Building envelope · Control Layers
Damp-Proof Barrier Continuity System
This entry treats the barrier beneath the floor, the course within the wall and the transitions between them as a single line, so a reader can see why each is ineffective wherever they fail to meet.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What damp-proof continuity is
At and below ground level, moisture arrives from a direction the rest of the envelope never deals with: upward and sideways out of the ground, drawn by capillary action and pushed by water standing against the construction. The layers placed against it are conventionally drawn as separate items, specified in separate sections and installed by separate trades.
Read that way, the corner where the floor meets the wall belongs to nobody. The barrier under the slab stops at the slab edge, the course within the wall starts above it, and the space between them is an uninterrupted path from the ground into the wall regardless of how well either product performs across its own area. Naming that path as the subject is why this entry exists.
The same logic runs through every interruption at this level: where a drain or an incoming service crosses the barrier, where the floor build-up rises to meet a door frame, where a cavity meets the ground, where the construction abuts a neighbouring structure, and anywhere external ground sits higher than the internal floor surface.
Cavity trays are the nearest relative, and the boundary is easy to test by asking where the water being turned back has come from. Ground moisture arriving at the base of the construction belongs to this entry. Water already inside a cavity, intercepted at a lintel, a floor edge or an abutment higher up the wall, belongs to the cavity tray and weep system.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- DPC and DPM continuity
- damp-proof course and membrane continuity
- moisture barrier continuity
- rising damp control arrangement
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Present one continuous barrier to ground moisture across the floor, the walls and every transition between them.
- Turn the junctions at slab edges, thresholds and service entries into designed connections rather than gaps between trades.
- Keep ground moisture away from the finishes, linings and insulation placed against the construction above it.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
This system is itself the junction between two others, so there is no build-up to read. The roles below are the conditions that have to be resolved where those systems meet, and no order is implied between them.
- Control layerHorizontal barrier beneath the floor
The layer laid under or within the ground floor construction to resist moisture rising through it. Its area performance is rarely the issue; what governs is how it is carried out to the perimeter and turned up to meet what continues in the wall.
- Control layerDamp-proof course within the wall
The layer built into the wall to interrupt the upward path through masonry. It has a defined position relative to external ground and internal floor, and that relationship, rather than the material chosen, determines what it can do.
- Jointing and sealingLinking lap between the two
The overlap that joins the floor barrier to the wall course, usually formed at the slab perimeter and buried within the first courses of masonry. It is the least visible part of the system and the one most often left out of both packages.
- Edge and terminationThreshold and opening transitions
Where a door opening interrupts the wall, the barrier has to be carried through the threshold construction and connected to the frame and the floor. Level access reduces the height available for that, which makes the detail tighter rather than simpler.
- Service zoneService entries and drainage penetrations
Incoming water, power, telecommunications and outgoing drainage all cross this line, usually before the floor is complete and often before the barrier is even placed. Each crossing needs a detail rather than whatever sealant is nearest at the time.
- Edge and terminationHandover to the above-ground weather line
Above ground the weather barriers of the wall take over from this line, and the two have to be joined so that each passes water to the other in the right direction. Where that join is omitted or lapped the wrong way, both lines remain sound while the path between them leads inward. The mechanism of the handover itself is described in the control layer transition system; what belongs here is the direction of the lap, which is set by the fact that one side is resisting ground moisture and the other is shedding rain.
- ProtectionProtection from following works
Barriers at this level are laid early and then walked over, loaded, drilled and covered by other trades. Damage happens before completion and is concealed by the very construction that follows it, so protection is part of the design rather than site conduct.
- SubstrateBackground the barrier bears on
The blinding, screed, concrete or masonry that the barrier is laid on or built into. A sharp, uneven or contaminated background punctures sheet barriers and prevents bonded ones from adhering, in both cases without visible evidence afterwards.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The floor barrier and the wall course only become a system through the lap between them, and that lap is formed at the moment when neither trade regards the work as theirs. If it is missed, both components remain intact and both continue to perform across their own area, while the assembly as a whole has a continuous path from the ground into the wall. This is the clearest case in the library of a complete component producing an incomplete system.
Levels govern everything else. The position of the course relative to the external ground and the internal floor decides whether the barrier is being asked to interrupt an upward path or to hold back water standing against the wall above it, and those are different demands. Later landscaping that raises external ground can change which of the two applies without anyone touching the construction.
Penetrations and thresholds fail for the opposite reason to the field. In the open floor the barrier is undisturbed; at a service entry it is cut, at a threshold it is compressed into the least available height, and at both it has to connect to something made of a different material that arrives at a different time. The sealing components used there depend on adhesion to surfaces that are frequently damp, dusty or freshly cast.
