The Importance of Reinforcements in Cold-Applied Liquid Roofing Systems

Cold-applied liquid roofing systems are widely used across the UK for their seamless application, fast curing times and suitability for both new builds and refurbishments. However, one crucial element often overlooked is reinforcement. Whether in the form of polyester fabric or fibreglass matting, reinforcement can significantly enhance the strength, flexibility, and long-term reliability of the finished roof.

Why Reinforcement Matters

When used correctly, reinforcements enhance key aspects of a liquid roofing system:

Tensile Strength

Reinforced membranes are significantly more resistant to splitting, tearing, or puncturing under mechanical stress. The embedded fabric provides structural stability across the entire surface, helping the system maintain its integrity even when exposed to movement or foot traffic.

Movement Accommodation

Buildings naturally expand and contract in response to temperature changes. Reinforcement allows the membrane to move with the structure by bridging small cracks and accommodating minor shifts without failure. This is especially important in climates like the UK, where thermal fluctuation is frequent.

Durability

The presence of reinforcement improves the system’s ability to cope with environmental stress. Roofs exposed to UV rays, standing water, and temperature extremes benefit from the additional layer of protection reinforcement provides. It helps slow the ageing process and extends the service life of the membrane.

Consistent Thickness

During application, reinforcement helps installers maintain an even membrane build-up. It acts as a guide for coverage, reducing the likelihood of thin spots that can become vulnerable over time. This consistency is key to ensuring the membrane performs as intended across the entire surface.

While reinforcement is not essential for every installation, its inclusion often leads to a more robust, longer-lasting system, particularly where substrates are aged, detailing is complex, or long-term performance is a priority.

Common Types of Reinforcement

The type of reinforcement used varies depending on the system:

Polyester Fabric

Polyester is one of the most commonly used reinforcement materials due to its excellent flexibility and ease of handling. It conforms well to awkward roof shapes and detailing, making it especially suitable for systems based on polyurethane, hybrid resins and elastomeric coatings. Its balance of strength and pliability makes it a versatile option across many applications.

Fibreglass Matting

Fibreglass or chopped strand matting offers higher tensile strength but less flexibility compared to polyester. It is typically used in GRP (glass reinforced plastic), Polurethane (PU) and some PMMA-based systems, where rigidity and structural integrity are prioritised. While more brittle, fibreglass provides a robust reinforcement when paired with compatible resins and is often selected for full-roof encapsulation systems.

Fleece-backed Rolls

These reinforcement options combine a fleece fabric with a carrier film or pre-formed sheet, allowing for faster coverage in larger areas. They are particularly useful in certain polyurea and spray-applied systems, offering a built-in reinforcement layer that integrates during application. This method simplifies installation while still delivering the structural benefits of traditional fabric reinforcement.

Each type of reinforcement serves a different purpose depending on the roof’s movement, detailing complexity, expected traffic, and system requirements. Selecting the right fabric is essential to ensure compatibility with the liquid system and to meet the performance expectations of the project.

Where Reinforcement Is Typically Used

Reinforcement may be used across the whole roof or only in specific areas, depending on the manufacturer’s guidance and the complexity of the installation. Key areas often include upstands and wall junctions, gutter outlets, substrate transitions, roof penetrations and any locations where the membrane will encounter movement or changes in level. In these areas, reinforcement helps manage stress and prevent membrane failure.

Comparing Reinforced and Non-Reinforced Systems

Both approaches can be valid; the key is choosing the right specification for the job.

Final Thoughts (TLDR)

Cold-applied membranes deliver exceptional waterproofing, and reinforcement is often the foundation that keeps it that way.

Reinforcement is not a one-size-fits-all requirement, but it remains a vital tool in the cold-applied roofer’s toolkit. Whether used throughout or only in vulnerable areas, it plays a critical role in supporting longevity, flexibility and resistance to real-world conditions.

Contractors and specifiers should assess each project on its merits. If in doubt, consult your system supplier to confirm the correct reinforcement method for your chosen product.

Reinforcement by System Type

Different cold-applied systems handle reinforcement in different ways;

GRP (Glass Reinforced Plastic) aka Fibreglass Roofing

Originally taken from the boat-building industry, this system is inherently built around reinforcement. It uses fibreglass matting embedded in liquid resin to form a hard, structural waterproof layer. GRP is rigid and highly durable, making it a popular choice for small flat roofs and areas with limited movement.

PU (Polyurethane)

Polyurethane systems can use a range of reinforcement fabrics depending on the manufacturer and specific system design. While polyester fabric is commonly specified for its flexibility, some PU systems like Fibrecoat also use a lower grammage fibreglass matting to achieve higher tensile strength or to meet particular installation standards. The reinforcement chosen must be compatible with the resin and suitable for the roof's movement and detailing requirements.

PMMA (Polymethyl Methacrylate)

PMMA systems cure rapidly and require precision during application. Reinforcement is typically specified to control shrinkage, support adhesion, and maintain integrity across joints or detailing. The result is a fast-setting, robust solution suited to projects with tight timelines.

Hybrid Liquid Systems

These systems may combine multiple resin technologies and can be adapted to different substrates. Reinforcement is often used in specific areas of stress or over entire surfaces, depending on project complexity and performance needs.

Polyurea

Although often spray-applied without a fabric layer, Polyurea can incorporate reinforcement where additional stability is needed, such as at roof junctions or over existing substrates. This ensures long-term adhesion and performance in vulnerable zones.

Application Best Practices

The most common application method is wet-on-wet embedment, where the fabric is laid directly into a fresh coat of liquid, then fully saturated with a second layer. Other key points include ensuring no dry spots remain in the fabric, overlapping joints by at least 50 mm, avoiding wrinkles or air pockets, wrapping fabric around upstands and edge trims, and following curing times precisely. Trained installers will ensure these steps are carried out to a high standard.

Common Failures When Reinforcement Is Neglected

Skipping or poorly applying reinforcement increases the risk of multiple failure modes in a cold-applied liquid roof. Cracks are more likely to appear around joints, upstands, and areas of movement, where reinforcement would normally help absorb stress. While delamination is often linked to substrate preparation and trapped moisture, the absence of reinforcement can compound these issues by reducing membrane stability. Unreinforced systems may also suffer from premature wear, especially if application results in thin or uneven coverage, making the roof more susceptible to blistering, pinholing, or UV-related degradation.

Reinforcement Is About Specification, Not Just Strength

What matters most is understanding when and why to use reinforcement, and ensuring any decision is backed by sound technical reasoning and system certification.

While reinforced systems are often viewed as stronger or more premium, non-reinforced systems are not inherently inferior. In simple roof layouts, with stable substrates and minimal detailing, a well-designed unreinforced system can be just as effective, particularly when installed correctly and maintained regularly.