DPC Meaning: What is a DPC?

What does DPC stand for?

DPC is an acronym for Damp Proof Course. It refers to a horizontal layer of water-resistant material built into the walls of a building to prevent moisture from rising up through the structure from the ground.

The DPC sits in the mortar bed between two courses of bricks or blocks, forming a continuous barrier around the building's perimeter. Its purpose is to interrupt capillary pathways that would otherwise allow groundwater to climb through porous masonry materials like brick, stone, and mortar.

Without a DPC, moisture from damp ground can rise to heights of 1-1.5 metres above ground level, causing damp patches, plaster damage, and timber decay in ground floor rooms.

How does a DPC work?

Porous building materials contain microscopic tubes (capillaries) that draw water upward by capillary action, similar to how a sponge soaks up liquid. In typical UK brickwork, this capillary rise can reach 1-1.5 metres above ground level.

A DPC breaks this capillary pathway by inserting a non-porous horizontal layer that water cannot pass through. The groundwater climbs through the masonry below the DPC but stops when it reaches the barrier. The wall above the DPC remains dry.

For a DPC to work effectively, it must:

• Be continuous around the entire building perimeter with no gaps.
• Sit at least 150mm above external ground level to avoid bridging by soil or paving.
• Connect to any damp proof membranes (DPMs) under floors to form a complete moisture barrier.
• Not be bridged by external render, internal plaster, or other materials that provide an alternative moisture path.

What is a DPC made from?

DPC materials have evolved over time. The choice depends on the age of the building and construction methods:

Modern DPCs (1960s onwards)

Plastic sheeting: Polyethylene or polypropylene membranes, typically 0.5-1mm thick. Flexible, durable, and cheap (under £5 per linear metre). This is the standard for all new builds.

Plastic DPCs comply with BS 6515:1984, which specifies that DPC materials must have water absorption below 0.03 litres per square metre and be able to accommodate building movement without tearing.

Mid-20th century DPCs (1920s-1970s)

Bitumen felt: Hessian fabric impregnated with bitumen. Effective when new but becomes brittle after 50-70 years. Many bitumen felt DPCs installed in the post-war period are now failing and require replacement.

Victorian and Edwardian DPCs (1875-1920)

Slate: Two courses of overlapping Welsh slate embedded in the mortar bed. Very durable but expensive and labour-intensive. Slate DPCs can last 100+ years if undisturbed.

Engineering bricks: Two or three courses of Class B engineering brick (water absorption under 7%). Still compliant with modern Building Regulations and used in high-specification work.

Pre-1875 buildings

Most buildings constructed before the Public Health Act 1875 have no DPC at all. They relied on breathable lime mortar, thick walls, and good external drainage to manage ground moisture.

DPC vs DPM: what's the difference?

DPC and DPM are often confused, but they refer to barriers in different locations:

• DPC (Damp Proof Course): Horizontal barrier in walls, typically 150mm above ground level.
• DPM (Damp Proof Membrane): Horizontal barrier under concrete floor slabs or vertical barrier behind basement walls.

In modern construction, the DPM under the floor slab connects to the DPC in the wall to form a continuous moisture barrier around the building's perimeter. This is required by Building Regulations Approved Document C (2022).

In older buildings, the terms are sometimes used interchangeably, but the principle is the same: a DPC blocks moisture movement through walls, while a DPM blocks moisture movement through floors or basements.

Where is a DPC installed?

Building Regulations Approved Document C (2022) specifies that DPCs must be installed at least 150mm above the highest level of adjacent external ground, paving, or drainage.

This 150mm clearance serves two purposes:

• It places the barrier above the typical height of ground splash from rainwater hitting the ground near the wall.
• It ensures that minor changes in ground level (settlement, soil build-up) do not bridge the DPC.

In practice, the DPC is built into the third or fourth course of brickwork above ground level, depending on brick height. On sloping sites, the DPC is stepped to maintain the 150mm clearance as the ground level changes.

When did DPCs become mandatory in UK buildings?

The history of DPCs in UK construction follows the development of building standards:

• Pre-1875: No DPC requirement. Buildings relied on traditional construction methods to manage moisture.
• 1875: The Public Health Act introduced minimum building standards for new construction. DPCs became common practice but were not strictly enforced.
• 1936: The Public Health Act was updated, making DPCs a de facto requirement for local authority approval, though enforcement varied by region.
• 1985: Building Regulations Approved Document C made DPCs a legal requirement for all new buildings. This is the first time DPCs were mandatory nationwide.
• 2004 and 2022: Approved Document C was revised to clarify DPC installation standards, including requirements for connection to DPMs and clearance above external ground.

Any building constructed before 1985 may have a substandard or missing DPC, depending on the local authority's enforcement at the time. Buildings from before 1875 almost certainly lack a DPC unless one has been retrofitted.

How do you retrofit a DPC?

If a building lacks a DPC or the existing DPC has failed, two retrofit methods are available:

Chemical injection DPC

Holes are drilled into the mortar bed at 100-120mm intervals, 150mm above external ground level. A water-repellent cream (silicone or silane-based) is injected under pressure. The chemical diffuses through the masonry and forms a water-resistant zone within 2-4 weeks.

Cost: £40-70 per linear metre. Most contractors offer a 20-30 year guarantee. This is the least disruptive method and is suitable for most solid-wall properties.

Physical DPC insertion

A horizontal slot is cut through the wall at DPC height using a specialist saw. A new plastic or slate membrane is inserted into the slot, and the slot is re-mortared. This creates a guaranteed physical barrier but is highly disruptive and requires temporary propping to prevent wall collapse during cutting.

Cost: £80-150 per linear metre. Used where chemical injection has failed, where walls are too thick for effective chemical diffusion, or where a structural guarantee is required (for example, to satisfy mortgage lenders).

What causes a DPC to fail?

Even when a DPC exists, rising damp can occur if the barrier is damaged or bypassed:

• Bridging: External render or raised ground levels create a moisture path that bypasses the DPC. This is the most common cause of rising damp in modern buildings with a DPC.
• Material degradation: Bitumen felt DPCs degrade after 50-70 years. Slate can crack due to structural movement.
• Poor installation: Gaps in the DPC or failure to lap joints properly leave capillary pathways open.
• Structural movement: Subsidence or settlement can tear a DPC, particularly plastic sheet types.

BRE Good Repair Guide 6 (2005) found that 60% of rising damp cases in buildings with a DPC are caused by bridging rather than DPC failure. Always check for bridging before assuming the DPC needs replacement.

Do I need to retrofit a DPC?

Not always. Many old buildings have lived without a DPC for over a century and show no signs of damp, particularly if they:

• Have breathable lime mortar and plaster that allows moisture to evaporate naturally.
• Maintain good external drainage with ground sloping away from the building.
• Have adequate clearance between ground level and wall base.
• Avoid impermeable cement render or gypsum plaster that traps moisture inside walls.

Historic England's guidance (Remedying Damp in Historic Buildings, 2016) advises against unnecessary DPC installation in traditional buildings, as it can disrupt the building's natural moisture management and cause more harm than the damp itself.

Only retrofit a DPC if you have confirmed rising damp (not condensation or penetrating damp from leaks) and have ruled out simpler solutions like improving drainage or lowering external ground levels.

Sources

• Building Regulations Approved Document C (2022): Resistance to moisture
• BS 6515:1984: Specification for polyethylene damp-proof courses for masonry
• BRE Good Repair Guide 6 (2005): Treating rising damp in houses
• Historic England (2016): Remedying Damp in Historic Buildings
• Public Health Act 1875 and 1936: Building standards legislation