, the key question is then; What is causing the Damp?

Well this opens up a range of options (however, the majority of the damp industry only looks for 'moisture' along the base of ground floor walls).

The damp could be caused by:

Hygroscopic salts. These are actually very common in walls and are often the cause of high readings. Salts can be introduced by the building process, water ingress and from the combustion processes associated with fireplaces. So if you have high readings around the base of the fire place / hearth this may well be salts not rising damp. The salts are 'hygroscopic' and that means that they attract moisture to them. So this moisture might be linked to condensation, being located in high humidity areas like kitchens / bathrooms, etc. So the moisture is probably associated with salt not 'rising damp'. So just keep dusting off the salt residues until they disappear and lo and behold you will have cured the damp. Simple.

Condensation. Virtually all external walls have a cold bridge along their base and this attracts moisture to condense there. This is natural and nothing really to worry about unless it is causing some mould issues. This condensation can also be caused by a lack of ventilation. So again you might need to look at ventilation / insulation rather than jump straight into a potentially unnecessary damp proof course. Note that thermal bridging can occur in a number of other places (ceiling edges, around window frames etc) where the wall is cold for some reason so again check issues like insulation cover and building defects.

Leaks. Old or current leaks can show up on the meters, so this is a case of fixing the cause, or just allowing the wall to dry naturally.

Material incompatibility. Old houses have walls that allow moisture to pass through them, however most modern materials are designed to resist moisture movement. Commonly you get a combination of materials that don't like each other in terms of moisture and a lot of problems stem from this. Gypsum plaster is particularly prone to becoming hygroscopic when it has been placed over old lime plaster.

Penetrating damp. This is the likely cause of most damp issues. The industry tends to recommend injecting more DPCs into the wall rather than addressing the actual problem. This tends to be simple and cheap maintenance issues like sealing around windows, cracked render, repointing etc.

So if you see a 'damp' meter, make sure it is attached to something that has an inquisitive brain and also nothing to sell you!!


  1. I live in an 1880s terraced house in Cardiff, and have experienced a peculiar issue since removing an internal door in a hallway.

    We had one of the walls that the door was attached to skimmed (with gypsum - having since learned that gypsum is not good for these houses!), and ever since the shape of where the door frame used to be is marked on the wall by a line of hygroscopic salts with a large damp patch at the bottom, which seems to get wetter/drier with changes in humidity.

    We had no dampness problems in this area prior to undertaking this work, so could it be that the render and plaster added to where the frame used to be is drawing salts out? The mortar in the internal walls is black ash.

    Any thoughts would be useful, as most builders don't appear to know what they are on about and will just want to hack it all off and repeat the problem!

  2. Hi Simon, quite often this was the area that was damp as the water could be drawn up behind the wooden frame, so it sounds as if there was a residual damp issue behind the wood and when this was removed and just skimmed over the salts / dampness had not had time to dry out, hence the salts have been pulled through the gypsum and are now just attracting water to the surface during periods of high humidity and then releasing the water when it is drier. You can just dust off the salts when dry and keep doing this until they have all reached the surface and been removed. Otherwise it would be a case of removing the salt affected gypsum, leave it to dry out, removing the salts from the walls as this process occurs and then once dry you can apply a new covering (gypsum or lime). There are salt resistant options out there as well. So sounds a bit like the job was a little rushed (no surprise there) and that the latent salts where not removed prior to replacement of plaster. This probably could not have been foreseen, so not really a mistake per se. Cheers, Peter