Damp management and remediation

When tackling dampness within occupied buildings, management of the problem or symptoms can lead to a cure of the dampness. For example, advice on the following can lead to a cure of the condensation problem at nil cost:

• minimising moisture production in the home;
• increasing ventilation;
• closing kitchen and bathroom doors during cooking, washing and bathing;
• increasing heat for longer periods and not below 15ºC;
• avoiding drying clothing, etc. over radiators or on airers in bedrooms and living rooms;
• avoiding use of liquid based fuels, i.e. calor gas;
• ensuring that tumble dryers are properly vented, if fitted with a hose that the appliance is vented directly to the outside air;
• pots and pans are covered with lids during cooking where it is safe to do so;
• having properly fitted covers over fish tanks; and
• regularly wiping off any excess moisture from surfaces around window areas.

Most importantly, that any escapes of water from internal plumbing or defects to the building that result in dampness penetration are reported or promptly investigated/remedied.

Remedies as part of diagnosis

Sometimes you will not be able to pin down a dampness problem to a single moisture source. Interpreting the visual evidence or the data you have collected can be difficult. Mainstream publications offer just a handful of diagrams to help you interpret the patterns of dampness associated with the 5 key moisture sources, and information about dampness sourced at the base of walls is particularly scarce. To make matters worse, 2-dimensional diagrams such as internal sketches of a room wall are rarely sufficient to help you to understand the complex situation you are faced with on site. For example, the 'typical rising damp pattern' (a tide-mark shown rising to a height of around 1m) could be the result of moisture from a number of sources, and not due to a failure of the existing dpc. There is a growing feeling among surveyors that rising damp rarely attains such a height; certainly it is not as common as was once thought.

Due to this lack of comprehensive guidance you have to rely on your own experience, recalling patterns of dampness you have come across over the years.

A house survey is just a snapshot view of the problem, and observations are not made over time. If you do establish increasing or decreasing concentrations of moisture at a wall base, it can then be difficult to confirm the actual direction the moisture has travelled. A long-standing leak can travel a considerable distance and could prompt you to recommend an over-reactive package of remedies. If there is a combination of sources, and sometimes even a combination of problems, you need to know which one is the lead source. Applying remedies can, in effect, be part of a diagnostic procedure.

Remedies must be applied in stages. In a dampness investigation, the first remedy might involve nothing more than basic maintenance: repair the leaking rainwater pipe; clear the blocked gully; reinstate the missing pointing. Then monitor the building's condition over time to assess whether the initial remedy reduces moisture to acceptable levels (see Invisible damp case study).

If basic maintenance does not solve the damp problem, more extensive work may be needed. The second stage of remedies might involve more substantial work; for example, reducing external ground levels, improving surface water drainage and so on.

The above example clearly shows how a combination of inspections and staged remedies can eventually lead to an understanding of causation. It also shows just how difficult it can be to track down dampness sources in an underground space. Jumping the gun by installing a new floor slab would not have addressed the root cause of the problem, and would only have masked the symptoms. Yet even this example is not clear-cut. The property owner was extremely disappointed at wasting money on the pond works, when the significant levels of coliforms, E Coli and faecal Streptococci in the sample tested ought to have alerted the laboratory staff to a potential foul drainage leak.

Do you need to retrofit a dpc?

People often jump to conclusions: they record or see moisture at the base of a building and assume 'rising damp' or failure of the dpc. Over years of investigating a huge list of properties with dampness problems, the author has never found a case of dampness that needed to be treated by the insertion of a chemical dpc. What was found was that many dampness problems are misdiagnosed and inappropriately treated.

If the Lewisham experience is symptomatic of what is likely to be found in towns and cities across the UK, then the waste of resources caused by poor building practice, misdiagnosis and ineffective remedial measures would run into millions of pounds.

Since the original publication of Diagnosing damp, Michael Parrett has been working with a number of leading social housing providers across the UK, extending the Lewisham experience to many other organisations. Work with Liverpool Housing Trust won the North West regional Construction Excellence Awards in the business and innovation category for the huge savings in their repairs budget. This was achieved through the ‘appliance of science’ training provided by the writer as to the methods of correct measurement and investigation of dampness, which has eliminated commercially driven solutions targeting their resources to the true causes of dampness. This had led to a decrease in complaints and a rise in customer satisfaction. The close monitoring of the team’s performance at LHT and the impact on budgets and performance have been hugely significant.

