The main cause of rising damp in modern properties is that the damp proof course has been bridged by mortar or debris within the cavity or damaged during construction.   

 

Earlier forms of DPC consisted of a layer of slate.  In older properties, built on shallow foundations, movement within the structure will often cause the slate DPC to break down and become ineffective. 

 

Rising dampness can also occur where an intact DPC is bridged by external plaster, raised ground levels or by items stacked up against the external walls. 

 

As the remedial works for rising damp as expensive it is important to ensure correct diagnosis before starting works.  Rising dampness can occur even when an effective DPC is in place and the assumption that a new DPC is required can be a costly one.  By following a systematic approach of inspection it is possible to avoid costly and ineffective repair works. 

 

Where external plaster has bridged the damp proof course it will be necessary to hack off the plaster to above the DPC and finish with a drip detail.  External ground levels are to be kept at least 150mm below the DPC.  Where existing ground levels are too high a cheaper alternative to lowering the entire ground level is to excavate a shallow channel adjacent to the external walls of the building. Where it is found that the DPC has been bridged by mortar droppings or construction debris at the base of the cavity it will be necessary to remove bricks/blocks in the external skin and rake out the mortar/debris. 

 

Where it is found that the DPC is defective or absent it will be necessary to install a new DPC. Traditional physical DPCs are the only certain method of introducing an effective barrier against rising damp.  The disadvantage to this approach is that the DPC has to be physical inserted into the brick/blockwork which lead to some disruption and can often be time consuming.  Thick stone or random rubble walls will often require some rebuilding.  The junction between the new DPC and any existing damp proof course to a solid floor is very important and care must be taken to ensure continuity of the barrier.  Where a suspended timber floor is in place it is important to inspect the joists for signs of decay, particularly the ends built into the damp walls. 

 

An alternative to the introduction of a traditional physical DPC is the use of a chemical injection system.  This approach involves injected a solvent or water based fluid into holes drilled into the brick/blockwork at approximately 300mm centres.  The effectiveness of the system depends on how deep the fluid penetrates into the wall and its successful curing.  Whilst this approach is cheaper the effectiveness can vary greatly. 

 

With both methods it will be necessary to remove the old internal plaster to at least 300mm above the highest level at which dampness was recorded and always at least 1m above the DPC.  The brick/blockwork should be cleaned down and the mortar joints raked out to a depth of 10mm to provide a key for the new plaster.  The walls should be left exposed to allow for residual moisture to dry out before re-plastering.  It is advisable to use a highly vapour permeable plaster to allow for residual moisture which in some cases can take months to dry out.