Diagnosing Rising Damp

This article on rising damp was first published on ‘surveyingproperty.blogspot’ and later reproduced in a SAVA technical bulletin, (issue 17, 2014). Since it is my own material then I thought it was long overdue for inclusion on my own blog site. I have further supplemented and updated the information by the inclusion of a damp investigation flowchart that I developed for inclusion in a book I am writing. Please note that this flowchart is the copywrite material of the author and should not be reprinted or posted elsewhere without the authors permission. 

Deep wall probes in use

Deep wall probes in use

My own level of expertise regarding rising damp stems from two years research carried out into rising damp that resulted in a dissertation entitled, “The Efficacy of DPC Injection”. I have been actively involved in surveying damp properties and more importantly, teaching damp investigation for a number of years now and think there have been a number of significant developments over the last ten years to merit an update on current thinking, controversies and industry developments.

There have been a number of commentators who have done nothing to move this issue forward over the last few years. In particular Jeff Howell’s book, “The Rising Damp Myth”. Stephen Boniface, former Chair of the RICS Building Surveying Faculty, has also gone on record to state his belief that rising damp is a myth.

NB. It is worth noting that since this article was first published, Stephen Boniface has gone on record in stating that he has was misquoted on this issue. 

Whilst I understand the sentiment behind their extreme view, it is perhaps a backlash to a DPC industry that promotes rising damp as a common occurrence.

During my research into rising damp, I came across a PCA examination paper for their National Certificate in Remedial Treatment from 2005 where a question started with the statement that, “Rising damp is a common problem”. Of course we know it is not a common problem but it demonstrates the second of two extremes when a rather more moderate approach needs adopting. Both views cause a number of problems:

 The view that rising damp is a myth may cause building surveyors to form a view that it is not worth learning how to properly survey for rising damp.

 The supposition that rising damp is a common problem has led to a glut of poorly trained industry surveyors and widespread misdiagnosis due to over reliance on hand held electrical moisture meters.

Even the poorly trained have a real sense of security gained in the knowledge that, even if you misdiagnose, the waterproof renovating plasters applied internally will give the appearance of a dry wall, thereby leading clients to conclude that your diagnosis was correct. After carrying out a substantial literature review on this question I can with confidence state two facts:

1. Rising damp does exist and is a scientifically proven phenomenon.

2. Although it exists, it is incredibly rare.

The more common academic view is that between 5% and 10% of damp properties will be affected by rising damp; my own research puts the incidence at less than 5%. (Note that we are talking about a percentage of damp properties here and not total properties in the UK.)

So what exactly is rising damp?

The simple academic description would describe rising damp as “an upward capillary migration of water in
masonry”. You will find the reference to capillary action in most text books and it is in this area that most text books are long overdue an update. Bricks contain capillaries or microscopic tubes that are small enough to allow inter-molecular attractive forces between the liquid and solid surrounding surface; these forces allow a liquid to flow in narrow spaces against gravity. The problem here is that we now know that rising damp has two moisture transfer mechanisms, i.e. capillary action and diffusion.

It is generally thought that molecular diffusion (Fickian) is the moisture transport mechanism for water molecules moving through cement paste. Some of you may remember this from your school physics lessons but in simple terms diffusion is the spreading of solutes from regions of highest to regions of lower concentrations caused by the concentration gradient. It is the same for concrete floor slabs; water moves up through the floor slab by a process of diffusion and not capillary action.

A new definition for rising damp

It is time to propose a new definition for rising damp and I would suggest the following description:

“Rising damp is an upward migration of groundwater in masonry walls. It will act in combination on the masonry units and their separating mortar joints or it will act primarily on the mortar joints. The moisture transfer mechanism in masonry is capillary action whilst the moisture transfer mechanism within mortar is diffusion. The major moisture pathway for rising damp is the mortar perps so it can be stated that there are dual moisture transfer mechanisms for rising damp, diffusion and capillary action.”

Maybe not as snappy as the original definition but it clears up a number of issues and in itself can be used as an aid to diagnosis and specification.

Since we know that the mortar joints are primarily affected then it serves very little purpose in drilling and injecting brickwork without also treating the mortar joints. We need to qualify this statement because we have something of an anomaly when it comes to discussing the mortar joints.

Tests were carried out at South Bank University a number of years back which failed to replicate rising damp in laboratory conditions. The tests were bound to fail because account was not taken for the fact that a new DPC mortar bed is impermeable to moisture. However, after 30-50 years of environmental exposure the mortar degrades and rather than providing an impermeable barrier, it then becomes the main moisture pathway.

For reasons of practicality and aesthetics we should have completely moved away from injecting brickwork and retrofit DPC injection should focus on the mortar bed and perp joints. However, we are getting ahead of ourselves because we’ve not yet discussed correct diagnosis.

It makes no sense whatsoever to install a retrofit chemical injection to a property that already has a physical DPC installed unless you can evidence failure of the existing DPC; to my knowledge, no one has yet done this.

