Most UK homes have something called a damp-proof course. It’s a layer of slate, bitumen, plastic, or engineering brick designed to stop moisture rising up through the walls. It’s one of the simplest and most effective parts of the structure, yet it often gets blamed for damp problems it hasn’t caused.
DPCs have been used in British homes since the late 1800s. After the Public Health Act of 1875 gave local authorities the power to tighten up construction standards, London introduced a byelaw in 1878 requiring DPCs in new builds. Other cities followed. Slate was common in some areas, lead or engineering bricks in others, but the principle stayed the same: create a moisture barrier at low level that water couldn’t cross. And unless something has bridged it or altered the original detail, most of these early DPCs still work perfectly well today.
Current Building Regulations haven’t changed the fundamentals. A DPC still needs to sit at least 150mm above external ground level, it must connect with the damp-proof membrane in the floor, and in cavity construction, you’ll usually find one in both the inner and outer leaves. Sometimes the DPC is a plastic strip, sometimes it’s a row of slate, and in some builds, it’s something more substantial. One example is engineering bricks. These dense, low-absorption bricks are often used as the DPC themselves. In some cases, they form the full moisture barrier. In others, they’re paired with a plastic or felt layer just above. It’s not excessive. It’s just good detailing. If you see engineering bricks near the base of the wall, they’re not decorative. They’re doing a job.
So if most homes have a DPC, and most of those DPCs are still intact, why is damp still such a common issue? The answer is simple. The DPC hasn’t failed. Something has been added or changed around it, and that change is allowing moisture to bypass it. That’s what we call bridging, and it’s the real cause in most cases where people are told they have rising damp.
Common culprits include raised patios, flowerbeds, or driveways that now sit higher than the DPC. Leaking gutters or downpipes can soak the wall below the barrier. External wall insulation or render can cover the DPC and allow moisture to track across. Steps or boundary walls may sit too close to the house. Even internal plaster can bridge a DPC if it runs all the way down to a solid floor. And inside cavity walls, it’s not unusual to find rubble or mortar droppings creating a solid link between the leaves. These aren’t failures of the DPC. These are external factors that make the DPC irrelevant.
This is where the “solution” often gets expensive. You notice damp, you call someone in, and before long they’re quoting for chemical injection. The idea is that by drilling holes into the wall and injecting a silicone-based cream, they can recreate the DPC and stop moisture from rising. But if the real issue is bridging, and it almost always is, then the injection won’t solve anything. Moisture will keep coming in because the route around the DPC is still there.
Independent surveyor Neil Marsden puts it well. “If a property has a physical DPC, and most do, it rarely fails. The cause is almost always bridging or water ingress elsewhere.” And he’s right. We’ve seen walls injected more than once by different companies using slightly different products, and the issue stayed exactly the same. As Neil points out, telling someone to lower their patio doesn’t pay nearly as well as selling them a chemical cream.
The actual solution is usually far more straightforward. Fix the leaking downpipe. Reduce ground levels. Cut back render or remove bridging plaster. Clear out debris in the cavity. It’s not glamorous, but it works. And it usually costs a fraction of what’s being quoted for injection work.
The other problem is trust. Most people offering to inspect your damp issue are also quoting for the fix. That’s not objective advice. It’s a sales pitch. At Survey Shack, we don’t treat damp and we don’t sell solutions. We provide insight, backed by proper research, so homeowners can see what’s really going on and decide their next move based on facts rather than fear.
So the next time someone tells you your DPC has failed, ask them what’s bridging it. If they can’t answer, or haven’t even looked, then they’re probably not giving you the full story.
Damp doesn’t appear out of nowhere. It’s usually the result of something being added, blocked, or overlooked. The DPC is rarely the issue. The problem lies in what’s been built around it.
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