# What is a damp proof course and how do you know if it has failed?

Source: https://dampanddry.co.uk/blog/damp-proof-course-explained
Published: 2026-10-12
Publisher: Damp and Dry

**In short:** A damp proof course (DPC) is a waterproof layer built into the base of a wall, usually just above outside ground level, to stop moisture rising from the ground. A damp proof membrane (DPM) does the same job under a solid floor. Most problems blamed on a failed DPC are actually caused by bridging, such as raised soil or paths, or by condensation or leaks.

Damp proof courses are a frequent source of confusion and, sometimes, of expensive work that does not solve the problem. Many older UK homes were built with a DPC of slate, bitumen felt or engineering brick; modern homes use plastic. Some very old buildings have none at all. This guide explains what a DPC and DPM are, how they can be bypassed, the signs that might point to a problem, and why a survey should come before any treatment.

## What a DPC is

A DPC is a horizontal barrier laid in the mortar joint of a wall, normally at least 150 mm above outside ground level. Brick and stone are porous, so without a barrier, ground moisture could soak upwards. In a cavity wall there is a DPC in each leaf. You can sometimes see it as a thin line of black material or slate in the mortar course near the bottom of the outside wall.

Many UK homes built from the late nineteenth century onwards have a DPC of some kind. Older buildings without one were often built of materials and with details, such as breathable lime mortars and renders, that let moisture evaporate freely.

## What a DPM is

A damp proof membrane is a sheet, usually heavy polythene, laid under a solid concrete floor to stop ground moisture coming up through it. It should link to the DPC in the walls so there is no gap at the edge. Timber suspended floors do not have a DPM but rely on a ventilated void under the boards, kept dry by air bricks.

## How a DPC gets bridged

A DPC only works if moisture has no way around it. Common bridges are flower beds, soil, patios or paths built up against the wall above the DPC; render or pebble dash taken down over the DPC to the ground; debris in the bottom of a cavity; and internal plaster that touches a damp floor. Blocked air bricks under timber floors also let moisture build up.

Fixing a bridge is often simple and cheap: lower the ground level next to the wall, cut render back above the DPC, or clear air bricks.

- Soil or beds piled against the wall
- Patios or paths less than 150 mm below the DPC
- Render taken down to ground level
- Debris in the cavity
- Blocked air bricks under timber floors

## Signs that might point to a DPC problem

Damp from ground moisture usually shows as a band of damp, tide marks and white salts on ground floor walls, rising from the skirting to roughly a metre, present all year and not just in winter. Timber skirting and floor edges may rot. A musty smell is common.

These signs overlap with condensation at the base of cold walls and with penetrating damp from leaking pipes or splashing rainwater. Our guide on [signs of damp walls](https://dampanddry.co.uk/blog/signs-of-damp-walls) shows how the patterns differ, and [damp meter readings explained](https://dampanddry.co.uk/blog/damp-meter-readings-explained) explains why meters can mislead on salty walls.

| Cause | Clues |
| --- | --- |
| Bridged DPC | Raised ground or render outside at that spot |
| Condensation | Worse in winter, cold corners, mould |
| Leaking pipe | Localised, near plumbing, constant |
| Splashback from rain | Outside surface wet after rain, no gutter or a leaking one |
| No DPC (older buildings) | Whole wall base affected, older construction |

Low-level damp: possible causes

## Get a survey before treatment

Injected chemical DPCs are widely sold, but if the cause is condensation, a leak or a bridge, injection will not help. Ask for an independent survey from a surveyor who does not also sell the remedy, for example a RICS or a qualified independent damp surveyor. For older solid-walled and listed buildings, specialist advice matters, because modern impervious materials can make damp worse.

While you wait, keep rooms ventilated and heated, and use a dehumidifier to control condensation so the true cause is easier to see.

[See dehumidifiers for damp rooms](https://dampanddry.co.uk/best/best-dehumidifier-for-condensation)

## What about the cost?

Costs for damp proofing vary widely with the property, the cause and the work needed, so we do not quote prices. Simple fixes such as lowering a flower bed or clearing air bricks cost little. Replastering after any treatment is often a large part of the bill. A good survey can save money by ruling out unnecessary work. Our guide on [how to treat damp walls](https://dampanddry.co.uk/blog/how-to-treat-damp-walls) covers the cheaper fixes first.

## Frequently asked questions

**How do I find my damp proof course?** Look at the outside base of the wall for a thin line of black material or slate in a mortar joint, usually at least 150 mm above ground. In rendered houses it may be hidden.

**Can a damp proof course fail?** Old materials can degrade, but bridging is a far more common reason for damp at the base of a wall.

**Do I need a new damp proof course?** Only if an independent survey finds the existing one has failed or is missing and other causes have been ruled out.

## Buying guides on this topic

- https://dampanddry.co.uk/best/best-dehumidifier-for-condensation
- https://dampanddry.co.uk/best/best-dehumidifier-uk
- https://dampanddry.co.uk/best/best-dehumidifier-for-garage-and-cellar

## Sources

- [RICS](https://www.rics.org)
- [Historic England](https://historicengland.org.uk)