Identifying the cause of the damp
There are several factors which can cause damp patches to appear on the inner surface of a wall. By doing a little detective work, it is relatively easy to work out what is causing the problem, and so take appropriate action.
There are some obvious things to check, but frequently these are overlooked.
Check to see whether there is a nearby downpipe which is fractured or leaking at a joint. This can cause water to drain out of the back of the pipe and down the wall, whilst hidden from general view.
A similar check can be made of the gutters. They can also become blocked by leaves and other debris causing them to overflow. The resulting flow of water over a prolonged period will soon begin to percolate through the brickwork and cause unsightly damp stains on the interior decoration.
Another common cause of dampness can be plants growing too close to the house. The problem with these is that moisture and rain gets trapped. With little airflow behind to help dry off the moisture when the weather is reasonable, this dampness can quickly become a problem.
Rising damp is usually easy to recognise. It is given the name because it is caused by moisture being drawn up through the wall from the damp ground below. This is referred to as capillary action. The reason that it does not go higher than about a metre, is that once this height is reached, the weight of water becomes too much for the capillary action to be able to draw it any higher.
Rising damp and the causes
So, if the damp area is only within the bottom 1 metre or so of the wall, it's a sure indication of rising damp. The cause of this is usually one of two things:
The damp proof course (dpc), an impermeable membrane set low down in the wall, may be bridged somewhere. In other words, something is providing a route for the moisture to get past the dpc. Examples of this might be soil in a flower bed which is too high along the wall, or a patio which has been laid too high, and without due consideration for the dpc. In both cases, remedial action will need to be taken to overcome the problem.
In some instances, the dpc may have broken down. An example of this might be an old property where slate was used as a dpc, and there has been some movement causing the slate to fracture. Once this has happened, the water can be drawn up into the brickwork above. In some very old properties, there may be no dpc at all.
The correct proceedure for dealing with a broken dpc is to have either a new section fitted in the area or, to have a chemical injected into the wall to make that section impervious. Both these jobs are normally carried out by specialists who should also be able to provide a guarantee on the work.