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A Guide to GRP Fibreglass Roofing Systems


10th April 2018

Our GRP fibreglass roofing guide is intended to help property owners understand more about the material, its uses and the how it can be applied to a flat roof. It's one of the most popular options available for the construction of flat roofs and brings with it numerous advantages, depending on the property type.

From the installation process through to its lifespan and cost, we answer all the important questions regarding GRP fibreglass below, so you can make the right choice for your property.

What is GRP Fibreglass?

Glass Reinforced Plastic (GRP) is comprised of polyester resin and chopped strand mat glass fibres, which are mixed together to form a laminate. GRP fibreglass has become one of the most popular forms of flat roofing systems because it is not only a strong material but doesn’t add too much extra weight to the structure.

GRP-Roof-Stores - GRP Fibreglass Roofing - Roof Stores
GRP Fibreglass Roofing

While it has been around in one form or another since the 40s and 50s, the modern version is far superior and used throughout the construction industry.

How to Install GRP Fibreglass Roofing

The first thing to consider when installing a GRP fibreglass roof is the weather. This is because it is a liquid material and can only be laid in dry, mild conditions.

If a GRP roof is laid in particularly cold weather, it is likely that the material will not cure. The temperature of the OSB boards being used should also be checked before proceeding.

Installing GRP Edge Trims

One side of these trims features a gloss finish and the other a matte finish for high adhesion. These need to be attached to the decking board with either staples or nails.

Preparing for Lamination

  1. The resin will need to be mixed and the first course of action is to add the catalyst before use. The mixing will ensure that any wax and styrene that may have settled at the bottom is mixed in, while the catalyst is an essential element required for the substance to harden once laid down. To avoid preparing too much resin for the day that may go to waste later, use a ration of 1.35kg of resin for every square metre of glass.
  2. Apply the resin to the corners and take time to bandage the trims first, providing a good idea of how long it will take to cure, which ideally should be between 20 to 30 minutes.
  3. Before the chopped mat can be laid out, thoroughly clean the deck and ensure all the trims are fixed firmly into place. Be sure to overlap the feathered edge of the mat on top of the cut edge, before laying it parallel to the drip trim. Overlap the trim by approximately 50mm but be sure not to go over the edge, leaving ends long for the time being.
  4. Each roll must follow the same pattern and the long ends can be cut off with a Stanley knife to create a straight and aesthetically pleasing edge. Then you should decide the best position from where the lamination can be finished.
  5. The mat can be rolled up as far away from the ladder as possible, leaving it on the roof to avoid any mix-ups later. Be sure to cut square pieces of the mat for corners and 200mm strips of bandage for the trim joints.


  1. ‘Wet out’ both sides of a 200m square mat piece with resin on the deck using a roller.
  2. Position the mat onto the face of the adjoining trims making sure the bottom edge is on the radius of the trim.
  3. Fold the mat around the corner, over the top of the trim itself and then back down to the deck.
  4. Cut the mat from the top of the trim upwards to make it easier to dress and feather.
  5. Using a paintbrush and a roller, the corners can then be feathered into place.
  6. With the same resin mix, bandage any joint in the trims in a similar way. Any trims or nails that are not due to be covered can be laminated with a small piece of mat.
  7. Before the bandages and corners have been cured, the deck can be laminated.

Laying Down the Laminate

  1. Place the mat along the lowest part of the roof in such a way that when it is unrolled it will not run off-line, before carefully rolling it back.
  2. 1/3 of the resin must be applied to the board, with two-thirds of the resin on the mat.
  3. The mat can then be unrolled onto the board which is now coated with resin.
  4. Let the resin soak into the mat for a few minutes to allow it to break the emulsion binder.
  5. Run the roller across the whole area of the roof to help the binding process. In colder weather, it will take a little longer.
  6. Make regular checks to see if it has consolidated, taking care to look out for any ‘pinholes’ that might lead to water penetration later.


Like the base resin, the topcoat needs to be cured with the addition of a catalyst. The topcoat should be applied as soon as possible when the laminate can be walked on without any stickiness. Or, at the very latest this should be applied within 24 hours.

What are the Advantages of Fibreglass Roofing?

There are numerous advantages to GRP Fibreglass roofing systems which explain their rising popularity in UK construction. These are the main reasons why so many people are opting for fibreglass roofing solutions:

3D-GRP-SIG - GRP Fibreglass Flat Roofing 3D Render - Roof Stores
GRP Fibreglass Flat Roofing 3D Render

  • Durability: GRP roofing systems have a reputation for their strength and durability which is one of the main reasons they are used. If you follow the correct guidelines or hire a professional company who will install the material correctly, your roof will remain in place without problems for years to come.
  • Weight: It is a great option for lightweight and green roofs. If other materials are too heavy for a structure then fibreglass will not only offer similar levels of protection, it also works well on more complex roof shapes.
  • No Joints: As the material is in liquid form it also means there are no joins or seams left on the roof surface after installation. This creates a smooth finish that other forms of roofing systems struggle to match.
  • Repairs: These are made incredibly easy thanks to the material being used. Not only is it a tough and durable solution but making repairs couldn’t be simpler. Maintenance can also be carried out by the property owner to a very high standard.
  • Aesthetics: GRP fibreglass comes in a wide range of colours which provides a great level of choice for property owners. If you have a specific colour scheme on the property you’re looking to match against, this is far more likely to be possible with GRP.

How Long Does GRP Fibreglass Last?

Professionals believe a well-installed GRP fibreglass roof will last for at least 25 years. You will have to also take into consideration the environment the surface is exposed to, the level of maintenance it receives over the years and the amount of foot traffic.

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Torch-on Bitumen Felt

This will all have an impact on the lifespan of the material but with good weather, low foot traffic and good maintenance, a GRP roof can last for far longer than the 25-year period.

How Much Does GRP Fibreglass Roofing Cost?

For the most up to date pricing for the products mentioned in this section, follow the links below.

Prices for GRP fibreglass systems will depend primarily on the scale of the job at hand. Professional companies will also factor in the cost of labour, disposal, additional materials and VAT. The best method is to contact at least three separate firms to receive a quote. This will detail all the costs involved.

To give you a rough indication, we would estimate that a single garage roof would cost approximately £1,500 to install. This would also include decking boards and all the materials required for the job.

If you wish to attempt the job yourself you will need to purchase the resin, topcoat and mixing catalyst for the roof surface. For the initial stages, CrysticROOF GRP Resin is our recommended choice when combined with the CrysticROOF GRP Topcoat. We also stock the Scott Bader Catalyst M Clear to help you complete your GRP Fibreglass roofing project with industry leading materials at a competitive price.


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