Water underground behaves differently from rain on a roof. Soil holds moisture against walls for long periods, groundwater can rise and push on slabs and walls under pressure, and once the structure is backfilled the waterproofing can rarely be reached again. Below-grade waterproofing has to work the first time and keep working for the life of the structure.
This guide explains the main concepts and options for basements, underground car parks, pits, shafts and tunnels: which side of the wall to waterproof, when the membrane goes on, how sheet and liquid membranes differ, and why protection, drainage and detailing matter as much as the membrane itself. It is general information. A waterproofing design for a real structure needs a qualified designer, a site investigation including groundwater conditions, the manufacturer’s data and the codes and standards that apply locally.
Key takeaways
- Positive-side waterproofing is on the face where water arrives and protects the concrete; negative-side is on the inside face and holds water back after it has entered the structure.
- Pre-applied (blindside) membranes go on before the concrete is placed; post-applied membranes go on after the structure is built and accessible.
- Sheet membranes give controlled thickness; liquid and spray-applied membranes have no laps and are easier around complex shapes.
- Protection boards and drainage protect the membrane during backfill and reduce water pressure on it.
- Tunnels use their own approaches, including sheet membranes between linings and sprayed membranes.
- Penetrations and joints need specific details designed with the membrane system.
Understanding the water
Before choosing a system, the designer needs to know what the structure will face:
- Soil moisture and capillary water: damp ground without free water.
- Surface water and perched water: water that collects temporarily in backfill or above less permeable layers, often after rain.
- Groundwater: a water table that may be permanently or seasonally above the base of the structure, applying hydrostatic pressure.
- Aggressive water: groundwater containing sulfates, chlorides or acids that can attack concrete; see why concrete deteriorates.
Site investigation, borehole data and long-term groundwater monitoring inform the design. A common recommendation is to design for a higher water level than measured, because levels change seasonally and over time and drains can fail.
Defining how dry it needs to be
Requirements depend on how the space is used. Some standards and guides define grades of protection. For example, the British standard BS 8102 describes grades ranging from spaces where some seepage is acceptable to those requiring a fully dry environment, and three types of protection: barrier protection (a membrane), structurally integral protection (watertight concrete) and drained protection (cavity drainage systems). Other countries use their own classifications. The point for owners is to agree the required dryness in writing early, because it drives the whole design.
Positive side and negative side
| Approach | Where it goes | Advantages | Limitations |
|---|---|---|---|
| Positive side | Outside face, where water arrives | Keeps water out of the concrete, protecting reinforcement and finishes | Must be installed during construction or after excavation; usually inaccessible later |
| Negative side | Inside face | Accessible for installation and repair after construction | Water still enters and saturates the concrete; does not protect reinforcement from the water; depends on bond to resist pressure |
| Blindside (a form of positive side) | Applied to shoring or formwork before concrete is placed | Allows waterproofing where there is no working space outside the wall | Must survive concrete placement; requires careful sequencing |
Negative-side systems are usually cementitious or crystalline coatings, or drained cavity systems with internal pumps. They are common in remedial work on existing basements where the outside cannot be reached. They can control water entry, but the structure itself remains wet.
Pre-applied vs post-applied
Pre-applied (blindside) membranes are installed before the concrete. On slabs, they are laid on the prepared sub-base; on walls, they are fixed to the excavation support (sheet piles, secant piles or lagging) before the wall is cast. Many are designed to bond to the fresh concrete as it cures, creating a fully bonded system. A full bond is valuable because if the membrane is punctured, water cannot travel along the interface to find a crack elsewhere.
Post-applied membranes are installed on the outside of walls after they are cast and before backfill, where there is enough working space around the excavation. They allow the concrete to be inspected and repaired before waterproofing, but need clean, dry and properly prepared surfaces.
Sheet membranes vs liquid and spray-applied membranes
Sheet membranes
Common types include self-adhered rubberized asphalt or modified bitumen sheets, torch-applied bitumen sheets, thermoplastic membranes such as PVC, TPO and HDPE, and bentonite clay composites that swell when wet.
- Strengths: factory-controlled thickness; many are designed for pre-applied use; thermoplastic sheets can be welded and tested.
- Considerations: laps and seams are potential leak paths; complex shapes and penetrations take time and skill; self-adhered products need clean, dry surfaces and suitable temperatures; bentonite needs confinement and suitable groundwater chemistry.
Liquid and spray-applied membranes
These include polyurethane, polyurea, rubberized asphalt emulsions and hot-applied rubberized asphalt, among others.
