Damp-Proof Course (DPC)

A horizontal moisture barrier built into the walls of a building, usually near ground level, to prevent rising damp — moisture from the ground travelling upward through masonry.

A damp-proof course (DPC) is a waterproof barrier built into the walls of a building to stop moisture from the ground rising up through the brickwork or stonework. In modern construction, it’s typically a strip of plastic (polyethylene) or bitumen felt laid between courses of bricks, usually about 150mm (6 inches) above external ground level. Older buildings may have a DPC made of slate, engineering bricks, or lead — or may have no DPC at all.

Why it matters

Without an effective damp-proof course, ground moisture is drawn upward through masonry by capillary action — a problem called rising damp. Signs include:

  • Damp patches or staining on lower walls
  • Peeling wallpaper or bubbling paint near the skirting
  • A “tide mark” line on walls, typically up to about 1 metre high
  • Musty smell in ground-floor rooms
  • Salt deposits (efflorescence) on wall surfaces
  • Deteriorating plaster at low level

When DPC issues arise during renovation

Damp-proof course problems often surface during renovation because:

  • Old or failed DPC — in older properties, the original DPC may have deteriorated or been bridged by raised ground levels, patios, or render that covers it
  • Ground floor changes — lowering floors, removing old concrete, or changing floor construction can expose DPC issues
  • Extensions — new walls must include a DPC that connects properly to the existing building

Types of DPC remediation

If your existing DPC has failed or doesn’t exist, solutions include:

  • Chemical injection DPC — a liquid is injected into the mortar course to create a new moisture barrier. This is the most common retrofit method
  • Physical DPC — a new physical membrane is inserted into a cut in the mortar (more disruptive but very effective)
  • Electro-osmotic DPC — uses a low electrical charge to repel moisture (less common, mixed effectiveness)

DPC vs. tanking

A DPC protects against rising damp in above-ground walls. Tanking is a different treatment used to waterproof below-ground structures like basements and cellars, where water pressure comes from all directions — not just from below.

Tips for homeowners

  • Check the DPC before starting any ground-floor renovation — it’s much easier to fix before new floors and plaster go in
  • Don’t bridge the DPC — ensure external ground levels, paths, and patios sit below the DPC line
  • Get a specialist survey if you suspect rising damp — not all damp is rising damp, and misdiagnosis leads to wasted money
  • Budget for DPC work in your contingency if you’re renovating an older property