Kemper System compares 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.



n many refurbishment and new-build projects, specifying a liquid system for a roof, walkway or balcony is an obvious choice. Not so obvious are the key considerations to be made before the specification of one liquid system 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.

Wet or dry?

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 topcoat. 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.

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 into 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 intercoat 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 topcoat are more prone to windborne 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.

Savvy specification

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.

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 manufacturers’ 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.

Time for training

In any building project, meeting quality standards is of paramount importance. As with all roofing systems that have varying application techniques, system training should be available to operatives new to using a product. With the lack of regulation on training in the roofing sector, operatives are less likely to be installing a safe, compliant system without appropriate training and should consider the long-term benefits. Many liquid manufacturers, such as Kemper System, offer free system training to approved contractors so operatives are confident in using their products.

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 as well as long-term performance of the cured membrane must all be taken into consideration. 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.

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