A waterproofing membrane rarely fails in the middle of a wall or deck. Leaks start where the membrane stops, turns a corner, meets a pipe, crosses a joint or changes to another material. These details take a small fraction of the area and a large share of the risk. They are also the parts most often drawn as a single line on a section, left for the site to work out.
This guide explains how the common details work for basements, decks, roofs, tanks and other structures: waterstops, movement joints, penetrations, terminations, corners and transitions, and how to check them before they are covered. It is general information. Details for a real project must be designed by qualified professionals using the membrane and accessory manufacturers’ published details and data sheets, and the standards and codes that apply locally.
Key takeaways
- Construction joints in concrete need waterstops or equivalent measures; the membrane is reinforced over them.
- Movement joints need details that allow the expected movement, usually a loop, a flexible band or a dedicated joint system.
- Penetrations are best planned before waterproofing, with sleeves or flanges cast in and sealed with collars made for the system.
- Terminations must be mechanically secured and sealed, and carried far enough above water level or finished surfaces.
- Transitions between different systems need written confirmation of compatibility from both manufacturers.
- Inspection and testing before cover, with photos and records, is the last chance to find detail defects cheaply.
Draw the waterproofing line
A useful first step is to trace the waterproofing on the drawings as one continuous line around the protected space: under the slab, up the walls, over the roof or deck, and back down. Every place the line is interrupted is a detail. Typical interruptions are joints, penetrations, changes of material, changes of direction, terminations and junctions with adjacent buildings.
For each one, the drawings should show how continuity is maintained, which materials are used and how the detail can be inspected. Where the line passes from one trade to another, such as from a below-grade membrane to a wall cladding system, the drawings should say who is responsible for the overlap. Many leaks occur in the gap between two scopes of work.
Construction joints
A construction joint is where one concrete pour stops and the next starts. Even well-prepared joints are planes of weakness that can crack and let water through.
| Measure | How it works | Points to check |
|---|---|---|
| PVC or rubber waterstop | Profiled strip cast half in each pour, lengthening the water path | Must be fixed in position so it does not fold during pouring; joints in the strip welded or spliced as the manufacturer specifies |
| Hydrophilic strip | Swells on contact with water to seal the joint | Must be confined by enough concrete; early wetting before casting can cause premature swelling |
| Injection hose | Hose fixed to the joint face, later injected with resin or grout if the joint leaks | Ends must be accessible and recorded; injection is done when needed or as standard |
| Steel or coated steel waterstop | Metal plate across the joint | Common in some markets for slabs and walls |
| Crystalline joint treatment | Slurry or strip at the joint face | Used where the manufacturer has a tested detail; see crystalline waterproofing |
Where a membrane covers the joint, it is usually reinforced with an extra strip or a thicker coat across the joint, because a joint that cracks will open the membrane locally if it is bonded tightly over it. BS 8102 discusses how waterstops and membranes fit into the different types of protection for below-ground structures.
Movement joints
Movement joints are designed to open, close and shear as the structure moves with temperature, shrinkage, settlement or load. A membrane bonded across them will tear.
Common approaches:
- Unbonded zone and loop: the membrane is debonded either side of the joint over a set width, and a loop or extra-flexible band takes up movement.
- Joint bands and tapes: flexible elastomeric bands bonded to the substrate on each side of the joint and overlapped with the membrane.
- Dedicated expansion joint systems: on decks and roofs, preformed joint systems carry traffic or foot loads and connect to the membrane with a detail from the joint manufacturer.
- External and internal waterstops with a central bulb for movement in below-grade joints.
The designer needs to state the expected movement. The detail is then chosen to accommodate it with a margin, according to the product data. A joint sealant alone, exposed to water pressure and weather, is a maintenance item, not a permanent waterproofing.
Penetrations
Pipes, ducts, cables, anchors and drains pass through the waterproofing. They introduce a different material, a curved surface and often vibration or thermal movement.
| Situation | Common approach |
|---|---|
| Pipes cast into concrete | Puddle flange or water bar collar on the pipe, cast into the wall, with the membrane sealed to the pipe or flange |
| Sleeves for later services | Sleeve with flange cast in; the gap between pipe and sleeve sealed with a mechanical seal or a system designed for it |
| Groups of pipes or cables | Spaced far enough apart to allow detailing around each one, or brought through a cast box with a dedicated seal |
| Pipes through liquid membranes | Pipe surface cleaned and primed with a primer for that material, then a reinforced collar of membrane carried up the pipe |
| Pipes through sheet membranes | Prefabricated boots or collars from the membrane manufacturer, sealed or welded to the sheet |
| Drains and outlets | Membrane dressed into the drain body or clamped under a ring so water on the membrane can drain |
| Anchors through roofs and decks | Supports designed to sit on the waterproofing or with their own flashing details, not bare bolts through the membrane |
Two practical rules prevent many leaks. First, coordinate all services before waterproofing starts and leave enough space between penetrations for the detail. Second, prohibit new penetrations after the waterproofing is complete unless a procedure for sealing them is agreed. Drilling for an extra cable after the membrane is buried is a classic cause of leaks that are very hard to find.
