What is a damp proof course?
A damp proof course, or DPC for short, is a physical barrier against the migration of moisture from the ground (groundwater). These barriers are formed by using an impervious material like slate (in the perfectly working example above from 1885), coal tar or bitumen tar and, nowadays, a thick polythene, which are built into the structure during its construction.
What does a damp proof course look like?
A damp proof course can be identified by a thin black line, ideally at least 150mm (6") above ground level. It may be difficult to see if a building has been re-pointed and often the damp proof course is covered by render which is bad for a couple of reasons. The first reason is that it hides the position of the damp proof course so ground levels may rise beyond it and, secondly, it may cause moisture to track behind the render and negate the properties of the physical barrier.
What causes a damp proof course to fail?
This is a frequently asked question and there are several answers. Bridging of the DPC by external ground levels can cause dampness at low level. Poor sub floor ventilation caused by blocking vents (high ground level again) can increase moisture and cause decay to floor timbers. Blocked drainage is never checked by damp proofers because they don't do drainage work (yet!), they only do damp proofing. As a material - they don't actually fail.
I've been told that my damp proof course has broken down
You have been told this by a damp proofing company who want to sell you their solutions. Your solution may be to buy a £25 spade from B&Q (other retailers are available) and reduce ground levels. DPCs do NOT break down. They are a physical material sandwiched between brick or stone and not subject too any form of decay or erosion. If you are told this, ask how it has happened and watch them squirm...
Does damp proofing work?
Damp proofing creates a physical barrier against the migration or moisture so, yes, damp proofing works. What doesn't work is when it's applied to a wall but not sealed to the floor, or not sealed to the adjacent wall or applied to a wall with a timber floor below. This allows moisture to migrate elsewhere and cause saturation.
Why is this a problem?
It is a significant problem because there is a 'source' of moisture that is never identified or dealt with. Where a house has existed for 20, 50 or 100 years or more, what has happened to suddenly make it damp? Something has happened. Whether it is caused by poor maintenance like drainage becoming blocked or some other occurrence, moisture levels have changed so surely it makes sense to find out what that 'something' is and fix it? Not so for the damp proofers who only make money by applying damp proofing.
Why is this a problem?
It is a significant problem because there is a 'source' of moisture that is never identified or dealt with. Where a house has existed for 20, 50 or 100 years or more, what has happened to suddenly make it damp? Something has happened. Whether it is caused by poor maintenance like drainage becoming blocked or some other occurrence, moisture levels have changed so surely it makes sense to find out what that 'something' is and fix it? Not so for the damp proofers who only make money by applying damp proofing.
What does damp proofing do to my house?
Damp proofing ignores the causes of the moisture so moisture is still entering or affecting your house behind the damp proofing and will eventually rise (damp proofing has created 'rising damp') beyond the damp proofing and continue to take effect. What is fundamentally significant is that moisture acts over a large area so if it is affecting lower walls, it is also affecting the floor structure. What is below the floor? Timber joists. Timber decays in the presence of moisture so covering walls in a waterproof membrane will make these timbers even wetter and ensure they decay. Where damp proofers also replace timbers, they will also rot since the moisture is still entering through the same defect that has never been remedied.