What Is a DPC (Damp Proof Course) and Why Every Building Needs One

A damp proof course is one of those details that's genuinely easy to skip on a tight budget, because a building without one looks completely fine on the day it's finished. The problem only shows up months or years later, slowly, as rising damp works its way up the wall — which is exactly why it's worth understanding what it actually does before deciding it's a line item you can cut.

What problem it's actually solving

Brick and concrete are both porous at a small scale, full of capillary-sized pores that can draw water upward against gravity, the same way a thin tube pulls liquid up through capillary action, or a paper towel wicks up a spill. A wall standing on damp ground, or in soil where the water table sits close to the surface, will slowly draw moisture up from the foundation into the wall above — a phenomenon called rising damp. Left unchecked, this shows up as damp patches, efflorescence (the white salt deposits that appear as moisture evaporates and leaves dissolved salts behind), peeling paint, and over the long term, actual deterioration of plaster and even the masonry itself.

What a DPC actually is

A damp proof course is a continuous, impermeable layer built into the wall specifically to block that capillary path — a barrier moisture simply cannot cross. It's not a coating applied to a finished wall; it's a layer built directly into the masonry itself, so the wall is physically discontinuous at that point from a moisture standpoint, even though it's structurally continuous.

Where it goes

The standard, most critical location is a horizontal DPC just above ground level and just below the floor level of the plinth — the layer that stops moisture from the ground travelling up into the walls above. But rising damp isn't the only path water takes into a building, so DPC (or DPC-equivalent waterproofing) shows up in several other locations too:

• Horizontal, at plinth level — the main defence against rising damp from the ground.
• Vertical, on the external face of a basement or retaining wall — protects against moisture pushing in sideways from wet soil in contact with the wall.
• Below window sills and above lintels/door and window openings — stops water that gets in around the opening from tracking sideways into the wall.
• At roof-to-parapet junctions and other roof penetrations — a different but related waterproofing detail, protecting against rainwater ingress from above rather than rising damp from below.

Common DPC materials

A few standard options, each with different cost and durability trade-offs:

• Cement concrete DPC — a dense, low water-cement-ratio concrete layer (often with a waterproofing admixture), typically 40–50 mm thick, commonly used across the full plinth width. Economical and straightforward with standard site materials.
• Bitumen felt / bituminous membrane — a flexible, rolled membrane laid across the wall width, effective and widely used, particularly where some flexibility for minor movement is useful.
• Plastic / polythene sheet — a low-cost option, though more vulnerable to puncture during construction than a felt or concrete DPC if not handled carefully.
• Stone / brick with waterproofing additive — historically used in some regional practice, dense stone laid with a waterproofing mortar layer.

What happens if it's skipped

The damage isn't cosmetic-only, even though it often starts that way. Persistent rising damp keeps masonry and plaster continuously wet at a low level, which accelerates deterioration of the plaster and, over enough time, can affect the masonry and any embedded metal (like door/window frame fixings) through ongoing moisture exposure. It also creates a persistently damp indoor environment at floor level, which has real implications for both the building's finishes and the comfort and health of whoever's living or working in it. None of this happens on day one, which is exactly what makes it easy to skip and expensive to fix retroactively — retrofitting a DPC into an existing wall (via chemical injection or physical insertion) is a real, disruptive, and costly repair, compared to building it in correctly the first time for a modest fraction of the wall's overall cost.