What Is a Damp Proof Course?

A damp proof course, or DPC, is a horizontal barrier built into a wall near ground level to stop moisture rising from the ground into the structure above.

What it looks like

In most houses built since the late nineteenth century, a layer of slate, bitumen or plastic membrane a few courses above ground. In older buildings there may be none at all, and the wall was designed to handle moisture differently.

Failure versus bridging

This distinction is worth understanding, because it determines whether an expensive treatment is the right answer.

Failure means the DPC itself has broken down. Less common than the volume of remedial work sold suggests.

Bridging means the DPC is intact but moisture is getting past it, external ground level raised above it, render carried down over it, debris in a cavity, or a path built up against the wall.

Bridging is common and is usually fixed by removing the bridge, not by injecting the wall.

Before agreeing treatment

Get a diagnosis from someone who does not sell the treatment. Damp is the most misdiagnosed problem in home improvement, and injection is regularly sold for conditions it cannot fix.