Diagnosing rising damp

There is a view within the damp proofing industry that rising damp can be diagnosed with nothing more than a hand held electrical conductance meter and a great deal of experience. There is not a shred of scientific evidence to support this view and in fact it is well documented that hand held electrical moisture meters are of limited use due to the fact that they are calibrated for timber and not masonry. They are also prone to giving false positive readings for damp wherever they encounter salts, carbonaceous materials or backing papers such as foil damp.

You need to confirm that three conditions are present to definitively confirm a case of rising damp:

1. You must have a rising damp moisture profile. That is a profile that is wetter at the wall base but gradually decreases with height to a theoretical maximum height of circa 1.5 m.

2. You must prove that moisture is present at depth in the masonry and it is not enough to take surface readings from the plasterwork. You will need deep wall probes or a calcium carbide (speedy) meter to confirm this on site.

3. You will need to confirm that nitrates are present in the damp apex of your moisture profile. This will involve doing on-site analysis or sending a sample off to the labs. You might have noted that I have ignored chloride salts because these can be present in tap water or building materials. A positive test for nitrates confirms that the moisture has leached up from the soil.

NB. I have since discussed and debunked the academic requirement for salts analysis in a later article entitled, ‘Rising Damp: An update for 2015.’

Unless you can confirm each of these three conditions then, your diagnosis is based on guesswork. On the upside, due to the use of waterproof renovating plasters no one will ever know you got it wrong. It is a fact that the application of renovating plaster provides the perfect cover up for bad surveying practice.

The following flowchart illustrates the damp investigation process that we teach on our damp investigation training course.

Damp Investigation Process Flowchart

Damp Investigation Process Flowchart

Do physical DPCs fail?

This was a key question asked in my research and I could not find a shred of evidence to support the view that physical DPCs fail though I accept that not enough research has been done in this area.

I did note that cracked slate DPCs had been found but as one of my contemporaries wisely pointed out: “A crack is a crack and a capillary is a capillary”, you will hopefully see the logic in this statement.

Slate DPC still fully functional in a building that is circa 140 years old.

Slate DPC still fully functional in a building that is circa 140 years old.

What is clear is that DPCs are regularly found to be bridged or compromised in some other way.

How has the damp proofing industry changed in the last ten years?
It is fair to say that the process of retrofit DPC injection has been taken out of specialist hands over the last ten years. In the past, expensive equipment and specialist training was required for injecting siliconate and stearate fluids into brickwork. These are still used, but the market has moved more towards the use of aqueous silane creams injected into mortar bed and perp joints.

The process is so simple that anyone with a reasonable degree of DIY skill can successfully carry out chemical injection. All that is needed is a hammer drill, a tube of your chosen water repellant cream and an application gun.

The cream is applied into 12 mm holes drilled at 120 mm intervals which will then diffuse into the wall via the mortar course to form a damp course to BS 6576. The drillings are simply made good with re-pointing rather than being sealed with plastic plugs, as used to be the case. Moreover, aqueous silane creams are far safer to use than the old types of injection fluids and come with far less chance of user error; anyone who ever used these fluids will tell you how they burned in contact with the skin.

It was not unusual for pressure injected fluids to be injected into voids within the brick and in any event these fluids were never designed to give full penetration that forms a continuous barrier to damp.

They worked by a process called viscous fingering which in basic terms means that you have fingers of waterproofing within the individual masonry unit rather than a complete barrier.

The best you could hope for was that you stop a fraction of water rising in the wall and restore moisture equilibrium. Moisture equilibrium is achieved when water is evaporating off the wall as fast as the damp is rising; thereby controlling any further rise in height of the damp. Silane creams are designed to give a complete impervious barrier to damp and on that basis alone outperform the old liquid systems.

Retrofit DPC injection has always been a two part management solution with the internal re-plastering being as, if not more important, than the injection work. Plaster becomes defective when chronic damp dissolves the calcium sulphate within the plaster, which make it extremely porous but salt contamination is the primary reason to hack off and replace the plaster. These salt contaminants are hygroscopic and will continue to absorb moisture from the atmosphere causing the wall to remain damp.

In the early days it was common for plaster to be hacked off and replaced with sand and cement render containing a waterproof additive that was then finished with a coat of Carlite finish. These days waterproof renders are rarely used with most contractors and specifiers opting for one in a range of waterproof renovating plasters that have become available.

For the record, I am neither anti damp proofing industry nor anti retrofit injection; I simply believe that the vast majority of damp buildings can be cured at source using nothing more than minor building works and the damp proofing industry would be best served by accounting for this fact.

I have both specified retrofit DPC injection and used it personally because pragmatically occasions do arise when you can do little else. What if a neighbour’s yard has higher ground levels than yours and is draining against you gable wall? It is unlikely that lowering your neighbour’s ground levels will be an option. A truly independent and competent damp surveyor will not hold with extremist views that rising damp is a myth but will also understand that rising damp is incredibly rare. It is this reasoned and pragmatic approach that will leave them best placed to appropriately specify works to achieve a cure or a management solution. Wherever possible, a cure should always be the preferred option and retrofit DPC injection falls firmly under the heading of management solution.

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