Envelope and protection · Sealants & Membranes
Pre-Applied Bonded Waterproofing Membrane Below Slabs
Separates a bonded pre-applied membrane from the loose-laid damp-proof membrane it is often confused with, and places its selection and detailing firmly with a qualified waterproofing designer.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What pre-applied bonded membrane is
A pre-applied bonded membrane is a factory-made composite sheet, typically combining a strong carrier film with an adhesive layer and a protective coating or mineral surfacing. It is manufactured so that concrete cast directly against it develops an adhesive bond as it hardens, and it is supplied with matched tapes, detail strips and collars for the interruptions that any real structure contains.
It differs from a damp-proof membrane in more than duty. A damp-proof membrane is a loose-laid sheet, and water that reaches it can travel along the space between sheet and concrete until it finds a way through. A bonded membrane removes that space: because it is stuck to the concrete itself, a breach tends to stay local rather than becoming a path across a whole area. That is the distinction designers are reaching for.
It is a specialist product used within designed waterproofing, not a general purpose sheet. Whether a bonded approach is appropriate, how it is arranged, how it terminates and how it relates to the rest of the waterproofing are decisions for a qualified waterproofing designer working alongside the structural engineer, and they follow site investigation rather than product literature.
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.
- pre applied membrane
- fully bonded waterproofing membrane
- under slab waterproofing membrane
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Below-ground concrete construction where the design team has chosen a bonded waterproofing approach for the structure.
- Sites with retained excavation faces, where the outside of the structure cannot be reached once the works progress.
- Structures where the designer wants to limit the consequences of a local breach rather than accept a continuous void.
- Projects where the waterproofing designer calls for a manufacturer's complete system rather than assembled components.
- Works whose sequence allows the membrane to be installed and fully detailed before reinforcement and concreting begin.
Consideration
Where it is commonly considered
- Where a qualified waterproofing designer has assessed the site and set out a bonded approach as part of the design.
- Where the outer face of the structure will be permanently buried and no later access to it is credible.
- Where the works can be sequenced so that detailing and inspection happen before reinforcement covers the membrane.
- Where the manufacturer's own detail components can be used at every penetration, junction and termination.
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.
- Where no site investigation has established what water conditions the finished structure will face.
- Where the membrane would be exposed to weather and site traffic for longer than the manufacturer describes as acceptable.
- Where piles, anchors or services interrupt the area and the detailing of those interruptions has not been designed.
- Where concrete will be placed in a way, or at a time, that the manufacturer does not document as compatible with the bond.
- Where an existing structure is involved, since these products belong to new construction rather than to remedial work.
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 whole argument for these products is the bond, which is intended to prevent water tracking along the interface. That argument holds only if the bond actually forms, which depends on how the membrane is treated before the pour.
- Installation dependency
- Laps, penetrations and terminations decide continuity and are formed with the manufacturer's own tapes and collars. Contamination of the bonding surface before concreting is the usual cause of trouble.
- Substrate dependency
- The prepared surface the membrane is laid over has to be firm, even and free of standing water and debris, and producing that surface is a groundworks activity with its own tolerances and programme.
- Documentation
- Ask for the manufacturer's system documentation, the detail drawings for every interruption, the stated limits on exposure before concreting, and evidence of installer training where it is required.
- Repairability
- Damage found before the pour can be made good with the manufacturer's components. After the pour the membrane cannot be reached at all, which puts the entire weight on the pre-pour inspection.
- Weathering and exposure
- Exposure on site before concreting is a genuine constraint: weather, standing water, mud and traffic all affect the bonding surface, and manufacturers set out how long exposure may continue.
- Maintenance
- Nothing is maintained once the structure is complete, so the meaningful legacy is the evidence of correct installation and the drawings showing where the membrane runs and where it terminates.
Exposure
Exposure and environmental conditions
- What has site investigation established about groundwater and its variation, and who has interpreted it?
- Will the membrane be exposed to weather between installation and concreting, and for how long in practice?
- Are ground contaminants or gases present at the site that change what the design has to address?
