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UKFR - ROOFING AND INSULATION SPECIALISTS
GRP roofing, also known as fibreglass roofing, is one of the most popular systems for flat roofs and is ideal for complex layouts with intricate detailing, pipes or skylights. In this blog, we’ll break down the biggest benefits of GRP roofing, as well as why it might not be right for you, to help you choose the system for your flat roof.
GRP stands for glass reinforced plastic, or more commonly known as fiberglass. A GRP roof is completely watertight and made up of a combination of resin and matting.
In most GRP systems, the first layer is the base coat/resin, which is formed using GRP resin and GRP matting which combine to create a waterproof lower base. This is applied in 3 parts all in the same process and drying time; apply the resin to the deck, roller in the matting when wet, and then apply a second layer of resin over the top. You should use a bubble buster to break down the matting and then allow it to dry.
However some systems, such as Flexitec 2020 require a primer prior to the base coat/resin application. For the Flexitec 2020 system the Flexitec 2020 Primer is most commonly used as it’s suitable for many surface types (unlike standard GRP which is only suitable for T+G OSB boards) including porous, bituminous, cementitious, timber and PVC single ply substrates. The primer must be mixed with a catalyst/powder hardener to initiate the curing process. There are also primers for use on EPDM & metal substrates.
After the resin and matting have cured, the surface should be slightly sanded and checked for any pinholes before applying the top coat.
The top layer is the top coat, which typically provides long-term UV protection and a modern colour finish. The Flexitec 2020 system differs here as the resin used for the base coat is also used as the top coat.
GRP is a highly versatile system for flat roofs, and is typically installed on garage roofs, shed roofs, extensions, or as balcony flooring.
It’s incredibly malleable and can form to any shape required during installation. This is particularly useful on complex roofs where you might have soil vents, roof pipes or skylights. In this situation, GRP is slightly easier to detail around things compared to EPDM.
GRP is very lightweight and thin, which reduces the strain on the roof joist and overall structure.
The finished waterproof layer typically has a thickness of around 1-2mm, and its weight generally averages to approximately 1.5 to 2.5kg per sq.m. This is similar to liquid roofing, which weighs around 1.5kg to 2kg per sq.m.
However, when comparing GRP to traditional roofing materials, the difference in weight is substantial. For example, Code 8 lead weighs around 40 kg per square metre, while built-up felt roofing systems typically weigh between 15 to 25 kg per square metre.
This makes GRP at least 10 times lighter than traditional roofing systems, which is ideal for the long-term performance of your roof.
Fibreglass roofing creates a thin, durable and puncture-resistant roof deck by combining the strength of the glass fibre matting with the hard, non-permeable layer of resin.
GRP systems are designed to be walked over, even in areas with high foot traffic. As a testament to its strength, they can also be used for balconies if used with an anti-slip grit.
A GRP roof’s lifespan depends on the system installed:
Compared to other roofing systems:
Like most modern roofing systems, GRP roofs have high resistance to UV, performing well in both hot and cold climates, making them less susceptible to cracks and blisters over time.
Some fibreglass roof systems can fade over time; this doesn’t cause damage to the roof and is purely an aesthetic issue. At UKFR Roofing & Insulation, we only sell fully UV-resistant fibreglass roofs.
It’s important to note that UV-resistant does not mean fade-proof. Whilst fading is very gradual and happens over time, it will not impact the waterproofing capabilities of the system as long as you have installed the compatible GRP topcoat. This only needs to be applied once at the end of installation, and you will benefit from years of UV protection.
GRP becomes 100% watertight once fully cured, preventing rainwater from seeping in.
The now hard, rigid surface becomes extremely durable and impenetrable to all weather conditions, including snow, ice, wind, rain, and subsequent physical impacts like hail or falling debris.
Fibreglass roofing is a wet-applied system, meaning it will mould to any roof shape, no matter how complex and will leave a seamless finish. The matting can also be formed to any shape, such as to suit upstands, allowing GRP systems to be used in conjunction with skylights.
The top coat used in a fibreglass roof is designed to cover the whole deck, including the trims and joints. This seamless finish is the reason many roofing contractors prefer to use fibreglass roofing, as it not only looks neat and professional, but it also reduces the time spent detailing around complex edges.
GRP can be used for green roofs, but it’s not the best membrane for them. This is because they do not perform well when exposed to standing water or damp conditions long-term; if it is, it will reduce their life expectancy.
This is why we recommend EPDM if you’re installing a green roof.
The cost is entirely dependent on the system you use. If you only need light access to the roof, then you would only need the 450g/600g matting and a basic top coat.
However, if you need a GRP roof covering for a warm deck roof or overlay or want a more flexible system, you would need the flexible 225g matting. This may increase material costs initially, but the longevity of the fibreglass roof ensures that a GRP roof is cost-effective.
Find out exactly how much your GRP roof will cost with our GRP Roofing Calculator.
We’ve built this ourselves, and it covers everything we need from you to provide you with an accurate quote. Simply enter your roof size, trim types, deck type and system type, and we’ll tell you exactly how much it will cost.
GRP is incredibly effective; however, it’s not right for every roof. These are some reasons why it might not be the right choice.
GRP is difficult to install. It requires a perfectly dry surface, the right temperature and must stay dry throughout the curing process. It is also very important that the correct ratios of any catalyst/hardener is used as well as resin to matting ratios. If this criterion is not met, it will lead to imperfections resulting in moisture. This is why we strongly recommend hiring a professional roofer to install it for you.
GRP is more expensive than other roofing systems, such as EPDM, in both material and installation costs. The GRP system includes the resin, matting, and specialist tools, and as we previously mentioned, GRP isn’t DIY-friendly, which is why we always recommend hiring a professional to install it. All of this together can become quite costly.
While GRP is incredibly durable, it is still subject to cracks, pinholes and other forms of damage. If this happens, you need to repair it; otherwise, it will result in water ingress and a subsequent failed roof.
The repair process is more complex and very involved compared to other roofing systems, with you needing to replace every layer of the roof.
We sell a GRP roof repair kit, which comes with everything you need for a repair job.
If GRP sounds like the right system, but the downsides are putting you off, RESITRIX can be a great middle-ground option. RESITRIX offers the best of EPDM and GRP, being both highly flexible and easy to handle, whilst using heat-welded seams for added security.
This makes it particularly effective for complex roof shapes or detailed areas where standard EPDM may struggle. It’s quicker and more straightforward to install compared to a full GRP system, but it provides extra confidence when dealing with more intricate sections of the roof.