External drainage and the barrier work against the same water from opposite sides. Where ground falls, land drainage and granular zones keep water from standing against the construction, the barrier is being asked to resist capillary movement only. Where those are absent or later altered, the same barrier is asked to hold back standing water, which is a materially different requirement and one that belongs with a qualified professional.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Control layer
Sheet, bonded and applied families are all commonly encountered forming barriers at this level, sometimes serving more than one purpose at once. Which is appropriate for a given ground condition is a matter for the manufacturer's documentation and for the designer.
Jointing and sealing
Components of these kinds are commonly encountered at laps, upstands and penetrations at this level. Whether any of them will adhere to a damp, dusty or freshly cast surface is established from the product documentation rather than from the role.
Drainage plane
These families are commonly encountered where the approach is to manage water reaching the construction rather than to resist it at the face. Which approach applies to a given site is a determination for a qualified professional assessing the ground.
Substrate
Backgrounds of these kinds are commonly encountered supporting or containing a barrier at this level. How a particular background has to be prepared before a barrier is placed on it is set by the product documentation and by the designer.
Protection
Boards of these families are commonly encountered laid over or against barriers at this level, where they also serve other purposes in the build-up. Whether a given board protects a given barrier is a matter for both sets of documentation.
Components
Building components that fill these roles
The discrete elements commonly occupying each role in this assembly. This is a different statement from the material families above: those name what a role is commonly made of, these name the positioned elements that occupy it. Commonly encountered, never recommended.
Edge and termination
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
Slab edge and perimeter
The slab perimeter is where the horizontal barrier has to be turned up and connected to what continues in the wall. Insulation, edge upstands and reinforcement all occupy the same corner, so the sequence in which they are placed decides whether the connection can still be made.
Read about Ground-Bearing Slab →Door thresholds
At a threshold the barrier, the frame, the floor finish and the external surface meet at the level where water gathers. The desire for a low or level threshold and the need for an upstand pull in opposite directions, and that conflict has to be resolved rather than deferred.
Read about Door Threshold Interface →Base of the cavity
At the bottom of a cavity, water running down the inner face of the outer leaf reaches the same zone as ground moisture rising. Which layer intercepts what, and where each discharges, is the point at which the two systems have to be drawn together.
Read about Cavity Trays →Transition to below-ground construction
Where the floor sits below external ground, the barrier stops being about capillary movement and becomes part of a water-resisting strategy for the whole structure. That strategy is designed as a whole by a qualified professional rather than assembled from ground floor details.
Read about Basement Structure →Course built into the wall
The wall course is laid by the bricklayer as part of the wall rather than as part of a damp-proofing package, which is why its position, its lap and its connection to the floor barrier have to appear on the wall drawings and not only in a specification.
Read about Load-Bearing Masonry →Drainage against the construction
Drainage placed outside the wall changes what the barrier has to resist by reducing the water standing against it. Because that drainage is often installed by a groundworks package early in the programme, its relationship to the barrier is easy to lose.
Read about Wall Base Drainage →Strip Foundation
This is the junction where buried and exposed parts of the wall meet, and where the relationship between the damp-proof course and the finished external ground level is set. Raising external levels later can defeat a detail that was sound when it was built.
Read about Strip Foundation →Raft Foundation
The barrier beneath the plate has to be connected to the damp control of the wall at the edge, which is the one place a buried plane becomes reachable. How the turn-up is protected during construction decides whether it survives to do its job.
Read about Raft Foundation →Pile Caps and Ground Beams
The damp-proof course starts on the beam rather than on the ground, so its relationship to the external ground level is set by the beam and by the finished landscape around it. Both have to be considered at the same time.
Read about Pile Caps and Ground Beams →Ground Gas Protection
The gas barrier turns up and has to be joined to the damp control at the wall, which is the one place both planes are reachable at the same time. A detail that satisfies one and ignores the other leaves a discontinuity that cannot be reached later.
Read about Ground Gas Protection →Mass Timber Structure
The base of a panel structure is where dry engineered timber meets construction that holds moisture. Separation at that line, and the ability to keep it clear of standing water and splashing, is one of the defining details of the whole system.
Read about Mass Timber Structure →Suspended Timber Ground Floor
Every point where timber bears on masonry depends on the damp course being present and unbroken. That is a continuity problem shared with the walls, and this floor is the place where a missing length of it does the most damage.