- How will surface water on the site be controlled while the membrane is exposed?
- Does the design account for the conditions that will exist after backfilling, not only during the works?
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.
- Surface preparation to the manufacturer's requirements is a groundworks activity, and it affects the programme before waterproofing begins.
- Reinforcement fixing and formwork erection follow the completed membrane, so protecting it during those activities has to be planned.
- Every penetration, pile head and anchor that interrupts the area is an individual detail resolved on drawings, not on site.
- The concrete placement method and the bond the membrane needs have to be compatible, which is a question for the manufacturer.
- Terminations, where the membrane stops or meets another element, are the designer's decisions and are drawn before work starts.
Appearance
Appearance and finish considerations
- The membrane is permanently concealed, so selection is about documentation and detailing rather than any visual quality.
- Surface markings and printed lap lines exist to help installers work accurately and are not a finish of any kind.
- Where the membrane terminates and becomes visible at an edge, how that edge is closed and protected is a drawn detail.
- The finished concrete is not affected in any way an occupant would ever see, so appearance is not a selection criterion here.
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.
- What evidence does the manufacturer offer about the bond, and under what conditions was that evidence obtained?
- How long may the membrane remain exposed before concreting, according to the manufacturer rather than the programme?
- What happens at the interfaces with other waterproofing elements over the long term?
- Is there any provision at all for intervention if water appears later, or is this a single chance installation?
- How does the product behave where concrete is placed in stages rather than in one continuous operation?
Upkeep
Maintenance and repair considerations
- What record will exist of the completed installation, including photographs taken before the pour?
- Who is responsible if water appears in the finished structure, and what is the process for investigating it?
- Are the terminations and interfaces documented in a way a future surveyor could actually follow?
- Does any insurance-backed arrangement apply to this installation, and what conditions come with it?
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.
- This is specialist installation, frequently requiring installers trained by the system manufacturer.
- Pre-pour inspection is the only check available, so it needs to be written into the programme and recorded.
- Weather during installation and before concreting affects both the detailing and the bond that is meant to follow.
- Coordination with groundworks, piling, drainage and the reinforcement trade has to be arranged well in advance.
- Damage caused by following trades is the most common problem and needs a protection plan rather than good intentions.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The waterproofing designer's proposals, explaining what approach has been chosen for this structure and why.
- The manufacturer's system documentation, including every detail component that will be used.
- The manufacturer's stated limits on exposure before concreting and on conditions during installation.
- Evidence of installer training or registration wherever the manufacturer requires it.
- A photographic and written record of the completed installation before reinforcement and concreting.
- Drawings showing terminations and interfaces with every other element of the waterproofing.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Who is the waterproofing designer, and what site information does the design rest on?
- Why has a bonded approach been chosen here rather than another form of protection?
- How are piles, penetrations and anchors being detailed where they cross the membrane?
- What are the manufacturer's conditions for the bond, and can the programme actually meet them?
- How will the completed installation be inspected and recorded before it is covered?
- What is the plan if water appears in the finished structure?
- How does this element relate to the rest of the waterproofing design for the building?
- Is the installer trained by the manufacturer, and can that be evidenced?
Blind spots
Commonly overlooked points
- The bond is the product's whole argument, and it is lost quietly if the surface is contaminated before concreting.
- Programme pressure is the usual reason a membrane sits exposed for longer than the manufacturer describes.
- Pile heads and service entries are where designs are weakest, because they tend to be drawn late and detailed on site.
- Once the concrete is placed the membrane is beyond reach, so the pre-pour record is the only evidence anyone will have.
- What is being bought is the waterproofing design rather than the sheet, and the two are very often confused.
What this page does not do
- Below-ground waterproofing is designed for the specific structure and site by a qualified specialist. This entry is background and specifies nothing.
- No sheet excludes water by itself. What is achieved depends on the design, the concrete, the detailing, the installation and the inspection standing behind it.
- How elements are arranged within a construction is assembly design, and it is not addressed here.
- This entry offers no advice on investigating or remedying water ingress in an existing building.
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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