Specifier Review
Kemper System

Liquid Waterproofing: Key differences between wet-on-wet and wet-on-dry applications

Stuart Hicks from cold-applied liquid waterproofing specialist, Kemper System, looks at the key differences and benefits of installing a wet-on-wet vs. wet-on-dry liquid waterproofing solution.

In many refurbishment and new build projects, specifying a liquid system for a roof, walkway or balcony is an obvious choice.  However, not so obvious are the key considerations to be made before the specification of one liquid solution over another.

Liquid waterproofing systems are made up of different chemistries, use alternative application techniques and have different curing times, meaning some are far better suited in certain applications than others. There are both wet-on-wet and wet-on-dry systems available and when specified from a manufacturer with appropriate BBA Certification, should offer a durable waterproofing solution for the client and end user alike.

Key Differences

When applying any liquid waterproofing products – whether it be by brush, roller or spray – it is often very difficult to achieve a consistent coverage rate. This means when the system begins to cure, whilst the average membrane thickness may be as required, the overall system could still be thin in parts and the creation of weak spots can occur.

It can be argued that achieving a consistent coverage rate is more difficult when using a wet-on-dry liquid waterproofing product, especially with the top coat.  If not applied correctly, this can lead to UV damage of the insufficiently protected base layer and subsequent system breakdown. A depth gauge or cut core samples may be required to prove the correct coverage has been achieved, but only represents the place in which they are taken. This process also presents a risk of weak spots where the performance of the whole roof is dependent on bonding between the two separate layers.

liquid waterproofing

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 optimum saturation is assured.

In addition, if a liquid manufacturer offers a warranty with its product, if a consistent depth of liquid has not been achieved, the warranty can be invalid, with any remedial work at the contractor’s or even client’s expense.

Another key consideration is ensuring the total integration of base and surface build-up once the liquid waterproofing is applied. Wet-on-wet systems typically involve two-thirds of the required resin being poured directly onto the prepared substrate and evenly spread with a brush or roller.  Reinforcement fleece is then laid directly in to the wet resin with the remaining one-third of the resin immediately applied on top to completely saturate the fleece.  This 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.

On any waterproofing project, there’s always a risk of surface contamination and the substrate must be clean, dry and stable before any product application to ensure a full, seamless bond. Wet-on-dry systems that require the base coat to cure, before application of the top coat, are more prone to wind-borne debris or moisture forming on the surface overnight. This is less likely with wet-on-wet applications, as these systems are laid in a single process and don’t require additional coats or secondary surface preparation.

Specification Considerations

Industry bodies such as the Liquid Roofing and Waterproofing Association (LRWA) advocates all liquid waterproofing systems should provide a reinforcement layer as part of the application to optimise the system performance, but there are some products on the market without this. However, contractors need to be aware that 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 wet-on-wet, such as the solution offered by Kemper System, or a fibreglass mat if using a wet-on-dry system for example.

Time and cost savings should also be taken into consideration during specification, and it can be argued that a wet-on-wet system offers quicker possession times and less disruption on site, as operatives do not have to wait until the first application of resin has dried before the final layer is installed.

liquid waterproofing

However, with any liquid system, the British climate can often delay a waterproofing project, as the substrate must be clean and dry before application. No high quality certified system should fail if installed correctly according to manufacturer’s instructions, training and with full diligence by the operatives.  The benefit of wet-on-wet systems is they remove some of the application issues and roof failures associated with multiple coatings and uneven layer thickness. Quality manufacturers should also offer a system warranty for clients, developed to build the confidence of the contractor, specifier and the end user.

A Matter of Choice

There’s no doubt there are varying specification and installation benefits of the different application techniques of using a wet-on-wet system in comparison to a wet-on-dry technique. Time and labour cost savings, consistent depth and coverage of resin and long-term performance of the cured membrane must all be taken into consideration.

As with all roofing systems that have varying application techniques, system training should also be available to operatives new to using a product. Many liquid manufacturers, such as Kemper System, offer free system training to approved contractors so operatives are confident in using their products.

Whichever liquid waterproofing solution is specified, it is always advisable to opt for an experienced, well established liquid manufacturer with full system training, specialised on and off-site support and system warranties to ensure a sound waterproofing solution for many years.