“You need a new damp proof course” is one of the most common recommendations homeowners and buyers hear, and one of the most expensive to get wrong. Sometimes it’s exactly right. Often the existing DPC is fine and the damp has another cause, in which case a new one costs money and solves nothing. Here’s how to tell the difference.
What a damp proof course is
A damp proof course, or DPC, is a horizontal barrier built into a wall just above ground level to stop moisture from the ground rising up through the masonry. Requirements for them spread through building byelaws in the late nineteenth century, so most houses from the Victorian era onward have one. You’ll usually find it roughly 150mm above external ground level, often visible as a thin line in the mortar joint.
The materials changed over time:
- Slate — two courses of slate bedded in mortar, common in Victorian and Edwardian houses. Durable, but can crack if the wall moves.
- Bitumen felt — common through much of the twentieth century.
- Engineering brick — dense, low-absorption bricks laid as a course near the base of the wall.
- Plastic (polyethylene) — the modern standard.
Many older buildings, particularly stone cottages and pre-Victorian properties, were built without one at all, and were designed to manage moisture differently.
Physical DPC vs injected (chemical) DPC
When a DPC is replaced or added to an existing wall, there are two broad approaches:
- Physical insertion — cutting into the mortar joint in sections and inserting a new membrane. Effective and permanent, but disruptive and not suitable for every wall.
- Chemical injection — drilling holes along the base of the wall and injecting a water-repellent cream (usually silicone-based) that forms a barrier in the masonry. Far more common, because it’s quicker and cheaper.
Chemical injection can work where the diagnosis is right and the installation is done well. But its effectiveness depends heavily on the wall — thick stone walls with rubble cores, and walls with irregular joints, are much harder to treat reliably than a standard brick wall.
The part that’s easy to miss: the replastering
An injected DPC on its own rarely fixes the visible problem. Years of rising moisture leave the wall plaster contaminated with salts, and those salts attract moisture from the air. So a DPC injection is almost always sold with hacking off and replastering the lower part of the wall, often in a salt-resistant render. A lot of the apparent success of a DPC job is actually the new plaster, which would sometimes have helped on its own.
When a new DPC genuinely is the answer
A new or replacement DPC makes sense when an investigation shows:
- the existing DPC has physically failed or is absent, and
- dampness is rising from the ground in the pattern you’d expect, and
- there’s no simpler cause — no bridging, no high ground, no leaking drain, no condensation or penetration being mistaken for rising damp.
When it isn’t
A new DPC is the wrong answer when the existing one is working but is being bypassed. The most common culprits are external ground or a path raised above the DPC, render carried down over it, or a solid floor laid higher than the original DPC. Injecting a new DPC above a bridged one doesn’t solve the bridge. Neither does it help with condensation or penetrating damp, which are more common than true rising damp.
It can also be the wrong answer in an older solid-walled building, where the real issue is often impermeable modern finishes trapping moisture that the wall was designed to release. We explain that in more depth in what rising damp is and how to fix it properly.
What drives the cost
If a DPC is genuinely needed, the price depends on:
- the length of wall to be treated, and whether it’s external walls only or internal walls too;
- wall thickness and construction — solid stone costs more than a single brick leaf;
- the extent of replastering, which is often the larger part of the job;
- access, including fitted kitchens, bathrooms and flooring that have to come out and go back;
- redecoration and making good afterwards.
Be wary of comparing quotes on the injection price alone. Two quotes can differ mainly in how much plaster they remove and what they replace it with.
Guarantees, and what they don’t cover
Treatment companies often offer long guarantees, sometimes 20 or 30 years. Read the terms. Many guarantees only cover the specific treatment installed, exclude damp from other causes, and depend on the company still trading. A guarantee is not a substitute for a correct diagnosis.
Get the diagnosis before the quote
The single most useful thing you can do before paying for a DPC is to have the damp diagnosed by someone who doesn’t sell the treatment. Our independent damp and timber surveys are exactly that: we identify the actual cause, tell you honestly whether a new DPC is needed, and recommend the simplest fix that will work. Send us the address and what you’re seeing.
Sources and further reading
- Approved Document C: site preparation and resistance to contaminants and moisture — Building Regulations guidance (Planning Portal)
- Damp in historic buildings — Historic England
- Homeowner advice hub — Property Care Association