ccording to LRWA (Liquid Roofing & Waterproofing Association) data, the use of liquid roofing systems has grown by an impressive 70% over the past five years. This confirms their popularity for a wide variety of projects; from flat and green roof applications to balconies and walkways.

There is a huge variety of systems on the UK market, including wet-on-dry systems with a basecoat and glass fibre reinforcement that cure prior to application of a topcoat and wet-on-wet membranes that are applied in a single process with a fleece reinforcement layer. Within these generic product types, there is also an array of specialist systems including solvent-free, low-odour and rapid cure systems.

With such a diverse choice of liquid roofing systems available to specifiers, a performance specification based on the service life of the product does not always consider the specific requirements of the building, the end-user or the building occupiers. It may also fail to tap into the technical expertise of the supplier to advise on the most appropriate product for the specific application.

In many cases, specifiers and contractors often have a preferred system that they have used previously. As a liquid roofing specialist with a range that encompasses wet-on-wet and wet-on-dry systems as well as a range of different chemistries, developed over the course of decades by our research and development teams in the UK and around the world, Sika Liquid Plastics is uniquely placed to offer an unbiased summary of specification best practice.

The human factor

All liquid roofing systems are designed to offer an effective, long-term waterproofing solution for the substrate, sometimes with added benefits such as non-slip maintenance walkways or root-resistance for green roof suitability. However, if the system is incorrectly specified, badly installed or compromised during installation it is unlikely to perform to its BBA accreditation.

Consequently, involving the system supplier in the specification is vital to ensuring the best possible outcome for the project. This is particularly valuable on refurbishment schemes where the existing building’s factors may affect the specification requirements and issues such as substrate compatibility with the system’s chemical formula, insulation upgrades, replacement of rooflights, awkward detailing and building occupation may all need to be factored into the specification.

Once the specification has been agreed, any roofing system is only as effective as the contracting team that installs it. As a result, it’s vital to select a system from a manufacturer that not only invests in training and accrediting contractors in the use of its systems, but also provides technical support throughout the entire programme.

For example, the depth and evenness of liquid coverage during installation is vital to the efficacy of the finished system, regardless of whether it is wet-on-wet or wet-on-dry. Consequently, it’s critical that contractors do not attempt to save material or labour costs by cutting corners on the amount of liquid used. Our technical specification includes stipulation of the volume of liquid required per m2 of substrate and our applications team make regular visits to site throughout each programme to advise on installation ensuring the installed roof meets the specified performance requirements.

Building in buildability

One of the major challenges that has beset roofing projects since the dawn of time is the weather, particularly in the unpredictable British climate. It’s important to check the temperature limitations for installation as some systems cannot be installed in extremely cold conditions. Similarly, UV resistance is vital both during and post-installation.

It is rain, however, that presents the greatest hazard to delivering the programme on time and to the integrity of the completed project. All roofing systems require a clean and dry substrate before installation begins. For wet-on-wet systems, rain during installation can compromise membrane performance so the contractor must ensure that there is sufficient time for both application and cure of the complete system for the relevant section of roof before the onset of rain.

This is not true of all systems, however. For example, the Decothane wet-on-dry system features a moisture triggered curing mechanism, ensuring that it repels water instantaneously on application, not just after full cure. The basecoat can then provide waterproofing and UV protection for the roof for several days, if necessary, until weather conditions are sufficiently favourable for installation of the grey topcoat. The fibre reinforcement of these types of systems also have the added advantage around the ease and speed of detailing. A fleece-based, wet-on-wet system can be as time-consuming as a sheet membrane in these situations.

On smaller projects with few details, a wet-on-wet system could be more beneficial. Here, a contractor could be on and off a project within a day without the need to topcoat the project at a later date.

Similarly, speed of cure can be just as vital as speed of installation or waterproof protection in some instances. This is particularly true of walkways and balconies in residential projects. As the cost and inconvenience of providing temporary accommodation is prohibitive, these programmes are often carried out while the building remains occupied so a rapid cure system ensures residents can access their properties quickly. For example, Sikalastic Rapid has been developed especially for applications like this, using PMMA (polymethyl methacrylate) technology to provide cure times as fast as just 20 minutes, as above, this system is wet-on-wet meaning projects of this type can be completed within a shorter application window.

Odour and occupation

Regardless of the varying chemical technologies used to formulate roofing membranes, any solvent-based system emits an odour during application and these odours can become a nuisance when used on an occupied building. As a result, there is growing demand for a low-odour solution that is just as effective as other roofing membranes.

Products like this are ideal for refurbishment projects and are particularly popular for hospital, school and residential installations. As a result, Sika Liquid Plastics has developed a low-odour system in both wet-on-dry (Decothane Ultra) and wet-on-wet (Sikalastic STP) formulations to answer the demands of the refurbishment market. Sika Liquid Plastics is the only independently-accredited manufacturer that certifies a low-odour solution.

We worked to develop our patented Decothane technology and continue to improve Decothane Ultra enabling us to remove hazard phases, such as harmful by inhalation and toxic to the environment which is unique to our product in the industry.

Impartial advice

Ultimately, specifier and contractor preference will always play an important role in the choice of liquid membrane for a roofing project. However, it’s advisable to take technical advice from a roofing systems expert and a supplier that is long-established and offers both wet-on-wet and wet-on-dry systems and a range of different chemistries to ensure you are provided with experienced, impartial guidance.