What you should know:
Rising damp is a relatively uncommon but serious form of damp. It affects buildings, primarily older ones, and occurs when moisture from the ground travels upwards through the walls via capillary action. This is similar to how a sponge soaks up water.
Causes of Rising Damp:
The primary cause of rising damp is a faulty, bridged, or completely absent damp-proof course (DPC). A DPC is a horizontal barrier, typically made of non-absorbent materials. These include slate, bitumen, or plastic, installed at the base of a wall to prevent ground moisture from rising.
Here’s a breakdown of the common causes:
Failed or Missing DPC:
In older properties, the original DPC may have deteriorated over time. However, some very old buildings might not have had one installed at all. This allows groundwater to be absorbed directly into the wall structure.
Bridged DPC:
Even if a DPC is present and intact, it can be “bridged” if something allows moisture to bypass it. Common ways this happens include:
High External Ground Levels:
If the ground outside the property (e.g., soil, paving, or landscaping) is higher than the DPC, moisture can simply travel over the DPC into the wall above.
Debris in Wall Cavity:
Building debris or inappropriate insulation material in a wall cavity that extends below the DPC can provide a path for moisture to rise past the DPC.
Overlapping Renders/Plasters:
Internal or external renders or plasters that overlap the DPC can create a pathway for moisture to wick up the wall.
Intersecting Masonry Structures/Abutting Garden Walls: Adjoining structures that are built against the wall and rise above the DPC can also bridge it.
Solid Floors:
In some cases, solid floors can allow moisture to rise if not properly damp-proofed.
High Groundwater Levels: While not a direct cause of DPC failure, consistently high groundwater levels increase the likelihood and severity of rising damp if the DPC is compromised.
Construction Materials:
Some older construction materials, like solid brick walls and lime mortar, can be more porous and susceptible to capillary action.
It’s crucial to differentiate rising damp from other forms of damp, such as penetrating damp (caused by water leaking through the exterior walls) and condensation (caused by excess moisture in the air). Misdiagnosis can lead to ineffective and costly treatments.
Curing Rising Damp:
The most effective way to cure rising damp is to establish or reinstate a functioning damp-proof course and address any bridging issues.
Here are the main treatment methods:
Chemical Damp Proof Course (DPC) Injection:
This is one of the most common and cost-effective methods.
- Holes are drilled into the mortar course at the base of the affected wall (usually about 150mm above ground level).
- A specialized damp-proofing cream or liquid is then injected into these holes.
- The cream diffuses into the mortar, creating a water-repellent barrier that prevents moisture from rising further.
This method is generally minimally invasive and can be done relatively quickly.
Physical Damp Proof Course (DPC) Installation/Replacement:
This involves physically inserting a new damp-proof membrane into the wall.
Sections of the brickwork or masonry are removed at the DPC level, and a waterproof material (like polyethylene or high-density polyethylene) is inserted.
This is a more invasive and labor-intensive process, typically done during major renovations or when the existing DPC is severely damaged or absent.
Electro-Osmosis Damp Proofing:
This is a less common, chemical-free method that uses a low-voltage electrical current to reverse the capillary action. This process drives the moisture back down into the ground.
Wires are installed within the wall, and an electrical current is passed through them.
It can be effective on various materials and is minimally invasive, as the wires can be hidden.
Addressing Bridging Issues:
This is a critical step in curing rising damp. It involves removing anything that allows moisture to bypass the DPC.
Lowering External Ground Levels:
If the ground outside is higher than the DPC, it must be lowered to at least 150mm below the DPC level.
Clearing Cavity Walls:
Any debris or inappropriate insulation in the wall cavity that bridges the DPC needs to be removed.
Removing Overlapping Plaster/Render:
Ensure that internal and external plaster or render does not extend below or over the DPC.
Post-Treatment Renovation:
Once the source of the damp is addressed, it’s essential to deal with the damage caused by the rising damp.
Removing Contaminated Plaster:
Plasterwork affected by rising damp will likely contain hygroscopic salts (nitrates and chlorides). These attract and hold moisture, even after the wall dries out. This plaster needs to be stripped off.
Re-plastering with Specialist Plaster:
The affected walls should be re-plastered using a specialist damp-proof render or a replastering system (e.g., with a cavity wall membrane) that prevents salt migration and allows the wall to dry properly.
Treating Timber:
Any decaying timber, such as skirting boards or floor joists, must be replaced or treated for rot (wet rot or dry rot).
Salt Neutralization:
In some cases, a salt neutralizer may be applied to the wall before re-plastering to help mitigate the effects of residual salts.
It’s highly recommended to consult with a qualified damp-proofing specialist to accurately diagnose the cause of the damp and recommend the most appropriate and effective treatment plan. Misdiagnosis can lead to wasted effort and continued damp problems.
