A damp proof course is important in preventing damp around the home. This article will cover what it is and other common questions around them.
What is Damp Proof Course?
A damp proof course (DPC) is a barrier installed in the walls of a building to prevent moisture from rising from the ground into the walls.
It is usually placed above ground level (150mm) and can be made from materials like plastic, rubber, or metal. It is often seen within the brickwork in the lines of mortar as a bitumen or plastic DPC sheet slightly sticking out.
The DPC stops water from moving up through the building materials by capillary action, helping to keep the structure dry and prevent damage from dampness.
DPCs are required in all new construction to stop rising damp.
Types of Damp Proof Courses
There are generally 3 DPCs considered for a damp proofing project:
Damp Proof Course Injection
Also known as chemical damp proof course injections. It involves injecting water-repellent chemical or cream (usually silicone-based solution) into the walls.
Holes are drilled at least 150 mm above the external ground levels and the solution is injected into the walls.
As it is water repellent, it stops damp rising through the walls.
Mortar Injection Damp Proof Course
This method uses a chemically-enhanced mortar instead of creams or liquids.
The mortar is caulked into holes drilled into the wall. It is particularly useful in walls constructed of random rubble, which often have voids within their structure.
The mortar fills these voids once inserted. The chemical additives are absorbed from the mortar into the surrounding masonry and block rising damp.
Electro Osmotic Damp Proof Course
If chemicals are not suitable or if the property owner prefers a non-chemical solution, then an Electro Osmotic Damp Proof Course can be installed instead.
It works by using titanium and copper wiring to send a small electric current through the wall. This reverses the capillary action that causes rising damp and keeps it below the damp proof course.
How Do I Know If I Have a Damp Proof Course?
Damp proof courses are standard elements within a building construction.
They should be placed at least 150mm above the ground. They usually appear between layers of brickwork in the mortar course as a bitumen or plastic sheet poking.
They were introduced as a compulsory measure in 1875. The initial DPCs were made from materials like slate, lead, or engineering bricks.
However, these early barriers often failed due to improper installation or deterioration over time.
More effective materials like bitumen felt were used over time, and modern properties now typically use a plastic membrane DPC.
Why Do Damp Proof Courses Fail?
Most existing properties should feature a damp-proof course that is visible at least 150 mm above ground level. This is usually seen as a line of mortar with a bitumen or plastic DPC sheet protruding.
These materials are generally effective unless they are compromised in some way:
- Building movement that causes cracking: When a building settles or shifts over time, it can lead to cracks in the structure. These cracks can disrupt the continuity of the damp proof course (DPC), allowing moisture to bypass the barrier and enter the building.
- The DPC is bridged: Bridging occurs when soil, debris, or construction materials accidentally cover the DPC, connecting the damp ground to the wall above the DPC. This allows moisture to travel across the bridge and ascend past the intended barrier, leading to damp issues.
- The material degrades due to age: Over time, the materials used in damp proof courses, such as plastic, bitumen, or metal, can deteriorate. This degradation weakens their ability to resist moisture, making them less effective and eventually leading to failure.
- Building renovations raising ground level above existing damp proof course: If the ground level around a building is raised it can end up higher than the installed DPC. This means the DPC no longer serves as an effective barrier at the new soil level, allowing moisture to rise above the DPC and into the walls.
How to Install a Damp Proof Course
- Prepare the Wall: Remove any damp plaster from the wall.
- Drill Holes: Drill the holes at least 150mm above the ground level into the wall.
- Install Damp-proof Course: Inject damp-proof cream into the holes, then seal them with mortar or plastic plugs.
- Apply Scratch Coat: Put on a rough coat of plaster about 5mm thick to help the next layer adhere.
- Apply Plaster Coat: Once the scratch coat has set, add a thicker plaster coat, between 15mm and 20mm.
- Finishing Coat: Finish with a smooth skim coat of plaster.
How Long Does a Damp Proof Course Last?
The lifespan of a damp proof course can vary depending on the materials used, the quality of the installation, and the environmental conditions.
Generally, a well-installed and high-quality DPC can last for the lifetime of the building (up to 50 years).
However, chemical DPCs injected into the walls have a more variable lifespan. This is typically around 20 to 30 years.
Regular inspections and maintenance can help extend the effective life of any DPC.
Signs That You Need a Damp-Proof Course
Several clear indicators suggest the presence of damp in walls, including:
- Black spot mold on interior walls, which often also indicates condensation issues.
- Watermarks on walls, which can be caused by either rising damp or penetrating damp.
- Peeling plaster and damage to other wall finishes like wallpaper.
- Deterioration of wood, such as skirting boards, due to moisture contact.
What Next?
If you suspect damp issues in your property or notice signs of damp, it’s important to have a professional conduct a damp survey.
Without proper training, it can be challenging to accurately identify the source of the damp problem.
When left untreated, the damp will inevitably become more severe. This can eventually cause structural damage in due course and ultimately means higher repair costs in the long run.
This is why it’s recommended to get an experienced and certified specialist to visit your home as soon as possible.
Your property is in safe hands when you contact us.