- Strengths: no laps or seams; fully bonded to the substrate; adapts to complex geometry, corners and penetrations; spray-applied systems cover large areas quickly. See what is polyurea for the spray elastomer family.
- Considerations: thickness depends on site control and must be measured; surface preparation and substrate moisture are critical; generally post-applied rather than blindside, although some systems are designed for sprayed application in tunnels between linings.
Choosing between them
There is no single best option. Designers weigh water pressure, the structure’s movement and cracking, access, sequencing, how penetrations will be detailed, compatibility with other systems and the experience of available installers. Whatever is chosen, it should be a complete system from one manufacturer, including primers, detail materials and accessories.
Protection boards and drainage
A membrane on a basement wall is vulnerable during backfilling. Stones, compaction equipment and settlement can puncture or tear it. Protection boards, such as polymer sheets, insulation boards or drainage composites designed for the purpose, are placed over the membrane before backfill.
Drainage reduces the water pressure on the membrane:
- Drainage composites (a dimpled core with a filter fabric) on the wall carry water down.
- Perimeter drains at the footing level carry water away to a discharge point.
- Free-draining backfill and a less permeable cap at ground level reduce surface water entering the backfill.
- Grading directs surface water away from the building.
Drains need a reliable outlet and should be accessible for inspection and cleaning where possible. Where groundwater is permanently above the slab, drainage alone may not be practical, and the structure must resist the full water pressure.
Tunnels
Tunnel waterproofing deals with large areas, continuous water pressure in some ground conditions and very limited access after construction. Approaches vary by tunneling method and ground conditions.
- Drained tunnels collect groundwater behind the lining with drains and carry it away. A sheet membrane (often PVC or other thermoplastic) with a protective geotextile is placed between the primary support (often sprayed concrete) and the final cast lining, with drainage at the base.
- Undrained or watertight tunnels resist full groundwater pressure, often with the membrane divided into compartments by waterstops so that a leak can be located and injected.
- Sprayed waterproofing membranes are applied between layers of sprayed concrete and bond to both. They follow irregular surfaces closely and avoid welded seams, but they need careful thickness control and suitable surface conditions.
- Segmental linings in bored tunnels rely mainly on gaskets between segments rather than a separate membrane.
Fire performance, drainage maintenance, compatibility with the tunnel’s ventilation and the long-term behavior of the membrane under ground pressure are all part of the design. Tunnel linings are also locations where spalling over traffic is a concern; see concrete spalling prevention. Our guide to tunnel waterproofing systems covers these approaches in more depth.
Detailing joints and penetrations
The field area of a membrane rarely leaks. Details do.
| Detail | Common approach |
|---|---|
| Construction joints in concrete | Waterstops (PVC, hydrophilic strips or injectable hoses) cast into the joint, with the membrane reinforced over it |
| Movement joints | Joint-specific details that allow movement without tearing the membrane |
| Pipe and cable penetrations | Sleeves or puddle flanges cast into the concrete, with membrane collars or detailing compounds |
| Inside corners | Fillets or reinforcement strips to avoid sharp folds |
| Outside corners | Reinforcement strips and rounded edges |
| Terminations at ground level | Membrane carried above finished grade and secured with a termination bar and sealant |
| Transitions between systems | Overlaps detailed with materials both manufacturers confirm are compatible |
| Tie holes and formwork defects | Plugged and repaired before membrane application |
Penetrations added after waterproofing is complete are a frequent cause of leaks. Coordinating all services before the membrane goes on, and recording where they are, prevents many problems.
Inspection and testing before backfill
- Check surface preparation, moisture and conditions before application.
- Measure thickness of liquid membranes as the specification requires.
- Inspect laps and welded seams of sheets; thermoplastic seams can often be tested with air pressure or vacuum methods.
- Look for damage, blisters and incomplete details.
- For some systems, electronic leak detection or flood testing of horizontal areas is possible.
- Photograph and record everything before backfill, especially details and penetrations.
Bottom line
Below-grade waterproofing starts with understanding the water and agreeing how dry the space must be. Positive-side systems protect the structure and are preferred where they can be reached; negative-side systems are a practical remedy where they cannot. Pre-applied and post-applied, sheet and spray-applied membranes all have their place. Protection, drainage and careful detailing of joints and penetrations decide whether the system works. Because these systems are hard to reach once buried, design them with qualified professionals, follow the manufacturer’s requirements and inspect thoroughly before backfill. Our comparison of protective coating systems explains the generic coating chemistries.