Surveying equipment and tests

Detecting salts

If salts – nitrates, chlorides and sometimes sulphates – are present in the materials being investigated, they magnify relative readings from moisture meters, making dampness appear more of a problem than it actually is. Use salts detectors whenever you record high relative readings because salts are not always visible at the surface where readings are taken.

Example Salts detectors are simple to use. One instrument tested during the preparation of this section required us to soak a piece of absorbent paper with de-ionised water so that we could first test the conductivity between metal studs on the detector unit. Then we had to undertake a second check of conductivity by placing onto the studs the same paper after it had been pressed against the wall to soak in any salts present. We then had to subtract the 2 readings obtained to assess whether salts were present that might affect relative readings from the moisture meter. (You could do a similar test using moistened blotting paper held across the pins of a digital resistance meter.)

A salts detector identifies the presence of salts, but it does not tell you the kind of salts present.

If hygroscopic salts are present, you can test them to find out what they are and at what concentration. This gives you valuable clues about the moisture source. And if you know that hygroscopic salts are present, you will realise that the wall may be difficult to dry out because the salts attract moisture from the air, whether or not there remains another moisture source.

Using a salts testing kit

A salts analysis kit is relatively simple to use and its results are useful but, according to one of the mainstream manufacturers, surprisingly few house surveyors use them. The kits are intended for site use, and it only takes minutes to carry out the test. You can then follow the diagnostic trail of evidence armed with useful insight into the possible origin of the moisture that may be causing a dampness problem.

Figure 1: You need distilled water, test tablets, clean containers, salt samples and a measuring spoon

Getting to grips with salts tests is another case of 'learning by doing'. Once you have observed a positive nitrates test from tap water, you begin to challenge the long-held myth that positive nitrates results are necessarily closely linked to 'rising damp'.

Undertaking a salts test involves some relatively minor destructive testing where, if you have permission, you scrape plaster or wallpaper samples from a small area. There should be little objection to scraping wallpapers that are already salt-damaged. You could also test drilled samples. For a test for nitrates and chlorides you need around 2.5ml of material.

The tests are mainly 'colourimetric'; that is, tablets are added to the sample in a distilled water solution to create a colour change (chlorides produce a yellow result, and nitrates a red). Break tablets up to speed up the reactions. The intensity of the colour produced in the test indicates the concentration of the salt in the sample. The concentration can be approximately assessed by comparing the test solution with a colour chart, or sometimes you can calculate the approximate concentration based on the number of test tablets you have added to the sample solution. Such field testing is not thought to be highly accurate at low concentrations of salt.

Remember to test a sample from at least 1 position well away from the area of salt damage, from the same wallpaper and plaster type as a control.

There are 2 potential problems in testing salts:

• the tests can take quite some time if colour changes do not readily occur; and
• when a colour change occurs, interpreting its intensity requires experience or a colour chart.

It is also possible to make mistakes when adding the various staged tablets. For these reasons you may prefer to conduct a second test at the office to confirm your site result.

Of course there may be occasions when a site test is particularly useful: the test result can focus further investigation on a problem while you are still on site. Early elimination of chlorides and nitrates can help focus the survey to likely sources of moisture.

One of the main difficulties in conducting salts tests lies in the interpretation of the colour changes. The yellow coloration of a chlorides test is not always easy to identify, whereas the cherry red expected for a positive nitrates test is obvious for all to see.

Site testing for sulphates is not as straightforward. In one particular site kit tested, a solution of the sampled salt placed into de-ionised water needed to be quite clear in order for the second phase of the test to be successful (where tablets had to be added, and a black dot viewed through a sliding chamber). To achieve the clear solution the test solution had to be left standing for some considerable time.

Interpreting the results

Surveyors normally test for 2 types of salts – nitrates and chlorides – because these salts have traditionally been used as evidence that the moisture has come from the ground (see figure 2). Some kits also include a test for sulphates. However, a number of writers and researchers have alerted us to the fact that these salts may originate from various sources, not just the ground. Be cautious in how you interpret salts tests. If you wish to draw conclusions on the origins of moisture from a confirmation of salts at the surface of a material, you should research what the sources might be, what their chemical constituents might include, and the nature of our building materials. This might include researching the chemical make-up of:

• tap water;
• groundwater;
• waste or soil water;
• garden soil-sourced moisture;
• surface water; or
• any other water.

Figure 2: A positive result for nitrates and chlorides

Example In most parts of the country quite significant concentrations of chlorides and nitrates are actually found in tap water. If such salts were confirmed at or near a wall's surface it would be easy to jump to the conclusion that the moisture was 'rising damp'. In London nitrate concentrations are around 30mg/litre (30 parts per million). In intensely farmed regions the concentration might be higher still. Chlorides are permissible in tap water at up to 250 mg/litre. (Permissible limits of a range of water constituents are set by law all over the UK.) In other words, the more we find out about salts in buildings, the weaker the diagnostic link between salts and rising damp becomes.

