Liquid waterproofing best practice

With the liquid roofing and waterproofing sector rapidly growing, Victoria Ramwell from Kemper System offers best practice advice on how to specify and install liquid systems.


The growth of the liquid roofing and waterproofing industry has provided specifiers and contractors with more product choice than ever before. The range of technologies now available also means liquids can be used for a huge variety of applications ranging from roofs and balconies through to walkways and car parks.

But regardless of the type of liquid solution being applied, there are some important steps to follow to ensure the system performs to the required standard and the building or structure remains waterproof.

Product research

With such a large number of liquid roofing products now available, specifiers must ensure they thoroughly research which one is most suitable for the application.

We’ve seen some new manufacturers offer liquids through distributors at low prices and claim guarantees of around 25 years. Specifiers need to tread carefully and establish whether a product is suitable for the purposes intended. If not, they risk a product being installed which may not perform effectively.

As a first step, consider requirements such as compliance with Building Regulations, including resistance to fire of the proposed roof build-up, as well as any specific performance characteristics, including the compatibility of the surfaces to be waterproofed, ability to withstand substrate movement, and resistance to damage from anticipated load levels or trafficking.

Independent product accreditation is used to effectively communicate conformity and suitability, so it is advisable to research the manufacturer’s current product certification. An example is BBA certification, but there are others. If in any doubt about the suitability of a product, the Liquid Roofing and Waterproofing Association (LRWA) may be able to assist.

Wet or dry

It is also important to establish whether a wet-on-wet or wet-on-dry system is appropriate. When specified from a manufacturer with appropriate third-party accreditations such as BBA or BDA certificates, both should offer a durable waterproofing solution, but there are key differences to consider before making a choice.

For example, when applying any liquid waterproofing product – whether it be by brush, roller or spray – it is often very difficult to achieve a consistent coverage rate. This means that although the average membrane thickness may be as required, when the system begins to cure, it could still be thin in parts, creating weak spots.

In addition, if a consistent depth of liquid has not been achieved, the liquid manufacturer’s warranty could be invalid and not conform with any third-party accreditation. Any remedial work required will then have to be carried out at the contractor’s or even client’s expense.

Achieving a consistent coverage rate can be more challenging when using a wet-on-dry liquid waterproofing product, especially with the topcoat. If not applied correctly, this can lead to UV damage to the insufficiently protected base layer and subsequent system breakdown.

Specifying a wet-on-wet system that utilises a reinforcement fleece, means it is easier to obtain a consistent depth and coverage as the liquid system completely saturates the reinforcement layer. Operatives know enough resin has been applied when the fleece is no longer visible, meaning it has been fully saturated.

The resin then cures to form a single, fleece-reinforced, chemically bonded membrane that is completely UV stable. This helps to avoid potential problems of inter-coat adhesion, thin spots or delamination.

Furthermore, as operatives don’t have to wait until the first application of waterproofing resin has dried before the final layer is installed, a wet-on-wet system can generate significant time savings and less site disruption.


The Liquid Roofing and Waterproofing Association (LRWA) advocates that all liquid waterproofing systems should provide a reinforcement layer as part of the application to optimise the system’s performance, but there are some products on the market without this.

Without a durable reinforcement, problems can develop. During specification, it is advisable to look for a liquid waterproofing solution with a reinforcement layer – typically a polyester fleece material when using a wet-on-wet solution or a fibreglass mat if using a wet-on-dry system, for example.

Thorough preparation

When it comes to installation, a crucial consideration is substrate preparation. If this is not carried out properly, there is a chance that the liquid installation will fail prematurely.

Before applying roof coatings, the surfaces to be coated must be firmly fixed and free from any contaminants that could prevent the primer or membrane from adhering properly. Most primers and coatings also require a dry surface to maximise the adhesion.

Surface profile should also be considered. Applying a liquid to a rough surface such as concrete will increase consumption and, therefore, your costs. In this situation, it may be more cost-effective to mechanically prepare the peaks in the substrate or fill the troughs with an approved material to flatten the profile. Generally, a coating will perform better under stress if it is applied to a uniform thickness.

Conversely, smooth metal surfaces may need to be abraded. This will normally improve adhesion.

Expert advice and guidance

Once the most appropriate liquid has been specified, operatives must also ensure they closely follow the manufacturer’s instructions. Each liquid waterproofing system is different, so the manufacturer will have its own set of instructions and recommendations for effective application, including suitable substrates, surface preparation, mixing and application.

It is advisable for contractors to take advantage of manufacturers’ training courses to ensure they’re fully up to speed on the product to be used and how to apply it, ensuring a robust waterproofing solution that will last for many years to come.

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