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Failed Damp Proof Course
You have been told the damp proof course has failed, and want to know what that means and whether it is true.
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What's happening?
A damp proof course is a continuous barrier built into a wall near its base to stop groundwater rising past it. The guidance describes what it can be made of — bituminous material, polyethylene, engineering bricks or slates in cement mortar, or any other material that prevents the passage of moisture — and requires it to be continuous with any damp proof membrane in the floors. That last point is where most real failures live. A slate or bitumen barrier does not decay across a whole wall in the way the phrase suggests; what usually happens is that it was breached at one point, or it was never joined to the floor, or it is still working perfectly and is being bypassed. Genuine failure is a local defect, and finding where matters more than accepting the label.
What usually causes it?
- Physical breach from later work — a new doorway, a re-laid floor, a service pipe or a wall tie retrofit cutting through it
- Slate damp proof course cracked by settlement or by movement at a bay or an extension junction
- No connection between the wall's damp proof course and the floor's damp proof membrane, leaving a continuous gap
- Original bitumen layer squeezed out or degraded where the wall was heavily loaded
- A previously injected chemical course installed incompletely, at the wrong height or into unsuitable masonry
- A cavity filled with rubble or mortar droppings above the barrier, giving water a route past it
- No damp proof course present at all, which is a design absence rather than a failure
Is it dangerous?
No immediate hazard, but a wall that is continuously wet at its base keeps plaster failing, salts migrating and any timber built into it decaying. The bigger practical risk is a wrong diagnosis: extensive injection work carried out on a wall whose barrier is intact leaves you with the original problem, replastered.
Can I fix it myself?
Establish the facts you can see. Find the damp proof course line on the outside of the wall — usually a visible mortar joint or a slate course two brick courses above the ground — and check whether the ground outside sits below it. Check where any recent work cut into the wall base. Then take the diagnosis to someone who will investigate. Do not accept a quote for injection based on a meter reading alone, and do not commission it as a precaution: it is remedial work for a specific defect, not maintenance.
When should you call a damp proofer?
Call a damp proofer for a survey that will tell you whether the barrier has actually failed, where, and why. Ask what alternatives were ruled out. If the answer is that the barrier is fine and the problem is a raised ground level, a bridged cavity or a leak, that is a much cheaper job — and a survey that never reaches that conclusion for any house is not a survey.
What will the damp proofer actually do?
- Locate the existing damp proof course and establish its type, height and condition
- Check whether it is continuous with the floor's damp proof membrane and whether anything has cut through it
- Measure moisture and salt distribution in the wall to establish whether water is genuinely rising
- Identify and remove bypass routes — bridging debris in the cavity, raised external levels, bridging render
- Install a remedial damp proof course only where the barrier is genuinely defective, to a recognised method and specification
- Specify replastering that manages residual salts, and state the expected drying time
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