The risk of associating the presence of nitrates and chlorides too exclusively with rising damp can be illustrated very clearly.

At the Built Environment Faculty of South Bank University in south London salt deposits on the stairwell walls tested positive for nitrate and chloride using a standard surveyor's testing kit. The positive result for both salts could not have been in any way indicative of 'rising damp' because the salts were sampled and tested up at fifth floor landing level. Salts at the surface had most likely leached through construction materials by lateral rain penetration due to faulty cladding joints.

Lewisham's Property Investigation Unit found that positive chloride and nitrate results are often indicative of a penetrating damp source where earth has been banked up against house walls, enabling salts contained within it to be transferred into the base of the house walls. In such a case, a less experienced surveyor could have made a false diagnosis of rising damp, leading to an unnecessary dpc injection.

Also remember that salts may build up over time, and that confirmation of moisture from one particular source does not mean that moisture from another source is not also present. If moisture from more than one source is present it may be difficult to establish which the lead source is. Salts in a wall or its finishes could also be evidence of a past dampness problem. Think of these sources as a number of water taps: you need to know which tap or taps need to be turned off to cure the damp problem (see The 334-day wall watch case study and the Buried hearth, buried pipes, buried truth case study).

Sources of nitrates, chlorides and sulphates in building materials, include:

• nitrates – from groundwater, tap water, other plumbing leaks or garden soil; sometimes sea water, flue condensation, urine contamination, some bricks, manures and fertilisers.
• chlorides – from groundwater, tap water, other plumbing leaks, accelerators in concrete (calcium chloride) and de-ice salts; also sometimes sea water, flue condensation, urine contamination, and some bricks.
• sulphates – from many building bricks and plasters and may be present in the ground.

Of course tap water is present in a range of pipework types – waste, foul, hot and cold service pipework and water mains – so a positive nitrates or chloride result from plasters or wallpaper in a property could easily be caused by water mains leaks, WC overflows, leaking taps and so on.

Remedial treatment companies occasionally appear to misinterpret the range of sources of nitrates and chlorides. One leading chemicals supplier advises that nitrates are normally indicative of rising damp and 'possibly from foul water, fertilisers, manures, etc.' and that chlorides normally indicate rising damp 'possibly from unwashed sand. Common in properties located close to the sea'.

There are only 4 possible results from the standard nitrates and chlorides test, as shown in table 1.

Table 1: interpreting salt tests

Nitrates + Chloride +	May be moisture from the ground, tap water, other plumbing leaks or garden soil	Further investigation required
Nitrates + Chlorides -	A rare result; moisture from the ground is likely; tap water unlikely; possibly an organic source
Nitrates - Chlorides +	Tap water possible; ground-sourced moisture very unlikely	This result may be obtained in a coastal property subject to salt spray
Nitrates - Chlorides -	Condensation could be the cause; penetrating dampness (rainwater) likely; probably not ground source or tap	Central heating leakage may need to be investigated

Salts are not restricted to wall plasters or wallpapers. It is not uncommon to find positive chlorides or nitrates results where samples have been taken from deep within walls subjected to soaking from water main leaks and subsequent repair to the mains has solved the dampness problem.

Longer term water mains leaks can cause a considerable build-up of both nitrates and chlorides. The 'symptom' of such a leak may be unusually high water bills. Laboratory tests of masonry samples that reveal the presence of detergents etc. have often enabled surveyors to track down the source of moisture to leaking waste pipes. (Detection of compounds other than chlorides, nitrates and sulphates requires tests in specialist laboratories.)

One supplier of tests kits says that 'condensation will not result in salts depositions'. This is true according to the laws of physics (the condensate will be pure water). Lack of any salts might therefore lead us to consider condensation as the most likely cause of a dampness problem. Most textbooks usually state that mould spores (of the type associated with condensation) do not germinate in salty environments. But mould growth has been seen on walls soaked persistently by tap water, which does contain nitrates and chlorides. And in experiments conducted at South Bank University, condensation-associated moulds developed on sample panels 'contaminated' with surface depositions of nitrates and chlorides. In this way a wall can exhibit mould development even if it is suffering from salts deposition, but the mould growth will probably tend to avoid areas of the wall plaster showing a greater concentration of salts.

Very positive sulphates results (often more than 200 mg/l) can be due to a wall suffering from significant dampness from a range of sources. Confirmation of sulphates alone could be indicative of a penetrating dampness source (e.g. rainwater). However, you must not forget that masonry materials themselves can contain sulphates.

Negative salt test results are, in many ways, more useful because more sources may be discounted. A negative chloride and nitrates test result might indicate moisture is either sourced from condensation or penetrating dampness via a rainwater plumbing leak or direct rainwater.