Read about Suspended Timber Ground Floor →Barrier Waterproofing
The below-ground barrier and the damp-proof provision of the walls and lowest floor have to lap into each other in the right direction and at a level related to where water can actually reach. This junction is the one place where the below-ground and above-ground moisture strategies are the same conversation.
Read about Barrier Waterproofing →Water Control Layer
At the bottom of the drained plane, water leaving the assembly meets the ground-level barriers coming the other way. The lap between them determines whether water is discharged clear of the construction or delivered behind the barrier below.
Read about Water Control Layer →Control Layer Transition
Near ground level the below-ground barrier and the above-ground control layers have to become one plane. This is the transition most often left between two packages that each assume the other has closed it, and it is difficult to reach once the ground is reinstated.
Read about Control Layer Transition →Twin-Leaf Cavity Wall
At the base the void, the ground outside and the internal floor all meet. Each leaf needs its own barrier against moisture rising from below, those barriers have to connect to whatever the floor uses, and the void needs a way out at its lowest point that the surrounding ground does not block.
Read about Twin-Leaf Cavity Wall →Solid Wall
With no void to drain, the foot of the wall is where ground moisture, splash from paving and any barrier in the masonry all meet in the same thickness. Ground raised against the wall, or an impermeable surface hard up to it, changes the behaviour of the whole base.
Read about Solid Wall →Timber Frame Wall
The sole plate sits close to the ground floor construction and to whatever is outside, so it needs separating from moisture below it while the wall's sealed layer is joined to the floor's. Both jobs are done at the same line, often by different trades.
Read about Timber Frame Wall →Internal Wall Insulation
Relevant at the foot of the lining, where the new construction meets the floor and whatever barrier the existing wall has or lacks.
Read about Internal Wall Insulation →Rendered External Insulation
Any damp-proof provision in the original wall passes at a particular level, and the render must not carry water across it or stop below it in a way that leaves an exposed edge in the splash zone. The starting line is set by that relationship, not by appearance.
Read about Rendered External Insulation →Direct Render
Render carried down past the level of a wall's damp-proof provision offers moisture a route around it, and render stopped at a raw edge lets water in behind. Where the coats terminate is therefore a moisture decision rather than a visual one.
Read about Direct Render →Brick Slip Facade
The lowest course sits close to splash and to any damp-proof provision in the wall behind. Where the facing stops, whether its bottom edge is supported and whether water can leave at that line together decide how the base of the elevation ages.
Read about Brick Slip Facade →Wall Lining
Where the wall carries a damp-resisting course near floor level, the lining and its support cross that line. What happens at the crossing - whether the support bridges it, stops short of it or is separated from it - is a detailing decision for a qualified professional and not a site improvisation.
Read about Wall Lining →Freestanding Wall
Where a boundary wall is bonded or tied into a house, the damp-proof lines of wall and house meet, and the wall can carry moisture inward or bridge the building's own barrier. The junction is also a conduction path through the external wall it meets, which is why it is usually designed rather than assumed.
Read about Freestanding Wall →Timber Deck Framing
A raised external surface changes the relationship between the ground outside and the moisture protection built into the wall. Whether the deck can abut the wall, stand clear of it, or need a separated frame is a question about the wall rather than about the deck.
Read about Timber Deck Framing →Mass Timber Wall Build-Up
The bottom of a timber wall standing on or near a slab is the point at which moisture from below, splash from outside and the end grain of the panel meet. How the panel is held clear, how it is separated from what it stands on and how the barrier turns down are all resolved at this line.
Read about Mass Timber Wall Build-Up →Adhered Facade
The bottom edge of a bonded facade is where splash, run-off and any moisture rising in the wall all arrive, and where the units are closest to freezing wet. Where the facade stops relative to the damp-proof layers, and how the bottom edge sheds water clear, is settled here rather than at the top.
Read about Adhered Facade →Underpinning
The horizontal and vertical damp-proof layers of the existing building are read as one barrier, and this work cuts through it. Where the line is remade, and whether the new element is inside or outside it, is decided as part of the scheme rather than afterwards.
Read about Underpinning →
Internal junctions
Junction conditions inside this system
Junctions between the components within this assembly, rather than at its boundary. An entry exists only where the condition between two elements governs how the assembly behaves — two parts touching is not on its own a junction worth naming.
Door Frame to Door Threshold
Where the uprights of a door frame land on the base of the opening, closing the corner at which a weathered surround meets a weathered and trafficked base.
Moisture · Weathering · Drainage · Buildability
Masonry Pier to Pier Cap
Where a masonry pier stops and the unit closing its top has to throw water clear of the masonry beneath it.
