A damp-proof course (DPC) is a barrier built into the base of walls to stop ground moisture from rising into a property. Modern homes normally have a DPC – usually a plastic or bituminous membrane bedded into the brickwork – to prevent water moving upwards through capillary action. Without it, moisture from the ground can travel up through bricks, mortar and plaster, causing peeling paint, damp patches, mould and rot.
DPCs have been required in new buildings since the 19th century, but many older homes either had no DPC or relied on early methods such as slate or bitumen, which often fail over time. If your walls are consistently damp at the base, it may indicate that your DPC is missing, damaged or bridged.
Common Types of DPC
Physical DPC
A physical DPC is built into the wall during construction. It might be made from slate, engineering bricks, bitumen felt or modern plastic sheeting. It is placed in the mortar bed and spans the full thickness of the wall. The purpose is simple: create an impermeable barrier that water cannot cross.
Chemical DPC
A chemical DPC (or damp-proof injection) is used when retrofitting damp-proofing to an existing structure. Small holes are drilled into the mortar joints, and a damp-proofing cream is injected under pressure. The cream penetrates the wall and cures to form a continuous water-repellent band. Chemical DPCs are installed in accordance with established industry standards, including the British Standard for rising damp treatment (BS 6576).
Physical DPCs are ideal during construction, while chemical injections are suitable for older homes where inserting a physical membrane would be too intrusive or impractical.
Proper Installation of a DPC
For a DPC to be effective, it must be installed correctly. Key principles include:
Continuous barrier
The DPC must run unbroken along the entire wall and extend fully through its thickness. In cavity walls, both the inner and outer leaves require their own DPC. Any gap or weakness allows moisture to bypass the system.
Correct height
UK Building Regulations require a DPC to be at least 150 mm above external ground level. This keeps it clear of splashback from rainwater and prevents the ground from sitting against the barrier. In many cases, installers aim for 150–225 mm depending on site conditions.
Avoiding bridging
Ground levels, soil, concrete, render or internal plaster must not cover the DPC. If the DPC line is buried or overlapped, moisture can “bridge” over it and rise into the wall. For this reason, the DPC must remain visible (or at least verifiable) and must not be obstructed.
Placement
The DPC should sit in the lowest course of bricks above the foundations. When adding extensions, steps or raised patios, care must be taken not to interfere with the existing DPC line. If ground levels vary or slope, the DPC can be stepped to maintain altitude above the ground.
UK Regulations and BS 6576 Guidance
UK Building Regulations (Approved Document C) require a DPC to be installed in all new masonry walls. The key requirement is clear: the DPC must protect the wall from ground moisture and be positioned at least 150 mm above ground.
For remedial work, the British Standard BS 6576 provides guidance on correctly diagnosing rising damp and installing chemical DPC systems. It outlines drill patterns, injection spacing, dosage and curing requirements to ensure a continuous and effective barrier. Reputable installers follow these standards to guarantee proper performance.
Detecting DPC Failure
A DPC can fail or become ineffective for several reasons, including age, damage, poor installation or bridging. Common signs of rising damp include:
Tide marks and salt staining
Look for a band of discolouration or white, powdery salts (efflorescence) up to around 1–1.2 m above floor level.
Peeling or blistering paint
Paint, plaster or wallpaper lifting near the bottom of walls is a classic symptom of rising moisture.
Mould and musty smells
Damp, stale odours or black mould near skirting height often indicate moisture rising from the base of the wall.
Rotting timber or plaster
Soft, rotten skirting boards, decayed floor joists or crumbling plaster are signs that the wall has been wet for a long time.
Bridging
Check outside: if soil, gravel, concrete or render sits above the DPC line, moisture will bypass the barrier. Landscaping and building work commonly create bridging problems.
Older homes
Pre-1900 houses often lack a proper DPC. Others may have early DPC materials that have deteriorated over time. If you live in an older property, a missing or ineffective DPC is a common cause of rising damp.
A professional damp survey is the most reliable way to diagnose rising damp accurately. Surveyors use moisture meters, thermal imaging and salt tests to distinguish rising damp from condensation or penetrating damp.
Replacing a Damp-Proof Course
Replacing a physical DPC is a major job. It involves removing bricks or blocks in stages, inserting a new membrane and rebuilding the wall section by section. This is labour-intensive, disruptive and expensive.
For this reason, most modern treatments use a chemical DPC injection. Holes are drilled, and a silicone-based cream is injected to form a new water-repellent barrier inside the wall. This approach is far less disruptive and generally very effective when carried out by experienced professionals.
DIY DPC creams exist, but incorrect spacing, uneven injection or insufficient dosage often lead to failure. The barrier must be continuous—any missed section allows moisture to rise again. This is why the task is best left to trained specialists.
New Builds vs Older Homes
New builds
Modern houses are constructed with a physical DPC installed at the correct level. Rising damp is rare unless the DPC becomes bridged by landscaping or poorly installed extensions. Most damp issues in newer homes come from leaks, cold bridging, or condensation rather than rising damp.
Older homes
Older buildings often have DPCs that deteriorate, were poorly made, or never existed at all. Retrofitting a chemical DPC is the most common approach, often combined with re-plastering or internal membrane systems to manage salts and moisture in historic walls.
When to Call a Professional
Rising damp is frequently misdiagnosed, so proper assessment is essential. If you notice tide marks, salt deposits, damp skirtings or peeling paint near the floor, book a professional damp survey.
A specialist will:
• assess moisture levels
• inspect the wall structure
• test for salts
• identify whether bridging, leaks or true rising damp is present
• recommend the correct treatment
Nordic Damp Proofing provides Damp Surveys and Rising Damp Treatment across Southampton, Dorset and Hampshire. Our team follows British Standards and industry best practice to diagnose and resolve rising damp properly.
Proper DPC installation and repair are not simple DIY tasks. Getting it wrong leads to wasted money, damaged walls and ongoing damp. Always consult an expert if you’re unsure.
We Find It. We Treat It. We Stop It Coming Back.
Whether your home is modern or historic, a functioning DPC is essential for a dry, healthy property. Catching early signs and ensuring your DPC is intact will protect your home for decades to come.
