Damp Proof Course Myth
Do you really need a new damp proof course, or is the problem actually bridging, condensation, high ground levels, leaks or another defect?
A chemical damp-proof course is often recommended when low-level damp is found, but it is not always the correct solution. Many properties already have a physical damp-proof course that is still doing its job, but the damp-proof course has been bridged or bypassed by later changes to the building.
Common causes include high external ground levels, render bridging the damp-proof course, internal plaster taken too low, poor sub-floor ventilation, condensation, leaking gutters, defective drains, penetrating damp and other moisture defects.
This page shows real examples where a new damp-proof course was recommended or installed when the true cause was something else.
The aim is simple: diagnose the damp properly before paying for treatment.
DPCs Often Still Work
Physical damp-proof courses such as slate, bitumen or plastic do not usually simply “break down”.
Bridging Is Common
High ground levels, render, plaster or debris can allow moisture to bypass an existing DPC.
Not Always Rising Damp
Condensation, leaks, penetrating damp and ventilation defects can mimic rising damp symptoms.
Survey Before Treatment
A proper diagnosis can help avoid unnecessary injection, replastering or membrane work.
Physical Damp Proof Courses: Real Examples
The photographs below show original physical damp-proof courses in place. In these cases, the problem was not that the damp-proof course had failed. The issue was bridging, high external ground levels or another moisture source.
100+ Year-Old DPC
~70 Year-Old DPC
Over 120 Year-Old DPC
The DPC Was Not the Problem
In the photographs above, the ground levels are too close to the damp-proof course, and in one example the render bridges the DPC. This is why the properties were suffering from damp issues. The original physical damp-proof courses were still present and should not automatically have been replaced or duplicated with a chemical injection.
Be Careful With “Free” Damp Surveys
Free damp surveys are often offered by companies that make their money from selling damp-proofing work. That does not automatically mean every free survey is wrong, but it does create a clear commercial incentive to recommend treatment.
A free contractor visit may be useful if you already know you want a quote for a specific job. However, if you need a proper diagnosis, a written report, or advice before buying or selling a property, a paid damp survey is usually the safer option.
The key question is: if no treatment is needed, how does the free survey company get paid?
Before agreeing to a chemical damp-proof course, replastering or membrane system, make sure the cause has been properly identified.
Unnecessary Chemical DPC: Real Example
The photo shows an original physical damp-proof course exactly where it should be, approximately 150mm above the external ground level.
A chemical damp-proof course was then installed below it. In this case, the new injection was unnecessary because the original DPC was already present. It was also positioned too close to the external ground level, meaning rainwater splashback and bridging could still cause low-level penetrating damp.
This is a good example of why a new damp-proof course should not be recommended without understanding the existing construction, ground levels and true moisture source.
Get the damp diagnosed before paying for injection work.
When a New Damp Proof Course May Not Be Needed
A new chemical damp-proof course may not be needed if the property already has a physical DPC and the issue is being caused by bridging, high external ground levels, render, internal plaster, condensation, leaks, poor ventilation or penetrating damp.
In these cases, the correct solution may be to lower ground levels, remove bridging plaster or render, improve ventilation, repair gutters or drains, fix a leak, allow the wall to dry, or address salt-contaminated plaster only where necessary.
A proper survey should explain why damp is occurring, not simply recommend a standard injection and replastering package.
Check for
- high ground levels
- render bridging
- low internal plaster
- blocked sub-floor vents
- leaking gutters
- condensation
- penetrating damp
Case Study: The Rising Damp Misdiagnosis
The photo shows a physical damp-proof course exposed at a property in Manchester.
The owner was selling the property when a pre-purchase damp and timber report claimed that the property had rising damp and required a new damp-proof course injection and replastering. The quotation was in the thousands, and the buyer withdrew.
The homeowner instructed Dampserve to carry out a full damp and timber survey. On inspection, the original damp-proof course was visible and present. Only small areas of low-level dampness were found. The cause was poor sub-floor ventilation, which was allowing condensation and moisture to affect the walls at low level.
The solution was to improve sub-floor ventilation by installing air bricks, allowing the affected areas to dry. No new damp-proof course injection or replastering was required.
The lesson: low-level damp should be diagnosed properly before treatment is agreed.
More Examples of Unnecessary DPC Injection
These examples show why the cause of damp should be understood before installing a chemical damp-proof course.
Chester
A damp-proof course injection had been installed just above the original physical damp-proof course. The issue was actually high external ground levels, not rising damp.
Wrexham
A damp-proof course injection had been installed into a damp-proof course brick. The internal damp issue was actually condensation, not rising damp.
Mold, North Wales
A damp-proof course injection had been installed just above a damp-proof course brick. The problem was high external ground level, not rising damp.
Another Client Avoids Costly Damp Proof Course Thanks to Dampserve - Part 1
Another Client Avoids Costly Damp Proof Course Thanks to Dampserve - Part 2
Damp Proof Course FAQs
Helpful answers about damp-proof courses, rising damp, bridging and chemical DPC injections.
Physical damp-proof courses such as slate, bitumen or plastic do not usually simply “break down.” Damp can occur where the DPC is bridged, bypassed or where another moisture source is present.
Not always. A new chemical DPC should only be recommended where the evidence supports it. Many damp problems are caused by high ground levels, bridging, condensation, leaks or penetrating damp.
DPC bridging happens when moisture bypasses an existing damp-proof course. This can be caused by high external ground levels, render, plaster, debris, solid floors or other construction details.
Yes. Condensation, cold surfaces, salts and ventilation defects can sometimes be mistaken for rising damp if the building is not assessed properly.
Yes, rising damp can occur, but it is less common than many quick diagnoses suggest. Proper inspection is needed before agreeing to treatment.
Arrange a proper damp survey or written report so the true cause is identified. This helps avoid paying for a new DPC where simpler maintenance, ventilation or de-bridging would be more appropriate.