Condensation damp has proven to be incredibly popular and with that in mind I was asked if I'd mind writing an update on rising damp. It may be worth mentioning that my own level of expertise stems from two years research carried out into rising damp that resulted in a dissertation entitled, 'The Efficacy of DPC Injection.' I've 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 10 years to merit an update on current thinking, controversies and industry developments. If I may, I'd like to clear up one key controversy from the outset...
Is Rising Damp a Myth?
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' and 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. 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 isn't 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 and 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%. Please note that we're 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'll 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; 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's 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 wasn't taken for the fact that a new OPC 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.