Common causes of Rising damp

Common causes of rising damp include:

No DPC (damp proof course) or failure of existing DPC

Depending on the age and construction of the property, this may have a slate, bitumen or polythene DPC, or the DPC may be missing entirely. DPC’s can fail for a number of reasons; these include deterioration of the original material or physical damage caused by building movement.

 

Only buildings built before 1900 were built without a DPC, although more prestigious buildings did sometimes include a traditional slate DPC.

Raised external ground level (bridging of the DPC)

A common cause of Rising damp in modern cavity wall buildings, with integral polythene DPC arises from DIY works, particularly alterations to include, resurfacing paths and drives or raising garden levels. These works may inadvertently increase levels near to or above the DPC thus allowing a direct path for water to travel vertically through capillary action.

Before you carry out any external works, which may affect ground levels, you should check to ensure you leave a minimum of 150mm clearance off the ground.

Splashing from down-pipes

Splashing can occur from rain or down pipes, this is particularly likely where there is a minimal overhang at the eaves and or on elevations that are particularly exposed to prevailing winds.

Render bridging DPC: 

A common mistake when rendering external walls, with e.g. a Tyrolean or Stucco finish, is to apply the render directly over the DPC. Cement based renders are fairly impermeable to moisture, however, they are also more rigid than lime based mortars, and can crack along the DPC, allowing water to enter.

Renders can also have the added problem of limiting evaporation from the wall surface, especially if applied to a direct wall, resulting in trapped moisture and damp problems.

Mortar build up in wall cavity:

A result of poor workmanship, mortar snots and or debris, which has built up within the wall cavity, may lead to water ingress and rising damp.

Mortar on wall ties may also cause another route for direct water penetration.

Bridging of floor screed:

Incorrectly built floor construction can result in the floor screed overlapping the DPC level, and therefore allow a path for moisture to rise vertically from the ground. In addition, water may rise vertically through punctures in the Damp proof membrane, through e.g. failure to provide a sand-blinding layer.