Terminations
A termination is where the membrane ends. It must stop water from getting behind the top or edge of the membrane.
- Height: on walls, upstands and parapets, the membrane is carried to a height above finished ground, paving or roof level set by the code or manufacturer, so splashing water and snow do not reach the edge.
- Mechanical fixing: a termination bar or clamping profile holds the edge, so that the membrane does not peel under its own weight, wind or movement.
- Sealing: a sealant at the top of the bar or a counter flashing over it sheds water away from the edge. Sealants need periodic renewal.
- Protection: where the edge is exposed to impact or UV, a cover flashing or protection strip protects it.
- Below ground: terminations at the base of walls and slab edges must connect to the slab waterproofing, not simply stop.
Corners and changes of direction
Sharp corners concentrate stress and make it hard to apply even thickness. Common practice:
- Inside corners: form a fillet or cove with a compatible mortar or sealant so the membrane does not bridge a void.
- Outside corners: round or chamfer the edge and add a reinforcement strip.
- Wall to slab junctions: often treated as a joint, with extra reinforcement because shrinkage cracks commonly form there.
- Liquid membranes: extra coats or fleece at corners, because liquids tend to run thin at edges.
Transitions between systems
Many structures use different waterproofing systems in different areas, such as a sheet membrane below ground and a liquid membrane on a podium deck, or a roofing membrane meeting a wall barrier. Each transition raises questions:
- Compatibility: some materials soften, harden or fail to bond when in contact with others. Bitumen, PVC and some sealants are well-known examples of combinations that need care.
- Overlap length and method: set by the manufacturers.
- Sequence: which system is installed first and which laps over the other, considering the direction of water flow.
- Responsibility: who installs and guarantees the overlap.
Ask both manufacturers to confirm the transition detail in writing. Many warranties exclude interfaces with other systems unless approved.
Details in specific structures
Specific structures raise their own issues, covered in separate guides:
- Basements and underground structures: below-grade waterproofing.
- Bridge decks under asphalt: expansion joints, drains, curbs and parapets in bridge deck waterproofing.
- Parking decks: joints, drains and wear zones in parking deck traffic membranes.
- Roofs: upstands, outlets and plant supports in liquid-applied roof waterproofing.
Inspection before cover
Once the details are covered by backfill, protection board, screed, paving or a second concrete pour, they cannot be inspected without demolition. Hold points before cover are worth the time.
| Check | What to look for |
|---|---|
| Waterstops before concrete | Correct position, secured against movement, joints in the strip made properly, no folds |
| Penetrations | Collars complete, primers correct for each material, membrane carried to the required height |
| Terminations | Height, bar fixing, sealant, connection to adjacent work |
| Corners and joints | Fillets formed, reinforcement strips in place, unbonded zones where specified |
| Liquid membranes at details | Thickness, fleece fully saturated, no pinholes or thin edges |
| Sheet membranes at details | Laps and welds complete; welded seams tested where the system allows |
| Horizontal areas | Flood testing or electronic leak detection where suitable |
ASTM D5957 gives guidance on flood testing horizontal waterproofing installations, and ASTM D7877 covers electronic methods for detecting and locating leaks in waterproof membranes. Whether either is suitable depends on the membrane, the deck and the slope; the specification should say which tests apply.
Photograph every detail with a reference to its location before it is covered. If a leak appears later, these records shorten the search and help decide whether to repair from outside, inject from inside or both.
When details leak
Leaks at details in existing structures are usually treated by:
- Injection of joints and cracks with polyurethane or acrylic resins; see crack injection.
- Re-injection hoses where they were installed during construction.
- Local excavation or opening up to repair the membrane from the water side.
- Negative-side measures on the inside face where the outside cannot be reached.
The repair should address why the detail failed, not only seal the visible leak.
Bottom line
Waterproofing succeeds or fails at its details. Trace the waterproofing line around the structure, and give every joint, penetration, termination, corner and transition a detail drawn from the manufacturer’s tested system. Allow movement where the structure moves, plan penetrations before the membrane goes on, confirm compatibility at transitions in writing and inspect and record everything before it is covered. These decisions belong to qualified designers, working from manufacturers’ data sheets and published details and the applicable standards.