Rising & Penetrating Damp
There are two types of damp that buildings can suffer from: rising damp and penetrating damp. With rising damp, moisture from the ground under and around buildings rises within external or internal walls that are made of permeable or absorbent building materials (for example, brickwork, stone or plaster). As it does so, the internal plaster finishes become saturated with moisture, leading to mould growth, the deterioration of the finish and eventually deterioration of the fabric of the wall itself. Typically, damp will be seen in the plaster finishes up to 1 metre above ground floor level.
The usual solution (and the one we used) for this type of damp is to drill holes at regular intervals around the perimeter of the affected area (just above external ground level) and apply a damp proof course (DPC) by means of a pressurised chemical injection into the holes. The chemical is absorbed into the masonry and forms an effective barrier to moisture travel by expanding into a line across the brickwork, in turn preventing the moisture from continually heading upward from the foundations.
The Affected Interior
Before the affected interior can be redecorated, the decorations and plasterwork have to be removed. This is extremely important in order to prevent residual moisture and salts from the underlying masonry traveling to the new surface. Walls take time to dry down and the lower part of the walls are always likely to remain damp because of the limitations of the chemical injection systems a) in terms of the injections only controlling the rising damp, and b) re-plastering to prevent future damage. You can’t have one process without the other.
The interior work was carried out by the individual landlord and at the moment, there have been no further problems reported (fingers crossed it stays that way!) because many disputes are centered upon whether any subsequent problems are caused by DPC failure or inadequate new plaster work. Should any damage reappear then both sets of work would have to be examined, rather than one or the other.
The most common defects that will lead to penetrating damp are:
- Blocked rainwater gutters, hoppers or down-pipes which overflow, causing the adjacent wall to become saturated;
- Defective concrete haunching around disused chimney pots that have been capped or sealed. This allows water to enter the unused chimney and then penetrate into the building;
- Defective flashings around chimneys, flues, soil-and-vent pipes and ventilation terminals, and at abutments of roofs to vertical walls. Whilst these defects usually cause leaks rather than damp, sometimes water finds its way into the roof construction and emerges elsewhere as penetrating damp;
- External wall cavities that have been bridged by rubbish left in the cavity or by mortar droppings left on wall ties or insulation batts during construction;
- Exposure to driving rain where water penetrates the brick outer leaf which may then be led across the cavity, locally saturating the inner masonry leaf and subsequently internal plaster or plasterboard finishes.
- A wall tie, which is designed not to allow moisture to cross it, being coated with mortar when the wall is built;
- Pipework which extends through the wall;
- Window, door frames and lintels which have been poorly fitted with damp proof courses.