Buildings may appear static, but they are constantly moving. Concrete expands and contracts as temperatures change, structures settle, roofs respond to daily heating and cooling, and traffic or loading can introduce further movement into decks and other surfaces. For waterproofing systems, this matters. A surface may be completely watertight when a project is completed, but if the waterproofing cannot accommodate the movement that occurs beneath it, cracks and separation can eventually create pathways for water ingress.
Effective waterproofing therefore involves more than creating a barrier against water. The system must also be selected and detailed according to how the structure is expected to behave over time.
Why do buildings and substrates move?
Structural movement can occur for several reasons, and understanding the source of that movement is an important part of waterproofing specification.
Thermal expansion and contraction
Materials expand when temperatures rise and contract as they cool. On exposed roofs, terraces and balconies, this cycle can occur every day. The waterproofing layer and the substrate beneath it may also respond differently to temperature changes. If a system is too rigid to accommodate this movement, stress can build at the interface between the waterproofing and the structure. Over time, this can contribute to cracking, splitting or loss of adhesion.
Concrete shrinkage and cracking
Concrete naturally changes as it cures and dries. Shrinkage can result in fine cracks developing within a slab, while movement can also occur around joints, penetrations and transitions between different materials. Not every concrete crack represents structural failure, but cracks can become weak points in the waterproofing envelope if they are not assessed and treated correctly before the waterproofing system is installed. Simply applying a coating over an existing crack without understanding whether that crack is static or still moving may only hide the problem temporarily.
Settlement and structural movement
Buildings and structures can also experience settlement over time. Different parts of a building may move at slightly different rates, particularly where extensions, changes in substrate or varying loads are involved. Where this movement is expected, waterproofing needs to be designed around it rather than assuming that the substrate will remain completely rigid.
Traffic and loading
Movement is not limited to buildings themselves. Balconies, car park decks, terraces and other trafficable surfaces are repeatedly exposed to people, vehicles, equipment and changing loads. These forces can cause small amounts of deflection within the structure. The waterproofing system needs to maintain its integrity while the surface beneath it responds to these loads.
Why rigid waterproofing can fail
A waterproofing material may have excellent resistance to water but still be unsuitable for a surface that experiences movement.
A very rigid system can perform effectively on a stable substrate, but when that substrate expands, contracts or flexes, the waterproofing layer may not be able to follow the movement. Stress is then concentrated at vulnerable points.
Typical signs can include:
- Cracking in the waterproofing membrane
- Splitting around joints and corners
- Delamination from the substrate
- Water ingress around penetrations
- Repeated failure in previously repaired areas
This is why flexible waterproofing is particularly important for applications where movement is expected. Flexibility does not mean that a waterproofing system can compensate for every structural defect. Significant structural movement, active cracks and movement joints need to be identified and correctly addressed.
The objective is to specify a system capable of accommodating the normal movement expected within its intended application.
Adhesion matters just as much as flexibility
For a waterproofing membrane to move effectively with a substrate, it also needs to remain properly bonded to it. Poor adhesion can allow water to travel beneath the waterproofing layer, sometimes making the eventual source of a leak difficult to identify.
A fully bonded waterproofing system creates close contact between the membrane and the substrate. This makes correct surface preparation especially important. Before installation, factors such as contamination, moisture, existing coatings, surface strength and cracking need to be assessed. The waterproofing system is only as reliable as the surface to which it is bonded.
Robex’s RX Protadeck, for example, is a fully bonded, seamless waterproofing system. Its specification includes a primer to seal and bond to the substrate together with a glass fibre membrane designed to facilitate movement and deflection.
The details are often where movement becomes a problem
Large open areas are not necessarily the most vulnerable parts of a waterproofed surface. Movement frequently concentrates around transitions and details such as:
- Corners
- Upstands
- Wall-to-floor junctions
- Drainage outlets
- Pipe penetrations
- Expansion and movement joints
- Changes between different substrates
These locations require careful detailing because different surfaces may move in different directions or at different rates.
A waterproofing specification should therefore consider the complete area rather than simply the main horizontal surface. Correct detailing around these interfaces helps maintain continuity of the waterproofing envelope as the structure moves.
Exposure conditions also influence movement
Temperature and weather exposure can significantly increase the amount of movement a waterproofing system needs to tolerate. A roof exposed to direct sunlight can experience substantial temperature changes between daytime and night-time. Balconies and terraces are exposed to similar cycles, together with rain, UV exposure and pedestrian traffic.
Material selection should take these combined conditions into account. RX AquaArmour 10, for example, forms a flexible waterproofing membrane and is suitable for applications including roofs, terraces and balconies. It also offers UV resistance and adhesion to wet and damp substrates, making it one of the systems within the Robex range used for exposed waterproofing applications.
The important point, however, is that the application determines the system. Flexibility, adhesion, UV exposure, traffic and substrate conditions all need to be considered together.
Can waterproofing stop existing cracks?
Waterproofing can protect a structure against water ingress associated with certain cracks, but it should not be treated as a way to conceal an underlying structural problem. Before waterproofing cracks, it is important to determine:
- What caused the crack
- Whether it is stable or active
- How wide it is
- Whether further movement is expected
- Whether structural repair is required
A static crack may require a different treatment from an active joint designed specifically to allow movement. Applying waterproofing without first understanding the crack can result in the same failure appearing again through the new system.
Why waterproofing specification matters
There is no universal level of flexibility that makes one waterproofing system suitable for every project. A flat roof, swimming pool, balcony, retaining structure and car park deck all behave differently. The substrate, temperature exposure, loading, traffic and expected movement need to be considered before the waterproofing system is selected.
At Robex, waterproofing specification starts with understanding these conditions. Systems are selected according to the application rather than applying the same waterproofing solution to every surface.
This technical approach allows factors such as movement, adhesion, exposure and substrate condition to be addressed before they become causes of failure.
Waterproofing needs to move with the structure
Structural movement cannot simply be designed out of every building. Expansion, contraction, settlement and loading are normal parts of how materials and structures behave. The objective of good waterproofing is therefore not to prevent movement, but to understand it and specify a system capable of maintaining protection as that movement occurs.
Correct substrate assessment, flexible waterproofing where appropriate, strong adhesion and careful detailing all contribute to a waterproofing system that performs beyond the day it is installed.
Contact Robex to discuss the waterproofing requirements of your project and let our technical team help specify a system suited to the substrate, exposure conditions and expected movement.
Frequently asked questions about flexible waterproofing
What is flexible waterproofing?
Flexible waterproofing uses a membrane or coating designed to accommodate a degree of movement within the substrate while maintaining a continuous barrier against water ingress.
Can waterproofing be applied over cracks?
Some cracks can be repaired and incorporated into a waterproofing system, but the cause and type of crack should first be assessed. Active structural cracks or movement joints may require specialist detailing rather than simply being coated over.
Why does waterproofing crack?
Waterproofing can crack when the movement of the underlying structure exceeds what the system can accommodate. Incorrect specification, poor preparation, inadequate detailing and loss of adhesion can also contribute to failure.
Where is flexible waterproofing most important?
Flexible waterproofing is particularly relevant to areas exposed to movement and temperature changes, including roofs, balconies, terraces, decks and certain water-retaining structures.
Does waterproofing prevent structural movement?
No. Waterproofing does not prevent a building or substrate from moving. The waterproofing system should instead be specified and detailed to accommodate the expected movement while maintaining protection against water ingress.
