When we talk about rising damp, we are referring to moisture from the ground that rises up into a wall through the capillaries in a brick or stone that is in contact with the ground. True rising damp will only affect the first two courses of brickwork or just above that which are in contact with the ground. This is one of the reasons why a damp proof course is installed 150mm above the external ground level, and internal floors should be a similar height above the external ground level.
On occasion, damp may rise in a wall above 150mm where there is no damp proof course, but this is usually caused by a leaking underground pipe or the diversion of an underground watercourse or river. The water escaping from the pipe or watercourse will be under pressure, and it is this pressure that forces the water into the wall and up higher than the usual 150mm.
Rising damp is probably one of the most misdiagnosed damp problems with buildings. For a long time, surveyors and specialist damp contractors have been using a moisture meter to detect moisture levels in plaster and brick, which can be a helpful diagnostic tool if used correctly. However, it is often misused, particularly by mortgage valuation surveyors and damp contractors who either do not fully understand its limitations or use it to sell their products.
Some RICS surveyors will recommend a further specialist investigation and report if a moisture meter gives even the slightest reading. In anything other than softwood, the reading should be taken into context as it will not truly reflect the moisture content of the material. However, given the litigious world in which surveyors operate, this may be appropriate, as it is not always the mortgage valuation surveyor’s job to diagnose the moisture-related problem. However, if the surveyor is undertaking a full building survey of a property, you should then expect the surveyor to undertake a full diagnosis instead of referring the matter for further investigation.
Furthermore, many mortgage valuers are required to use certain phrases when completing valuation reports. If they find evidence of or suspect a damp problem, they are often required to refer the matter for specialist investigation. A phrase similar to the following is usually inserted into the mortgage valuation report:
“Evidence of dampness and timber infestation was noted. A further inspection of the property should be undertaken by a specialist contractor who is a member of the Property Care Association (PCA), and all remedial work is to be covered by an appropriate insurance-backed guarantee.”
When the surveyor first recommends a damp surveyor’s report, this is when the problem starts, and it usually leads to work that is often unnecessary and a complete waste of the client’s money. The mortgage companies usually require that such investigations be undertaken by a company belonging to the Property Care Association (PCA), most (but not all) of which are treatment companies. These companies offer free or cheap damp surveyors as a window to selling their products.
Although there are many competent RICS surveyors and other professionals who are not members of the PCA and can assess dampness properly without a vested interest in any specialist treatment, the problem persists while there are companies that sell damp proofing products.
Investigation of Moisture & its Effects on Traditional Building should be read by anyone who has been advised to obtain a timber and damp contractor report as a condition of a mortgage valuation. A copy of the document can be found here for free.
What does hygroscopic mean in terms of building materials?
All traditional building materials are hygroscopic. Hygroscopic means “water seeking”. So brick, plaster, stone and wood will naturally absorb water and the amount of water absorption will vary depending on humidity or vapour pressure. In old buildings with rattly old sash windows and open fireplaces and timber floorboards the internal relative humidity of the building will follow rises in external vapour pressure. If the internal vapour pressure is constant the internal building materials will be at equilibrium. However, if vapour pressure changes then the building materials will naturally take up or release moisture until the new equilibrium is established.
Where do hygroscopic salts in building come from?
Hygroscopic salts in buildings come mainly in the form of carbonates, sulphates, chlorides and nitrates. These salts can come ground water, salts from the sea contained in driving rain, clean products, the spreading of salts during the gritting of roads, cleaning products, horse manure and most commonly from chimney soot.