Since the work was undertaken in Lewisham, the savings across all of the organisations the writer has worked with has produced similar results of huge savings and increased customer satisfaction, none more so than with Barnet and Brent Councils. A significant and typical case study was published in the RICS Building surveying journal (January/February 2009):

The Society for the Protection of Ancient Buildings has run a number of seminars to address this question of retrofit dpcs in the context of historic building preservation. The seminars highlighted important principles of building behaviour that apply to a wide range of building ages and types. Most traditional masonry buildings are designed on the 'overcoat principle' where buildings are designed to breathe, taking in moisture and losing it by evaporation in a carefully balanced manner. Upset this equilibrium and the building's performance might suffer. Traditional repair remedies in keeping with the age and style of construction will usually solve dampness problems. If the building has an existing dpc, applying basic repairs could be all that is needed to bring the building back to original working order and solve the dampness problem.

Even if you do finally conclude that, in all probability, dampness is rising, you are still not in a position to recommend a retrofit dpc.

There are the 2 options described earlier: managing or curing the dampness. In the context of a below-ground source, curing the moisture at source might involve:

• reducing ground levels near the base of the wall to reduce water levels, and to help the wall to breathe;
• improving site drainage;
• choosing wall finishes that are tolerant of a little moisture;
• changing the use of space within a building, e.g. basements for storage rather than habitation.

You might even consider 'siphon tubes', although there is a strong argument for removing siphon tubes if you think they may be causing more dampness than they are in fact alleviating (see Evaporation improvement methods).

All these alternatives must be viewed in the light of the likely damage, disturbance, health and safety issues and cost implications. If you decide finally that you need a retrofit dpc,  consider carefully whether it is likely to work on the basis of the construction and site conditions, and in the light of your knowledge of the system you are specifying. There is much debate, for example, on the effectiveness of chemical-injected dpcs.

Coleman alerts us in his book Guide to identification of dampness in buildings to the inherent difficulties of injecting chemical fluids into masonry:

'When fluids are injected into a heterogeneous substrate such as brick/mortar they do not totally fill up the porous structures, and do not completely push out the water in front of the advancing fluid as is so often claimed. Instead, the fluid tends to “finger” within the substrate, a process known as “viscous fingering”….

In practice this suggests that it is unlikely that, in the case of water repellent type dpcs, the diffuse band will be totally complete. The resultant chemical dpc may therefore not stop rising dampness by causing an immediate cut-off of rising water above the dpc like that effected by a physical dpc. Instead, a relatively rapid decline in the capillary moisture gradient should occur above the inserted chemical dpc. Thus, in practice, the rising ground water should be reduced to such a level that, in association with specialist replastering, it should no longer cause decorative spoiling or damage.'

The problem of viscous fingering has been known for some time, and is thought to be more likely when the chemicals are injected under greater pressure.

Whether or not chemical-injected dpcs actually work is very difficult to prove in practice because injected walls are usually masked by associated new plastering and you cannot observe changes in conditions in the wall thickness behind. Commonly, for many years chemical injected walls have been re-plastered with waterproof cement-based renders that produce a dry wall finish in just a few days after plastering, whether or not the substrate behind is itself 'dry'.

Diagnosing damp in buildings is as much about understanding the pathology of buildings through the ages as it is technical and surveying skill. When you consider the evolution of building regulations even over the last 200 years, considerable changes in construction technique and material usage have taken place. Many of the buildings you survey are from times past and knowledge of modern methods of construction may not necessarily help practitioners to understand the way older buildings were put together or the use and behaviour of materials (e.g. use of hydraulic limes).

It therefore follows that providing effective advice on the decision to manage or cure the dampness problem requires a more in-depth knowledge of construction through the ages and across regions.

Further information

Coleman, G.R., Guide to identification of dampness in buildings (2nd edition), Wessex Publishing, 1990.