Exploring dampness

There may be no problem for a building or any users if, for instance, a brick at the wall face has 10% moisture content after rain and then dries out later. However, problems could result if a similar brick, located in the inner thickness of the wall (and much nearer to the habitable space) developed a persistent moisture content of above, say, 3%. Symptoms could include damp plasters, damaged decoration, damp timbers from contact with the masonry (leading to timber rot), etc.

Is this 'significant dampness'? This is the first question in any dampness investigation. The second question is: where has the moisture come from? 'Significant' dampness exists where there is enough moisture in a material to cause problems for the material (e.g. material degradation), adjacent materials, the building, an adjacent building, the users of the building, the client, or other people.

But what is 'significant'? To know this you must first establish thresholds for the moisture content of the key materials within the subject property. If the moisture content of a material is found to be above the threshold, then you will be alerted to an actual or potential risk in the property.

Typical thresholds for building materials are often based on established guidance in research reports or textbooks. Some textbooks set out typical values of moisture content for materials when 'air dry', 'damp', 'wet', 'very wet' or 'saturated'. Although such general guidance is useful in principle, it may prove problematic when you are faced with atypical building materials in a survey. And 'typical' thresholds are not property-specific.

Some organisations that carry out dampness investigation reports generally set in stone the thresholds at the beginning. (Typical percentages quoted are 4% or 5% for bricks.) But setting such a threshold could mean that the surveyor, or the client, fails to appreciate the fact that the bricks at the subject property could actually be saturated at a seemingly low moisture content.

A brick that has been oven-dried in a laboratory will have a total moisture content (TMC) of 0%. If the brick were then located in an inner house wall in the UK, and that wall were not subject to any extraneous moisture source other than the air at normal conditions of humidity (i.e. air dry), the brick would probably have a moisture content of, say, 0.5%. The small amount of moisture in the brick would be due to hygroscopic moisture content (HMC). If the brick were subjected to a source of moisture other than the normal amount of moisture found in the air, its moisture content would inevitably increase. At some point (typically between 3% and 5%) the dampness could be considered a problem.

But materials vary in the amount of moisture they could potentially contain. A dense engineering brick could be 'very wet' at a moisture content of only 2%, whereas a more porous fletton would only be thought 'damp' at the same percentage. Also, bricks containing more than the typical air-dry 0.5% moisture content could either be getting wetter or be getting drier. So the condition of the wall needs to be monitored.

Some publications link moisture content with a source (e.g. if a brick has a moisture content of 5%, is it rising dampness?). Logically, you would first need to find out how much moisture the brick could potentially hold, and then assess whether 5% represents 'significant dampness' – which it probably would. Then you would pursue further investigations in an attempt to find out where the moisture had come from. This is the kind of logical thinking that is needed for effective diagnosis.

Property-specific thresholds can be determined by a little additional research. If you know the masonry material(s) used in the property (e.g. sand-faced fletton; colour dark russet; manufactured by Bedlam Brick Company) you could refer to the manufacturer's data on the brick used. Knowing the maximum absorption potential of that material would give you an idea of the likely HMC of such a brick, so that you could set a threshold moisture content for your survey. If you have no such data you could test samples of the material to find out how much moisture it could potentially hold.

As described in the case studies section, you can drill out and test a control sample from a dry part of the building, and the moisture content you determine (done most easily with a carbide meter) would be indicative of the material's HMC, since in a dry part of the property there would be no other moisture source present. See Damp detective (Building control journal, October/November 2016, p. 28) for more information.

Table 1 shows how a fletton brick may go through changes in condition from oven-dry to complete saturation.

Moisture content (%)	Condition	Comments
0	oven dry	only found in laboratory conditions
0.5	air dry	typically found in a 'dry zone' (below RH 75%)
2	damp or slightly damp	some moisture present that should not be there: monitoring advisable; further investigation may be needed
3	'significantly damp'	source of moisture needs to be traced (is the dampness progressive or regressive?)
8	very damp	probably damp to the touch; source of moisture needs to be traced; remedy required
24	saturated	visibly wet (obtainable in laboratory conditions)

Table 1: Fletton brick changes

A brick’s absorption characteristic, expressed as the potential moisture content (PMC), is usually contained in the manufacturer’s literature. Engineering bricks must comply with British standards. For example, the previous British Standard 3921 stated that Class A bricks must have a maximum of 4.5% water absorption, and Class B bricks a maximum of 7.0%. Bricks tested to BS 3921 were boiled for 5 hours to test their capacity for absorbing water.

Mainstream texts contain little information for surveyors on the absorption capacities of mortars and plasters, although these materials are regularly tested in laboratories. Property investigators seem to assume either that bricks are usually more dense materials than mortars, or that mortars are generally more porous than bricks. In reality, both materials can have similar moisture absorption capabilities. However, it is more difficult to test mortars and plasters than bricks because they are often friable and may break up even when soaked in cold water. (They would suffer even more degradation if boiled for 5 hours.) Oven-drying mortars and plasters also requires considerable care, because heating materials such as gypsum to over 45°C induces chemical changes in the materials and therefore distorts the calculations on water absorption. Assessing the moisture absorption potential of mortars is also difficult because they will generally have been hand-mixed and can vary from brick course to brick course.

Ascertaining moisture content in timbers is usually less of a problem, as most common moisture meters are calibrated for this material (see Measuring moisture below). Manufacturers of moisture meters even publish correction data so that surveyors can adapt their readings to allow for a range of timber species. But there are 2 main problems: knowing which species is under investigation and, for larger timbers, knowing the moisture content deeper within the material. Even standard skirting boards will vary in moisture content from the visible front to the concealed back of the board, and especially where the skirting is in close contact with damp plaster. Standard softwood sections can contain up to 30% of their dry weight in moisture.

It is not practical for surveyors to undertake laboratory-style tests of every brick they drill in a survey, but knowing the absorption parameters for common materials would help in most investigations. See table 1 in Drilling for samples for the absorption potential of some common materials. Using the controlled ovens, sensitive calibrated laboratory weighing equipment and the dedicated humidity chamber of the physics laboratory at South Bank University to test a few common bricks, plasters, mortars and softwood samples – and the results are very revealing.

Cement-based mortars, for example, were found to be far more absorbent than figures quoted elsewhere. Even plasters seemed to be far more able to take in water than expected. Some Carlite Bonding samples caused the moisture meter to indicate a relative reading of 8–10, even when the material had been oven-dried!

It was not surprising, however, that an oven-dried sample of a pulverised fuel ash (PFA) lightweight block would cause the moisture meter to react positively, because PFA includes carbonaceous material, which is conductive and therefore sways the readings (see Measuring moisture below).

In the attempt to establish percentage data for HMC at 75% RH, the tests certainly showed that the denser solid materials are extremely reluctant to take in the air moisture inside a 75% humidity chamber – even after several months of testing.