With Autumn just around the corner and the prospect of wetter weather, Building Surveyor Beth Round considers the critical building defect of rising damp in her latest blog.
Rising damp is one of the most talked about types of damp. It occurs when moisture from the ground moves upwards through the walls of a building. As building surveyors, rising damp is one of the critical defects we look for when surveying a property.
Generally rising damp is first noticed by the damage it causes to the internal walls of a building. Plaster and paintwork deteriorate and wallpaper loosens. A visible stain often appears on the wall as a tide mark at the point where the groundwater has reached. You may also see salts on the internal surface.
Damp-proof courses are installed in properties forming a barrier to stop the progress of moisture. We inspect any damp-proof course in place and see if it has been breached.
The Evolution of the Damp Proof Course
There are two categories of DPC: traditional and non-traditional. A traditional DPC is a physical damp-proof course installed in brickwork or coursed stonework. A non-traditional DPC is a chemical injection used in most wall types but should only be employed when the traditional route is impossible.
Damp-proof courses became compulsory back in 1875 when materials such as slate, tar and sand or hessian soaked in tar were common. In more prominent buildings, lead, copper or layers of slate were often used.
A damp-proof course should be flexible to allow for building movement. Many early materials were discounted; for example, slate was prone to cracking as detailed in the Marshall & Worthing publication, The Construction of Houses.
From the 1950s, the damp-proof courses tended to be a bitumen-impregnated material, usually cloth, which often failed because it could be easily punctured.
The chemical injection damp course came into play in the 1960s. The injected liquid must link together to form a water-resistant barrier to be effective. The fluid should line each pore or void, but if a hole is too big, it will likely flow away, and the damp proof layer will not form.
In the 1970s, the electro-osmotic damp proof system was introduced, which involves drawing moisture from an anode to a cathode. It is no longer commonly used and often fails if the homeowner turns off the electricity or the cabling is damaged.
In the later 1970s, a controversial ceramic tube system was introduced. The tubes were intended to allow water to evaporate and stop it from rising up the wall. The theory was that the sloped ceramic tubes would draw the dampness out of the walls, and the moist air would flow down the slope and be replaced by fresh air to continue the process. However, the system has been widely discredited.
Currently, there should be a continuous damp-proof course, usually a form of plastic sheeting, for a wall’s entire length and thickness. It should be a minimum of 150mm above the finished ground level to avoid it becoming breached by a build-up of materials against the wall and to stop splashing water wetting the brickwork above as indicated in Barry’s Introduction to the Construction of Buildings.
Modern physical damp-proof courses are essential for preventing rising damp in most houses and have a long lifespan with no maintenance.
However, they can be bridged, which could result in rising damp. When surveying buildings, we check for the presence of a damp-proof course and for where it could be breached.
Here is a list of things to look out for when establishing if a damp-proof course has been compromised:
- Is the external ground level higher than the internal floor level?
- Has the damp-proof course been pointed or rendered over?
- Is there a build-up of soil against the wall?
- Has paving or a ramp been fitted outside over the damp-proof course line?
- If there are vents, they should be above the damp-proof course. If the vent is just above the ground, then it is likely to be that the ground is breaching the damp-proof course.