Contrary to popular belief, true rising damp is actually quite rare. With rising damp water is sucked up from the ground and travels through the brickwork via capillary action. This water may also contain salts that can create visible blooms on the inside of your walls.
Adjoining porous materials may also be affected. Timber floor boards and skirting boards will also absorb moisture and may exhibit signs of wet rot.
Why is Rising Damp Rare?
Most buildings have a damp proof course that should be positioned below the floor timbers and 150mm about the exterior ground level. Damp proof courses are designed to prevent water from rising, and the materials used to form this barrier have evolved through time. Early DPCs were often made out of slate, bitumen, or pitch polymer. More modern DPC’s use plastic or injectable chemical creams. If the damp proof course is damaged, incorrectly installed, missing, or has failed due to age, then rising damp could be a problem.
Bridged Damp Proof Courses
In some cases, a damp proof course may become bridged by remedial building works. Once bridged the previously effective damp proof course can no longer prevent water ingress. Some common causes of bridged DPC’s are:
- Wall cavity insulation.
- External rendering that covers the damp proof course.
- Raising external ground level about the damp proof course.
- Garden walls intersecting with the property.
Wall cavity insulation can sometimes allow lateral water ingress on weather prevailing elevations. Wall cavities are designed to create a physical barrier between the inner and outer wall, preventing lateral moisture ingress. When this void is filled with insulation, the barrier becomes bridged. Wall cavity insulation can often exasperate damp problems caused by exterior cement render.
Damp Caused by Cement Render
Cement render is probably the most common cause of damp in old houses. Stone and old bricks expand and contract as the building warms and cools, but unfortunately cement render does not. This constant expansion and contraction causes the render to crack, allowing water ingress. Since cement is non-porous, the moisture is trapped against the masonry and serious penetrating damp problems can develop which are frequently misdiagnosed as rising damp.
Cement render should never be used on old buildings. If you want to render an old house or to replace your existing cement render, use a high quality breathable lime render system such as Puracalce K710. You can find a wide range of high quality breathable renders at www.amaroc.co.uk”.
Why Damp Surveys Often Misdiagnose Rising Damp
Most damp surveyors use meters that work by measuring the salt content of your walls. The theory behind this practice is that if you have rising damp, salts will be drawn up from the ground along with the water. Unfortunately this method fails to take into account that more often than not, these salts originate from other sources.
One of the leading sources of salts found in masonry are toxic fumes and pollution which form acids. These acids attack the bindings components of brick, stone, and mortar leading to the formation of salts. These highly soluble salts can permeate masonry fairly deeply, and high salt readings are often found for several meters around a chimney or fireplace. Unscrupulous damp surveyors know this, and often make a beeline for the chimney with their damp meter in order to sell expensive but unwarranted damp proof remedies.
If you are thinking of booking a damp survey, it’s usually better to go with an independent surveyor who does not provide any remedial damp proofing services.