Envelope and protection · Sealants & Membranes
Damp-Proof Course (DPC)
Describes the damp-proof course as a built-in walling component and separates it from the damp-proof membrane, while leaving all positioning, diagnosis and remedial work to qualified professionals.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What damp-proof course is
A damp-proof course is a moisture-resisting strip or course incorporated into walling as the wall is built. Modern versions are most often a flexible polymer strip supplied in rolls to suit the walling, although other forms exist, including courses of dense engineering brick and the slate courses found in many older buildings.
The distinguishing feature is that it is part of the wall, laid in as construction proceeds rather than applied to a finished surface. That makes it a component of the masonry sequence: it is bedded, lapped where lengths meet, and connected to whatever the design requires it to connect to at openings, at abutments and where a floor construction meets the wall.
It is constantly confused with a damp-proof membrane. A membrane is a continuous area layer associated with floors and slabs; a damp-proof course is a linear component within walling. Both control moisture, both fail through discontinuity, and the junction between them is one of the details a qualified professional draws. Where a damp-proof course goes, and how it relates to the ground around the building, is design work for the specific building rather than a general rule.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- DPC
- Damp proof course
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Moisture-resisting courses built into masonry walling as the wall is constructed.
- Cavity trays and linear components associated with openings, abutments and changes in construction.
- Preformed units used at corners, stop ends and other points where a straight run changes direction.
- Strips used where a wall meets a different form of construction and the design calls for separation.
- Courses of dense engineering brick or slate serving the same role in some traditional walling.
Consideration
Where it is commonly considered
- Where new masonry walling is being built and the design team has set out the moisture control strategy.
- Where an extension or a new opening interrupts existing walling and continuity has to be re-established by design.
- Where the wall meets a different construction type and a designer has specified a separating component.
- Where a manufacturer's cavity tray or preformed component matches the walling actually being used.
- Where the design shows how the walling and floor moisture control layers are joined to each other.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- As a response to damp observed in an existing wall, which requires a qualified professional to inspect and identify the cause.
- Where existing walling is being altered without anyone establishing what is already built into it.
- In below-ground walling, where waterproofing is a specialist design discipline rather than a component choice.
- Where ground levels, drainage or paving around the building have changed and nobody has assessed the effect.
- Where the walling material and the component material have not been checked for compatibility.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The component is intended to interrupt the movement of moisture within walling. Whether it does depends on continuity, on how it is lapped and terminated, and on the wider design, not on the strip considered in isolation.
- Installation dependency
- It is built in as the wall goes up, so it cannot be added afterwards without opening the wall. Bedding, laps and stop ends are the points at which continuity is won or lost.
- Substrate dependency
- The component has to work with the walling it sits in, including the mortar, the units and any cavity construction. Compatibility and bedding are matters for the design and for the manufacturer.
- Documentation
- Because it is buried within the wall, the drawings and the site record are usually the only evidence of what was installed and where it runs.
- Repairability
- Repair is intrusive by nature, because access means taking walling apart. That is one reason damp problems in existing buildings are investigated by specialists rather than treated by assumption.
- Maintenance
- The component itself cannot be maintained, but the conditions around the wall can be: drainage, ground surfaces, gutters and the way water is shed away from the building all matter.
- Weathering and exposure
- The exposure of the wall as a whole, including driving rain and splash from adjacent hard surfaces, shapes how that wall behaves. That is an assessment for a professional looking at the actual building.
Exposure
Exposure and environmental conditions
- What is the exposure of this wall to driving rain, and which elevations are worst affected?
- Have external ground levels, paving or planting changed around the building over the years?
- Is water being discharged near the wall by gutters, downpipes or hard surfaces alongside it?
- Does the site slope towards the building, and what does that do to surface water in heavy rain?
- Is any part of this wall retaining ground, which changes the design discipline that applies?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The component is part of the masonry sequence, so bricklaying practice determines how well it is built in.
- Cavity construction, insulation and wall ties all interact with where and how the component runs.
- Openings, abutments and changes of construction are where continuity has to be re-established by detail.
- Junctions with floor construction, and with any membrane there, are detailed by the design team.
- Existing walls may contain older forms, or none at all, and that has to be established rather than assumed.
Appearance
Appearance and finish considerations
- In finished walling, little or nothing of the component is normally visible from outside.
- Where an edge shows in a mortar joint, its appearance says nothing about whether the run is continuous.
- Rendering, painting or cladding over walling can conceal the component from view entirely.
- Traditional slate courses are visible in some older buildings and read as part of the wall's character.
- Bridging by later work such as paving, render or planting is often the visible change, not the component.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- How is the component protected during construction while walling continues above it?
- Are laps and stop ends formed as the design requires at every change of direction?
- What happens at openings, where continuity most often breaks down in practice?
- Is there an inspection point while the wall is still open to view?
- How might alterations by future owners affect what has been built into this wall?
Upkeep
Maintenance and repair considerations
- Is there a drawing or record showing what was built into this wall and where it runs?
- Who would investigate if damp appeared in this wall at some point in the future?
- Are the gutters, downpipes and surface drainage around the wall being maintained?
- Has anything been added outside the wall since it was built, such as paving or a raised bed?
- What access would be needed in order to inspect or repair the component at all?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- It is laid as the wall is built, so it depends entirely on the bricklaying sequence and supervision.
- Laps, stop ends, corners and junctions follow the design details rather than site improvisation.
- Manufacturer guidance governs bedding, jointing and which accessories are compatible.
- Interaction with cavity trays, weep provision and insulation is set out by the design team.
- Damage during construction is easy to cause and hard to see once the wall is complete.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's information for the strip or component that was actually supplied.
- Design drawings showing runs, laps, stop ends and junctions with other construction.
- Photographic records taken while the walling was still open and the component visible.
- Details of any cavity trays and associated components used alongside it.
- Any survey information about what already exists in an older wall being altered.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What does the design show for this wall, including at openings and at abutments?
- How does the walling component connect to the moisture control layer in the floor construction?
- What is actually known about what already exists within the walls of this building?
- Is any part of this wall retaining ground, and if so who is designing for that condition?
- How will external ground levels and drainage around this wall be handled?
- Who is inspecting the walling while the component is still visible and can be checked?
- If damp has been observed, who will carry out the investigation before any work is specified?
- What record will be handed over describing what was built into the wall and where?
Blind spots
Commonly overlooked points
- It is a linear component within a wall, not the continuous area layer people picture when they hear the term.
- Later work outside the building, such as new paving or planting, can change how the wall behaves.
- Continuity at openings and abutments is where the detail matters most and is easiest to get wrong.
- Older buildings may have been built with a different approach entirely, or with nothing of the kind.
- Because it is buried, the only later evidence is the drawing set and the construction record.
What this page does not do
- Where a damp-proof course sits within any wall is design work for a qualified professional and is not stated in this reference.
- Damp in an existing building must be investigated on site by a qualified professional; rising damp, penetrating damp and condensation are routinely confused and cannot be separated from a description.
- Below-ground walling and retaining construction involve specialist waterproofing design rather than the selection of a walling component.
- No single component makes a wall dry; behaviour depends on the whole wall, its exposure, its drainage and its detailing together.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Sealants & Membranes
Reference entries for the thin control layers — joint sealants, damp-proof courses and membranes, vapour control layers, breather membranes, tanking systems, tapes and foams.
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