When referring to rising damp, that is, moisture wicked up from the ground through capillary action which has become evident at the base of a wall above and below the floor, some care needs to be given to projected remedies

 

THE ISSUE

 

Very simply put, rising damp will often bring with it salts from both the ground and the building materials. These salts as they make their way to the face of the wall, dry out and leave efflorescence.
If the salt laden moisture evaporates within the wall it may expand as it dries out, often causing spalling

It is often assumed that when moisture or spalling is visible, it is solely because the damp proof course has failed. This is not always the case. Inadequate or dysfunctional drainage to the perimeter of the building, poor or non-existent subfloor ventilation, leaking gutters, deteriorated external pointing or dirt built up against a wall creating a bridge over the dpc are just as likely to contribute to the issue. Dealing with only one of these issues without considering the others may result in considerable use of resources only to find the problem still evident.

Moisture at the wall/floor junction can also have a negative effect on timber floor framing, floorboards and skirting boards, causing damaging fungal growth and rot.

THE TREATMENT

Before any works take place, a thorough investigation should be made of the contributing factors.

If stormwater or surface water is making its way under the building and leaving the subfloor area wet this is obviously a problem. The remedy can often be as simple as grading the ground outside the building so as to direct the surface water away from the building. This done in conjunction with a subterranean strip drain makes for a very effective perimeter drainage system. However, in an extreme situation a moisture apron may need to be considered.

Should sub-floor ventilation prove to be lacking, the introduction of sub-floor vents to the perimeter of the building and increased internal sub-floor cross ventilation with the introduction of penetrations in the sub-floor walls will aid in the remedy.

If the dpc has failed or is non-existent, it can re-instated with either a physical barrier or a chemical dpc.

Physical DPC: Once the correct position for the dpc has been established, (preferably below the structural floor framing) the mortar bed joint is cut out through the full thickness of the wall, and a modern dpc is inserted into the cut. Once in position, the mortar joint is re-pointed so as not to allow any settlement of the structure during the process. As a visible barrier, and when carried out professionally, there can be no doubt as to the effectiveness of this method.

Chemical DPC: The most common method of installation is to remove the skirting boards and the affected plaster, and inject the wall just above floor level. Bulky application equipment can often prohibit the use of this product in a confined sub-floor area. As a result, moisture to the floor framing members may need to dealt with as a separate issue. When the walls are of very sound construction, and good penetration is achieved, this method is effective as a dpc.

Once a method of DPC has been chosen and has been carried out, the issue of moisture and salt affected wall finishes still remains. Sacrificial renders and poultices are effective and have their place. One product regularly used by us is a paper pulp poultice. Clay poultices have also been used by many with effective results. These techniques can be supported by a series of before and after tests to confirm that salt levels are being reduced and to determine the need for repeat applications.