Weathering · Moisture · Movement · Buildability
Slab Edge to External Wall
Where a floor meets the external wall, forming a line that runs around the whole building at every level.
Thermal continuity · Moisture · Fire continuity · Structural transfer
Slab Edge to Sole Plate
Where a framed wall is stood and secured on the perimeter of the floor slab, at the point its load path, its insulation and its moisture separation all have to be resolved together.
Structural transfer · Moisture · Thermal continuity · Air leakage · Buildability
Sleeper Wall to Wall Plate
Where the intermediate bearing inside a suspended floor void carries the plate the floor members sit on, in a space that depends on air moving through it.
Structural transfer · Moisture · Movement · Buildability
Threshold to External Ground
Where a door opening meets the ground outside, and where keeping water out and letting people in are in direct conflict.
Moisture · Drainage · Thermal continuity · Buildability
Wall Base Damp-Proofing Junction
Where a wall's damp-proof course and a floor's damp-proof membrane must be joined so neither has an open end.
Moisture · Thermal continuity · Buildability · Structural transfer
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- Nothing in this system is damp-proof on its own; resistance to ground moisture is a property of the completed and correctly connected line, and how much moisture arrives depends on the ground, which is assessed by a qualified professional.
- Durability
- A break at this level does not announce itself immediately. Moisture moves into linings, skirtings and insulation over seasons, and by the time a stain appears the source is usually several elements away from where it shows.
- Buildability
- This line is built at the busiest and least tidy stage of a project, under weather, alongside groundworks and often before a drawing package is complete. The connections it depends on are made before anyone is watching them closely.
- Interfaces
- The floor barrier, the wall course, the threshold and the service entries each sit in a different package, so the junctions between them are contractual boundaries as well as physical ones.
- Movement
- Slabs, walls and thresholds move relative to one another as they dry and settle, so a connection made rigidly between two of them may open even where nothing was damaged.
- Documentation
- A record of where the barrier runs, what it is and how it was connected is what allows any later alteration, a new opening, a raised patio, an added service, to be judged rather than guessed at.
- Maintenance and access
- Almost none of this line remains reachable after completion, so anything that could reasonably be inspected later, such as external levels and drainage, carries a disproportionate share of the attention.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Where the floor barrier and the wall course lap into one another, and who installs that lap, should be drawn rather than described.
- Whether the relationship between external ground level and the internal floor has been fixed on the drawings is a decision that constrains everything else.
- Which service crossings are expected at this level, and whether they were known when the barrier was designed, is worth checking early.
- How thresholds reconcile a low step with the barrier that has to be carried through them is a conflict to resolve at design stage.
- Whether the ground conditions have been investigated well enough to say what the barrier is resisting is a question for the outset.
- How the barrier will be protected between being laid and being covered is a sequencing decision as much as a specification one.
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A damp-proof course is treated as a product that solves rising damp, when the path it interrupts continues wherever the course does not connect to anything.
- Ground moisture problems are assumed to come from the wall itself, when raised external levels, blocked drainage or a bridged cavity are more often the route.
- The barrier under a floor is thought of as separate work from the course in a wall, when the corner between them is the part that decides the outcome.
- Internal treatments are seen as a substitute for continuity, when applying a coating to a wall face does not restore a line that was never joined up.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- How does the barrier beneath the floor connect to the course within the wall, and at what point in the programme is that connection made?
- What is the relationship between external ground level, the course and the internal floor level in this design?
- Which services cross this line, and how is each crossing detailed rather than sealed on site?
- What has the ground investigation established about water levels and about how much water may stand against the construction?
- How is the barrier protected from damage between installation and being covered by the following trades?
- If external levels were raised later by landscaping, what effect would that have on this arrangement?
What this page does not do
- No membrane, course or coating is damp-proof in isolation; behaviour belongs to the completed, correctly connected and correctly protected assembly.
- Ground conditions, water levels and contamination vary by site and are established by investigation rather than assumed from the type of construction.
- Where ground gas or contamination may be present, the barrier at this level forms part of a separate protection strategy designed by qualified professionals.
- Diagnosing damp in an existing building requires investigation of the whole construction, since treatments applied to a symptom rarely address the path carrying moisture.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Commonly built alongside
Systems routinely present in the same building without a junction recorded between them and this one. Where two systems do meet, the junction is set out above instead.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Building Envelope · Envelope
- Ground Floor · Substructure
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Envelope Control Layers and Continuity
The continuous planes running around an enclosure — air, vapour, water and heat — and the interruptions that decide whether that continuity actually exists.
Browse all control layers entries →