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Liquid roofing chemistries explained

This month we’re delighted to welcome Sarah Spink as our guest blogger. Sarah, CEO of the Liquid Roofing and Waterproofing Association (LRWA), explains the different cold applied liquid chemistries and offers advice on which ones to use for certain applications.

Liquid waterproofing technology is continuously evolving and the systems we use today are a result of decades of innovation. These systems are made up of a wide range of chemistries and that means some may be more suitable for certain applications than others.

So, what are the liquid roofing chemistries and when should you use them?

Glass reinforced plastic

This technology is often referred to as GRP, glass fibre or fibreglass, and consists of a glass fibre reinforced polyester resin. It is non-corrosive and durable and offers excellent waterproofing properties.

GRP systems are predominantly used for domestic properties with small flat roofs of less than 100 sq m. This is because the system requires a quality plywood or oriented strand board substrate although it can also be applied to surfaces such as concrete with the use of primers. Contractors would typically use GRP for foot trafficked areas such as walkways or balconies. It is also popular for garage roofs, outhouses and housing extensions.

The installation of GRP involves laying the substrate to form a structurally sound deck. Preformed edge trims are then fitted around the perimeter of the roof, at any significant junction or change in pitch. The waterproof laminate of a GRP system is applied onto the deck. This is made up of roofing resin combined with a catalyst which is used to saturate a reinforcement mat – typically a chopped strand mat (CSM) material. Once the laminate has cured, a coloured topcoat mixed with a catalyst is applied over the surface using a brush or soft roller.


Polyurethane systems are one of the most popular chemistry types as they can be used in a multitude of applications. The chemistry can provide a cost effective option for metal roofs, gutters, flat roofs of any size, warm roofs and green roofs, as well as trafficked walkways, balconies and terraces.

Products are available as both single and twin pack and may use glass fibre matting or polyester fleece reinforcement for single (wet-on-wet) application or multi-layer application. They can also be reactive systems, moisture triggered, or moisture cured. Some have an odour, and some are odour free, making them ideal for use in sensitive areas such as nurseries, hospitals and schools.

Polyurethane coatings are elastomeric allowing high flexibility to mitigate movement in the underlying structure, reducing the risk of potential cracks. As with many liquid waterproofing systems, polyurethanes also give inherently good chemical resistance and excellent durability against long term UV radiation, heat and water ponding.

Flexible polyester

This technology consists of an unsaturated polyester resin reinforced with either fibreglass mat or polyester fleece. It is very fast curing allowing for quick walk-on times.

Flexible polyester systems can be applied to a wide range of substrates and are equally suited to refurbishment or new build projects. Various colours and finishes can be incorporated.

The polyester resin incorporated within the system must be specially formulated to provide a high degree of flexibility and elasticity combined with fire performance. This technology can be used on all sizes of roofing projects as well balconies. Its component qualities make it quick and easy to apply.

Cold-applied bitumen emulsions

Bitumen is now one of the most popular construction materials and can be used in a reinforced membrane or adhesive. It is versatile, easy to apply and maintain, and durable and multi-functional.

Although five-year guarantees of bitumen-applied roofs were the norm in the 1970s when it became extremely popular as a liquid waterproofing solution, there are systems that have remained in service, untouched, for more than 35 years. Now, rubber-modified bitumen emulsions and solutions are being used commercially to provide cost effective, high performance waterproofing with a lifespan of 20 years or longer.

Bitumen has experienced many developments over time. The introduction of highly insulated structures meant systems needed to cope with greater temperature extremes. Hotter summers and colder winters resulted in higher levels of thermal expansion and contraction which meant rubber-based polymers were developed into product formulations.

PMMA (polymethyl methacrylate)

Widely used in the construction industry, PMMA has rapid curing qualities. It can dry in 30-40 minutes and, in some cases, even quicker by using an additional catalyst.

PMMA-based systems tend to have a minimum of two components, including a resin and a catalyst or activator. The solvent-free, cold-applied liquid resin cures rapidly even at temperatures around 0°C or lower. It is installed wet-on-wet which means the polyester reinforcing fleece is first saturated with liquid resin, bonding the membrane to the substrate. The surface layer is then applied onto the bonding coat while still wet to form a single, seamless membrane of uniform thickness.

PMMA is ideal for waterproofing projects where site access is very limited and work must be completed extremely quickly. This includes communal walkways, stairways, car parks or balconies on occupied housing blocks. On smaller roof areas, a one-day application may be possible as there is no need to wait for drying between coats.

Polyurea technology

Polyurea technology is an advanced liquid coating system. Its main benefit is that it is quick curing – ideal in applications where access can’t be restricted for many hours. It is often tack-free within 10 to 60 seconds after application and can be trafficked within the hour.

In addition to roofing applications, it is often specified for podium decks, balconies, bridge coatings, wastewater treatment plants, landscape and water containment, aquarium linings, water parks and playgrounds.

Polyurea systems provide excellent adhesion to substrates including concrete, cementitious screeds, metal, plywood, asphalt and bitumen sheets.

The technology can be used in any new build or refurbishment project as systems are often quick and easy to apply, durable and UV stable.

Impartial advice

With so many technologies now available, it is always advisable to consult an impartial organisation, such as the LRWA, who are launching their new Product Register. This new tool will provide contractors, distributors and specifiers with a tool enabling them to assess the suitability of liquid systems quickly and easily, based on information contained in third-party accreditations.

Accreditations provide the data that is needed to confirm that individual products will meet specified requirements and perform as expected. For the liquid waterproofing industry, agrément certificates and European Technical Approval (ETA) certificates are commonly used.

However, understanding agrément and ETA documents can be challenging and one of the biggest concerns is that people don’t read agrément certificates properly. This can lead to people making inaccurate assumptions or relying too heavily on manufacturers’ own product claims, some of which are misleading.

The LRWA Product Register aims to resolve these issues by compiling product information from agrément and ETA documents into a format that is quick and easy to read. This will ensure that anyone seeking to specify or install a liquid waterproofing solution can make an informed decision and that the product will perform as expected.

At CRS, we are experienced in applying all types of liquid roofs and can advise the most suitable system to meet your requirements. Get in touch with our experienced team if you’d like to discuss liquid roofing solutions.

Sarah Spink

CEO, Liquid Roofing and Waterproofing Association (LRWA)